Energy efficiency in Thermal utilities
(Chapter 3: Steam System)
Introduction
Steam has been a popular mode of conveying energy since the industrial revolution. Steam is used for generating power and also used in process industries such as sugar, paper, fertilizer, refineries, petrochemicals, chemical, food, synthetic fiber and textiles. The following characteristics of steam make it so popular and useful to the industry:
¢ Highest specific heat and latent heat
¢ Highest heat transfer coefficient e Easy to control and distribute Cheap and inert
Properties of Steam
Water can exist in the form of solid, liquid and gas as ice, water and steam respectively. If heat energy is added to water, its temperature rises until a value is reached at which the water can no longer exist as a liquid. We call this the “saturation” point and with any further addition of energy, some of the water will boil off as steam. This evaporation requires relatively large amounts of energy, and while it is being added, the water and the steam released are both at the same temperature. Equally, if steam is made to release the energy that was added to evaporate it, then the steam will condense and water at same temperature will be formed.
Liquid Enthalpy
Liquid enthalpy is the “Enthalpy” (heat energy) in the water when it has been raised to its boiling point to produce steam, |and is measured in kcal/kg, its symbol is h,. (also known as “Sensible Heat”)
Enthalpy of Evaporation (Heat Content
of Steam)
The Enthal f tion is the heat energy to be added to the water (when it has been raised to its boiling point) in order to change it into steam. There is no change in temperature,
the steam produced is at the same temperature as the water from which it is produced, but the heat
energy added to the water changes its state from water into steam at the same temperature.
When the steam condenses back into water, it gives up its enthalpy of evaporation, which it had acquired on changing from water to steam. The enthalpy of evaporation is measured in kcal/kg. Its symbol is h,,. Enthalpy of evaporation is also known as latent heat.
The temperature at which water boils,
also called as boiling point or saturation temperature increases as the pressure increases. When water
under pressure is heated, its saturation temperature rises above 100 °C. From this, it is evident that as
the steam pressure increases, the usable heat energy in the steam (enthalpy of evaporation), which is
given up when the steam condenses, actually decreases.
The total heat of dry saturated steam or enthalpy
of saturated steam is given by sum of the two enthalpies h, +h, (Refer Table 3.1 and Figure 3.1). When
the steam contains moisture the total heat of steam will be h, =h,.+ X h,, where X is the dryness
fraction.
The temperature of saturated steam is the same as the water from which it is generated, and corresponds
to a fixed and known pressure. Superheat is the addition of heat to dry saturated steam without increase
in pressure.
The temperature of superheated steam, expressed as degrees above saturation corresponding
to the pressure, is referred to as the degrees of superheat.
The Steam Phase Diagram
The data provided in the steam tables can also be expressed in a graphical form. Figure 3.1 illustrates
the relationship between the enthalpy and the temperature at various different pressures, and is known
as a phase diagram.
As water is heated from 0 °C to its saturation temperature, its condition follows the saturated liquid
line until it has received all of its liquid enthalpy, hf (A - B).
If further heat continues to be added, it then changes phase to saturated steam and continues to increase
in enthalpy while remaining at saturation temperature, hfg (B- C).
As the steam/water mixture increases in dryness, its condition moves from the saturated liquid line to
the saturated vapour line.
Therefore at a point exactly halfway between these two states, the dryness
fraction (X) is 0.5. Similarly, on the saturated vapour line the steam is 100% dry.
Once it has received all of its enthalpy of evaporation, it reaches the saturated vapour line. If it continues
to be heated after this point, the temperature of the steam will begin to rise as superheat is imparted
(C -D).
The saturated liquid and saturated vapour lines enclose a region in which a steam/water mixture exists
- wet steam. In the region to the left of the saturated liquid line only water exists, and in the region to
the right of the saturated vapour line only superheated steam exists.
The point at which the saturated liquid and saturated vapour lines meet is known as the critical point.
As the pressure increases towards the critical point the enthalpy of evaporation decreases, until it
becomes zero at the critical point.
This suggests that water changes directly into saturated steam at the
critical point.
Above the critical point only gas may exist. The gaseous state is the most diffuse state in which the
molecules have an almost unrestricted motion, and the volume increases without limit as the pressure
is reduced.
The critical point is the highest temperature at which liquid can exist. Any compression at constant
temperature above the critical point will not produce a phase change. Compression at constant
temperature below the critical point however, will result in liquefaction of the vapour as it passes from
the superheated region into the wet steam region. The critical point occurs at 374.15°C and 221.2 bar
(a) for steam. Above this pressure the steam is termed supercritical and no well-defined boiling point
applies.
Steam Distribution
The steam distribution system is the essential link between the steam generator and the steam user.
Whatever the source, an efficient steam distribution system is essential if steam of the right quality
and pressure is to be supplied, in the right quantity, to the steam using equipment. Installation and
maintenance of the steam system are important issues, and must be considered at the design stage.
As steam condenses in a process, flow is induced in the supply pipe.
Condensate has a very small
volume compared to the steam, and this causes a pressure drop, which causes the steam to flow through
the pipes. The steam generated in the boiler must be conveyed through pipe work to the point where
its heat energy is required. Initially there will be one or more main pipes, or ‘steam mains’, which
carry steam from the boiler in the general direction of the steam using plant. Smaller branch pipes can
then carry the steam to the individual pieces of equipment. A typical steam distribution system is shown
in Figure 3.2.
The working pressure
The distribution pressure of steam is influenced by a number of factors, but is limited by:
¢ The maximum safe working pressure of the boiler
¢ The minimum pressure required at the plant
As steam passes through the distribution pipe work, it will inevitably lose pressure due to:
¢ Frictional resistance within the pipe work
¢ Condensation within the pipe work as heat is transferred to the environment.
Therefore allowance should be made for this pressure loss when deciding upon the initial distribution
pressure. Having determined the working pressure, the following factors have to be considered in the
design of a good Steam Distribution System.
¢ General Layout
¢ Pipe sizing and Design
¢Pressure Reducing and Des-superheating Station
¢ Air Venting
¢ Steam Pipe Insulation
General Layout
General layout and location of steam consuming equipment is of great importance in efficient distribution
of steam. Steam pipes should be laid by the shortest possible distance rather than to follow a building
layout or road etc. However, this may come in the way of aesthetic design and architect’s plans and
a compromise may be necessary while laying new pipes.
Any modification and alteration in the existing steam piping, for supplying higher quality steam at
right pressure and quantity must consider the following points:
Pipe Sizing
The objective of the steam distribution system is to supply steam at the correct pressure to the point
of use. It follows, therefore, that pressure drop through the distribution system is an important feature.
Proper sizing of steam pipelines help in minimizing pressure drop. The velocities for various types
of steam are:
Superheated 50-70 m/sec
Saturated 30-40 m/sec
Wet or Exhaust 20-30 m/sec
For fluid flow to occur, there must be more energy at Point | than Point 2 (see Figure 3.3). The difference in energy is used to overcome frictional resistance between the pipe and the flowing fluid.
For fluid flow to occur, there must be more energy at Point | than Point 2 (see Figure 3.3). The difference in energy is used to overcome frictional resistance between the pipe and the flowing fluid.
¢ The friction factor (f) is an experimental coefficient which is affected by factors including:
o The Reynolds Number (which is affected by velocity).
o The reciprocal of velocity’.
Because the values for ‘f’ are quite complex, they are usually obtained from charts.
Example — Determining the pipe size
A process requires 5 000 kg/h of dry saturated steam at 7 kg/cm? (g). For the flow velocity not to
exceed 25 m/s, determine the pipe size.
Since the steam velocity must not exceed 25 m/s, the pipe size must be at least 130 mm; the nearest
commercially available size, 150 mm, would be selected.
In practice whether for water pipes or steam pipes, a balance is drawn between pipe size and pressure
loss. The steam piping should be sized, based on permissible velocity and the available pressure drop
in the line. Selecting a higher pipe size will reduce the pressure drop and thus the energy cost. However,
higher pipe size will increase the initial installation cost.
By use of smaller pipe size, even though the
installation cost can be reduced, the energy cost will increase due to higher-pressure drop. It is to be
noted that the pressure drop change will be inversely proportional to the 5" power of diameter change.
Hence, care should be taken in selecting the optimum pipe size.
Pressure Reducing De-superheating Station
A reduction in steam pressure through a pressure reducing valve (PRV) is an isenthalpic process.
Saturated steam when reduced to a lower pressure results in superheated steam. Since the process requires only saturated steam, desuperheating is often required, to compensate for superheat gained
in PRV application due to isenthalpic expansion. Pressure reduction and desuperheating of steam or
conditioning of steam is done in many process industries to suit process requirement. This is due to
the fact that steam is produced in a boiler economically at higher pressure and temperature. Generally,
the temperature of the steam after desuperheating will be closer to saturation temperature for heat
transfer applications.
The desuperheating of steam is done by spraying water through a spray nozzle into a pipe. Normally,
the desuperheating is done by automatic control system, using a control device for spraying water,
which takes the feed back from the temperature control loop.
Proper Air Venting
When steam is first admitted to a pipe after a period of shutdown, the pipe is full of air. Further, amounts
of air and other non-condensable gases will enter with the steam, although the proportions of these
gases are normally very small compared with the steam. When the steam condenses, these gases will
accumulate in pipes and heat exchangers. Precautions should be taken to discharge them. The
consequence of not removing air is a lengthy warming up period, reduction in plant efficiency and
process performance. Air in the steam system will also affect the system temperature. Air will exert
its own pressure within the system, - -
which will add to the pressure of the
steam to give a total pressure.
Therefore,
the actual steam pressure and
temperature of the steam-air mixture
will be lower than that shown by pressure gauge.
Automatic air vents for steam systems
(which operate on the same principle
as thermostatic steam traps) should be fitted above the condensate level so that only air or steam-air mixtures can reach
them. The best location for them is at
the end of the steam mains as shown in Figure 3.4.
In addition to air venting at the end of
a main, air vents should also be fitted:
o In parallel with an inverted bucket trap or, in some instances, a thermodynamic trap. These
traps are sometimes slow to vent air on start-up.
o In awkward steam spaces (such as at the opposite side to where steam enters a jacketed
pan).
o Where there is a large steam space (such as an autoclave), and a steam/air mixture could
affect the process quality.
Example — Effect of air in steam system
If 20% of air is entrained in a steam system at 5 kg/cm? (g) then the effect of air will be as follows
Steam quality = 80% Steam + 20% Air
Pressure = 0.80 x 5+0.20x 5
= 4 kg/cm2 (g) + 1 kg/cm?’ (g)
(steam) (air)
Temp. of steam at 5 kg/cm? (g) = 158°C
Temp. of vapour mixture is = 152 °C (equivalent to steam at 4 kg/cm2 (g))
Steam Pipe Insulation
The insulation of steam conveying pipes and the steam consuming equipment is very essential to retard
the flow of heat from the system to the environment. Broadly the purpose of steam pipe insulation is
as follows
1. Conserve energy by reducing heat loss
2. Facilitate temperature control of a process
3. Prevent condensation of steam
4. Prevent or reduce damage to pipe from exposure to fire or corrosive atmospheres
5. Control surface temperature for personal protection and comfort
The following table 3.2 indicates the effect of insulating bare pipes
Heat can be lost due to radiation from steam pipes. As an example while lagging steam pipes, it is
common to see leaving flanges uncovered. An uncovered flange is equivalent to leaving 0.6 metre of
pipe line unlagged. If a 0.15 m steam pipe diameter has 5 uncovered flanges, there would be a loss of
heat equivalent to wasting 5 tons of coal or 3000 litres of oil a year. This is usually done to facilitate checking the condition of flange but at the cost of considerable heat loss. The remedy is to provide
easily detachable insulation covers, which can be easily removed when necessary. The various insulating
materials used are cork, Glass wool, Rock wool and Asbestos.
Effect of insulation of flanges:
12 Flanges of 150 mm diameter.
Heat loss in the following 2 cases:
Case (I) — Bare flanges
Case (II) — Flanges with 50 mm insulation and aluminum cladding
Note: Calculation procedure to find out the economic thickness of insulation is given in chapter-5:
Insulation and Refractories.
In a steam system the major scope for improving energy efficiency lies in the utilization part. As shown
in Figure 3.5 the steam generation and distribution efficiencies are fairly high at more than 80% whereas
the utilization efficiency is only 47%.
When viewed from the standpoint of being a heat medium, steam has superior properties not offered
by other heat mediums. Among those, the following two are the most notable:
¢ Provides even heating
¢ Provides rapid heating
In the case of saturated steam, if the steam pressure is known then the steam temperature may be
determined. Pressure changes instantaneously within a space. When saturated steam condenses, it
condenses at the saturation temperature, and the saturated water (condensate) formed is of the same
temperature as the saturated steam. This means that if the pressure at the heat transfer surface (the
jacket or coil interior of the equipment) is held at a constant, continuous heating will be able to take
place at the same temperature at every part of the heat transfer surface.
The amount of the heat transfer is indicated by the heat transfer coefficient (= film coefficient of heat
transfer). The unit is [W/m2 K].
W = J/sec, so if heat exchange takes place on the same heat transfer surface area and with the same
temperature difference, the larger the heat transfer rate the shorter the time required for heating.
The rough values for the heat transfer rates of hot water and steam are as follows:
o The rate at which heat is transferred to the heat transfer surface of a heat exchanger using hot
water as the heat source:
1000 — 6000 [W/m2 K]
o The rate at which heat is transferred to the heat transfer surface of a heat exchanger using
steam as the heat source:
6000 — 15000 [W/m2 K]
In actual heating situations, the
heat transfer process will be a combination of the mechanism of heat transfer within the walls of the heat exchanger and the mechanism of heat transfer from the wall surface of the heat exchanger to the product being heated (Figure 3.6). Evaluations of heating must use the overall heat transfer coefficient [W/m2K] to indicate this combination. This coefficient varies greatly from heat exchanger to heat exchanger, but even so, steam heating shows
numbers 1.5 — 2 times those of
hot water heating.
Steam provides rapid heating because the transfer of heat caused by the process of condensing. The
latent heat contained in steam is released in the instant the steam condenses into liquid phase. The
amount of latent heat released is 2 — 5 times greater than the amount of sensible heat in the hot water
(saturated water) after condensation. This latent heat is released instantaneously and is transferred by
means of a heat exchanger to the product being heated.
In contrast, hot water and oil are used in convective heating, which does not involve a phase change.
Instead, the heat medium reduces its own temperature in order to transfer heat to the product being
heated. A mainstream in industry is the use of forced-convection by means of equipment such as a
pump to create the flow against the heat transfer surface.
Example —
Steam Utilization
1) A milk evaporator uses a steam jacketed kettle, in which milk is batch-processed at atmospheric
pressure. The kettle has a 680 kg per batch capacity. Milk is heated from a temperature of 26 °C to 100
°C, where 25% of its mass is then driven off as vapor. Determine the amount of 1 kg/cm? (g) steam
required per batch, not including the heating of the kettle itself. (Specific heat of milk is 0.90 kcal/kg
°C). The latent heat of steam at 1 kg/cm? is 525 kcal/kg.
Thermocompressor
In many of the steam utilization equipment where condensate
comes out at high pressure, a major portion of it flashes
into low pressure steam which goes wasted. Using a
thermocompressor (Figure 3.7) it becomes feasible to compress
this low pressure steam by high pressure steam and reuse it as
a medium pressure steam in the process. The major energy in
steam is in its latent heat value and thus thermocompressing
would give a large improvement in waste heat recovery.
Thermocompressors are designed to accurately mix
lower-pressure steam with higher-pressure steam. The
higher-pressure motive steam entrains the lower pressure steam
and increases its pressure. The motive steam is introduced
through the nozzle of the thermocompressor. As the nozzle
opens, the high velocity motive steam draws the lower-pressure
steam into the thermocompressor body. An exchange of
momentum occurs as the steam flows are mixed and the mixed
flow is accelerated to high velocity with a uniform profile in
the mixing chamber of the thermocompressor. As the mixed
flow enters the diffuser section, the diffuser flow area gradually increases to allow the velocity of the mixed flow to be reduced. As the velocity is reduced, the steam
pressure increases. At the end of the diffuser, the discharge steam pressure is higher than the lowerpressure suction flow entering the thermocompressor. A figure of thermocompressor is shown in Figure
3.7. A typical application is in evaporators where the boiling steam is recompressed and used as heating
steam.
Advantages of thermocompressors
¢ No condensation losses take place
¢ Thermal efficiency of the system is extremely high
¢ Entrainment of low pressure steam results is substantial savings
¢ No moving parts and hence maintenance needs are minimal
¢ No major operational charges
¢ Low space requirement
¢ Insensitive to fouling
¢ High operating reliability
Dryers
Drying is a process by which a liquid (commonly water) is removed from a material. This is usually achieved by applying heat, typically steam and/or the flow of carrier gas (commonly air) through or over the surface of the material (Refer Figure 3.8). The objective
of drying is to form a product that meets a water-content specification, so the amount of water removed depends on the desired product.
The basic drying energy requirement is the latent heat needed to evaporate the water. Clearly, this
depends on the amount of water being evaporated. In most cases, the product material, the carrier gas
and the equipment also need to be hot. So the total energy required includes:
¢ Heat leaving the dryer in the exhaust flow. This includes the latent heat of the water evaporated,
but the sensible heat of the hot gas can also be significant.
¢ Heat lost from equipment and ducting. However good the insulation, there is always some heat
loss.
¢ Heat leaving the dryer as hot product.
¢ Motive power for fans and conveyors.
Common factors resulting in excessive energy use
o Excessive drying load — for example, unnecessarily wet feed material, or off-specification
product that needs to be reprocessed
oExcessive airflows
oUnnecessarily hot exhaust flow
oHot air leaks
oPoor insulation
oExcessive fan power (for example, over specified fans restricted by dampers)
oSteam system inefficiencies
Example - Heat Energy in Air Drying
A food containing 80% water is to be dried at 100°C down to moisture content of 10%. If the initial
temperature of the food is 21°C, calculate the quantity of heat energy required per unit weight of the
original material, for drying under atmospheric pressure. The latent heat of vaporization of water at
100°C and at standard atmospheric pressure is 2257 kJ/kg. The specific heat capacity of the food is
3.8 kJ/kg °C and of water is 4.186 kJ/kg °C. Find also the energy requirement/kg water removed.
Calculating for 1 kg food
Initial moisture = 80%
800 g moisture is associated with 200 g dry matter.
Final moisture = 10 %,
100 g moisture is associated with 900 g dry matter,
Therefore (100 x 200)/900 g = 22.2 g moisture are associated with 200 g dry matter.
1 kg of original matter must lose (800 - 22) g moisture = 778 g = 0.778 kg moisture.
Heat energy required for | kg original material
= heat energy to raise temperature to 100°C + latent heat to remove water
= (100 - 21) x 3.8 + 0.778 x 2257
= 300.2 + 1755.9
= 2056 kJ.
Energy/kg water removed, as 2056 kJ are required to remove 0.778 kg of water,
= 2056/0.778
= 2643 kJ.
Steam is often used to supply heat to air or to surfaces used for drying. In condensing, steam gives up
its latent heat of vaporization; in drying, the substance being dried must take up latent heat of
vaporization to convert its liquid into vapour, so it might be reasoned that 1 kg of steam condensing
will produce | kg of vapour, neglecting minor losses.
Proper Selection, Operation and Maintenance of Steam Traps
The purpose of installing the steam traps is to obtain fast heating of the product and equipment by
keeping the steam lines and equipment free of condensate, air and non-condensable gases. A steam
trap is a valve device that discharges condensate and air from the line or piece of equipment without
discharging the steam.
Functions of Steam Traps
The three important functions of steam traps are:
¢ To discharge condensate as soon as it is formed
¢ Not to allow steam to escape.
¢ To be capable of discharging air and other incondensable gases.
Types of Steam Traps
There are three basic types of steam trap into which all variations fall, all three are classified by
International Standard ISO 6704:1982.
Thermostatic (operated by changes in fluid temperature) -
he temperature of saturated steam is
determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat),
producing condensate at steam temperature. As a result of any further heat loss, the temperature of the
condensate will fall. A thermostatic trap will pass condensate when this lower temperature is sensed.
As steam reaches the trap, the temperature increases and the trap closes.
Mechanical (operated by changes in fluid density) -
This range of steam traps operates by sensing
the difference in density between steam and condensate. These steam traps include ‘ball float traps’
and ‘inverted bucket traps’. In the ‘ball float trap’, the ball rises in the presence of condensate, opening
a valve which passes the denser condensate. With the ‘inverted bucket trap’, the inverted bucket floats
when steam reaches the trap and rises to shut the valve. Both are essentially ‘mechanical’ in their
method of operation.
Thermodynamic (operated by changes in fluid dynamics) -
Thermodynamic steam traps rely partly
on the formation of flash steam from condensate. This group includes ‘thermodynamic’, ‘disc’, ‘impulse’
and ‘labyrinth’ steam traps.
Some of the important traps in industrial use are explained as follows:
Inverted Bucket
The inverted bucket steam trap is shown in Figure 3.9. As its name implies, the mechanism consists
of an inverted bucket which is attached by a lever to a valve. An essential part of the trap is the small
air vent hole in the top of the bucket. In (i) the bucket hangs down, pulling the valve off its seat.
Condensate flows under the bottom of the bucket filling the body and flowing away through the outlet.
In (ii) the arrival of steam causes the bucket to become buoyant, it then rises and shuts the outlet. In
(iii) the trap remains shut until the steam in the bucket has condensed or bubbled through the vent hole
to the top of the trap body. It will then sink, pulling the main valve off its seat. Accumulated condensate
is released and the cycle is repeated.
In (11), air reaching the trap at start-up will also give the bucket buoyancy and close the valve. The
bucket vent hole is essential to allow air to escape into the top of the trap for eventual discharge through
the main valve seat. The hole, and the pressure differential, is small so the trap is relatively slow at
venting air. At the same time it must pass (and therefore waste) a certain amount of steam for the trap
to operate once the air has cleared. A parallel air vent fitted outside the trap will reduce start-up times.
¢ The inverted bucket steam trap can be made to withstand high pressures.
¢ Like a float-thermostatic steam trap, it has a good tolerance to water hammer conditions.
¢ Can be used on superheated steam lines with the addition of a check valve on the inlet.
¢ Failure mode is usually open, so it’s safer on those applications that require this feature, for
example turbine drains.
Disadvantages of the inverted bucket steam trap
¢ The small size of the hole in the top of the bucket means that this type of trap can only discharge
air very slowly. The hole cannot be enlarged, as steam would pass through too quickly during
normal operation.
¢ There should always be enough water in the trap body to act as a seal around the lip of the
bucket. If the trap loses this water seal, steam can be wasted through the outlet valve. This can
often happen on applications where there is a sudden drop in steam pressure, causing some of
the condensate in the trap body to ‘flash’ into steam. The bucket loses its buoyancy and sinks,
allowing live steam to pass through the trap orifice. Only if sufficient condensate reaches the
trap will the water seal form again, and prevent steam wastage.
Float and Thermostatic
The ball float type trap operates by sensing the difference in density between steam and condensate.
In the case of the trap shown in Figure 3.10A, condensate reaching the trap will cause the ball float to
rise, lifting the valve off its seat and releasing condensate. As can be seen, the valve is always flooded
and neither steam nor air will pass through it, so early traps of this kind were vented using a manually
operated cock at the top of the body. Modern traps use a thermostatic air vent, as shown in Figure
3.10B. This allows the initial air to pass whilst the trap is also handling condensate.
The automatic air vent uses the same balanced pressure capsule element as a thermostatic steam trap,
and is located in the steam space above the condensate level. After releasing the initial air, it remains
closed until air or other non-condensable gases accumulate during normal running and cause it to open
by reducing the temperature of the air/steam mixture. The thermostatic air vent offers the added benefit
of significantly increasing condensate capacity on cold start-up.
In the past, the thermostatic air vent was a point of weakness if water hammer was present in the
system. Even the ball could be damaged if the water hammer was severe. However, in modern float
traps the air vent is a compact, very robust, all stainless steel capsules, and the modern welding
techniques used on the ball makes the complete float-thermostatic steam trap very robust and reliable
in water hammer situations.
In many ways the float-thermostatic trap is the closest to an ideal steam trap. It will discharge condensate
as soon as it is formed, regardless of changes in steam pressure.
Advantages of the float-thermostatic steam trap
o The trap continuously discharges condensate at steam temperature. This makes it the first
choice for applications where the rate of heat transfer is high for the area of heating surface
available.
o It is able to handle heavy or light condensate loads equally well and is not affected by wide
and sudden fluctuations of pressure or flow rate.
o As long as an automatic air vent is fitted, the trap is able to discharge air freely.
o It has a large capacity for its size.
o The versions which have a steam lock release valve are the only type of trap entirely suitable
for use where steam locking can occur.
o It is resistant to water hammer.
Thermodynamic
The thermodynamic trap is an extremely robust steam trap with a simple mode of operation. The
trap operates by means of the dynamic effect of flash steam as it passes through the trap, as depicted
in Figure 3.11. The only moving part is the disc above the flat face inside the control chamber or
cap.
On start-up, incoming pressure raises the disc, and cool condensate plus air is immediately discharged
from the inner ring, under the disc, and out through three peripheral outlets (Figure 3.11, 1).
At the same time, the flash steam pressure builds up inside the chamber above the disc, forcing it down
against the incoming condensate until it seats on the inner and outer rings. At this point, the flash steam
is trapped in the upper chamber, and the pressure above the disc equals the pressure being applied to
the underside of the disc from the inner ring. However, the top of the disc is subject to a greater force
than the underside, as it has a greater surface area.
Eventually the trapped pressure in the upper chamber falls as the flash steam condenses. The disc is
raised by the now higher condensate pressure and the cycle repeats (Figure 3.11, iv).
Thermostatic
Thermal-element thermostatic traps are temperature actuated. On startup the thermal element is in a
contracted position with the valve wide-open, purging condensate, air, and other non-condensable
gases. As the system warms up, heat generates pressure in the thermal element, causing it to expand
and throttle the flow of hot condensate through the discharge valve.
When steam follows the hot condensate into the trap, the thermal element fully expands, closing the trap. If condensate enters the trap during system operation, it cools the element, contracting it off the seat, and quickly discharging condensate (Figure 3.12).
Thermostatic traps are small, lightweight, and compact. One trap
operates over extremely broad pressure and capacity ranges.
Thermal elements can be selected to operate within a range of
steam temperatures. In steam tracing applications it may be
desirable to actually back up hot condensate in the lines to extract
its thermal value.
Bimetallic Type
Bimetallic steam traps operate on the same principle as a heating thermostat. A bimetallic strip or wafer
connected to a valve bends or distorts when subjected to a change in temperature. When properly
calibrated, the valve closes off against a seat when steam is present, and opens when condensate, air,
and other noncondensable gases are present (Figure 3.13).
o Relatively small size for the condensate loads they handle
o Resistance to damage from water hammer
A disadvantage is that they must be set, generally at the plant, for a particular steam operating pressure. If the trap is used for a lower pressure, it may discharge live steam. If used at a higher steam pressure, it can back up condensate into the system.
A disadvantage is that they must be set, generally at the plant, for a particular steam operating pressure. If the trap is used for a lower pressure, it may discharge live steam. If used at a higher steam pressure, it can back up condensate into the system.
Thermostatic traps are often considered a universal steam trap; however, they are normally not
recommended for extremely high condensate requirements (over 7000 kg/hr). For light-to-moderately
high condensate loads, thermostatic steam traps offer advantages in terms of initial cost, long-term
energy conservation, reduced inventory, and ease in application and maintenance.
Installation of Steam Traps
In most cases, trapping problems are caused by bad installation rather than by the choice of the wrong
type or faulty manufacture. To ensure a trouble-free installation, careful consideration should be given
to the drain point, pipe sizing, air venting, steam locking, group trapping vs. individual trapping, dirt,
water hammer, lifting of the condensate, etc.
1) Drain Point
The drain point should be so arranged that the condensate can easily flow into the trap. This is not
always appreciated. For example, it is useless to provide a 15mm drain hole in the bottom of a 150
mm steam main, because most of the condensate will be carried away by the steam velocity. A proper
pocket at the lowest part of the pipe line into which the condensate can drop of at least 100mm diameter
is needed in such cases.
2) Pipe Sizing
The pipes leading to and from steam traps should be of adequate size. This is particularly important
in the case of thermodynamic traps, because their correct operation can be disturbed by excessive
resistance to flow in the condensate pipe work. Pipe fittings such as valves, bends and tees close to the
trap will also set up excessive backpressures in certain circumstances.
3) Air Binding
When air is pumped into the trap space by the steam, the trap function ceases. Unless adequate provision
is made for removing air either by way of the steam trap or a separate air vent, the plant may take a
long time in warming up and may never give its full output.
4) Steam Locking
This is similar to air binding except that the trap is locked shut by steam instead of air. The typical
example is a drying cylinder. It is always advisable to use a float trap provided with a steam lock release
arrangement.
5) Group Trapping vs. Individual Trapping
It is tempting to try and save money by connecting several units to a common steam trap as shown in
Figure 3.15A. This is known as group trapping. However, it is rarely successful, since it normally
causes water-logging and loss of output.
The steam consumption of a number of units is never the same at a moment of time and therefore, the
pressure in the various steam spaces will also be different. It follows that the pressure at the drain
outlet of a heavily loaded unit will be less than in the case of one that is lightly or properly loaded.
Now, if all these units are connected to a common steam trap, the condensate from the heavily loaded
and therefore lower pressure steam space finds it difficult to reach the trap as against the higher pressure
condensate produced by lightly or partly loaded unit. The only satisfactory arrangement, thus would
be to drain each steam space with own trap and then connect the outlets of the various traps to the
common condensate return main as shown in above Figure 3.15B.
Dirt is the common enemy of steam traps and the causes of many failures. New steam systems contain
scale, castings, weld metal, piece of packing and jointing materials, etc. When the system has been in
use for a while, the inside of the pipe work and fittings, which is exposed to corrosive condensate can
get rusted. Thus, rust in the form of a fine brown powder is also likely to be present. All this dirt will
be carried through the system by the steam and condensate until it reaches the steam trap. Some of it
may pass through the trap into the condensate system without doing any harm, but some dirt will
eventually jam the trap mechanism. It is advisable to use a strainer positioned before the steam trap
to prevent dirt from passing into the system.
7) Water Hammer
A water hammer (Figure 3.16) in a steam system is caused by condensate collection in the plant or
pipe work picked up by the fast moving steam and carried along with it. When this collection hits
obstructions such as bends, valves, steam traps or some other pipe fittings, it is likely to cause severe
damage to fittings and equipment and result in leaking pipe joints.
The problem of water hammer can be eliminated by positioning the pipes so that there is a continuous
slope in the direction of flow. In case of steam mains, a slope of at least 1 m in every 100 meters is
necessary, as also an adequate number of drain points every 30 to 50 meters.
8) Lifting the condensate
It is sometimes necessary to lift condensate from a steam trap to a higher level condensate return line
(Figure 3.17). The condensate will rise up the lifting pipe work when the steam pressure upstream of
the trap is higher than the pressure downstream of the trap.
The pressure downstream of the trap is generally called backpressure, and is made up of any pressure
existing in the condensate line plus the static lift caused by condensate in the rising pipe work.
The
upstream pressure will vary between start-up conditions, when it is at its lowest and running conditions,
when it is at its highest.
Backpressure is related to lift by using the following approximate conversion: | metre lift in pipe work
= 1 m head static pressure or 0.1 bar backpressure.
If a head of 5 m produces a backpressure of 0.5 bar, then this reduces the differential pressure available
to push condensate through the trap; although under running conditions the reduction in trap capacity
is likely to be significant only where low upstream pressures are used.
In steam mains at start-up, the steam pressure is likely to be very low, and it is common for water to
back-up before the trap, which can lead to water hammer in the space being drained. To alleviate this
problem at start-up, a liquid expansion trap, fitted as shown in Figure 3.17, will discharge any cold
condensate formed at this time to waste.
As the steam main is warmed, the condensate temperature rises, causing the liquid expansion trap to
close. At the same time, the steam pressure rises, forcing the hot condensate through the ‘working’
drain trap to the return line.
The discharge line from the trap to the overhead return line preferably discharges into the top of the
main rather than simply feed to the underside, as shown in Figure 3.17. This assists operation, because
although the riser is probably full of water at start-up, it sometimes contains little more than flash steam
once hot condensate under pressure passes through. If the discharge line were fitted to the bottom of
the return line, it would fill with condensate after each discharge and increase the tendency for water
hammer and noise.
It is also recommended that a check valve be fitted after any steam trap from where condensate is
lifted, preventing condensate from falling back towards the trap. The above general recommendations
apply not just to traps lifting condensate from steam mains, but also to traps draining any type of
process running at a constant steam pressure. Temperature controlled processes will often run with
low steam pressures. Rising condensate discharge lines should be avoided at all costs, unless automatic
pump-traps are used.
Maintenance of steam traps
Dirt is one of the most common causes of steam traps blowing steam. Dirt and scale are normally
found in all steam pipes. Bits of jointing material are also quite common. Since steam traps are connected
to the lowest parts of the system, sooner or later this foreign matter finds its way to the trap. Once
some of the dirt gets logged in the valve seat, it prevents the valve from shutting down tightly thus
allowing steam to escape. The valve seal should therefore be quickly cleaned, to remove this obstruction
and thus prevent steam loss.
In order to ensure proper working, steam traps should be kept free of pipe-scale and dirt. The best way
to prevent the scale and dirt from getting into the trap is to fit a strainer. Strainer (Figure 3.18) is a
detachable, perforated or meshed screen enclosed in a metal body. It should be borne in mind that the
strainer collects dirt in the course of time and will therefore need periodic cleaning. It is of course,
much easier to clean a strainer than to overhaul a steam trap.
At this point, we might mention the usefulness of a sight glass fitted just after a steam trap. Sight
glasses are useful in ascertaining the proper functioning of traps and in detecting leaking steam traps.
In particular, they are of considerable advantage when a number of steam traps are discharging into a
common return line. If it is suspected that one of the traps is blowing steam, it can be quickly identified
by looking through the sight glass.
In most industries, maintenance of steam traps is not a routine job and is neglected unless it leads to
some definite trouble in the plant. In view of their importance as steam savers and to monitor plant
efficiency, the steam traps require considerably more care than is given.
One may consider a periodic maintenance schedule to repair and replace defective traps in the
shortest possible time, preferable during regular maintenance shut downs in preference to break
down repairs.
Guide to Steam Trap Selection
Actual energy efficiency can be achieved only when
a) Selection
b) Installation and
c) Maintenance of steam traps meet the requirements for the purpose it is installed
The following Table 3.3 gives installation of suitable traps for different process applications.
Steam trap performance assessment 1s basically concerned with answering the following two questions:
¢ Is the trap working correctly or not?
¢ If not, has the trap failed in the open or closed position?
Traps that fail ‘open’ result in a loss of steam and its energy. Where condensate is not returned, the
water is lost as well. The result is significant economic loss, directly via increased boiler plant costs,
and potentially indirectly, via decreased steam heating capacity.
Traps that fail ‘closed’ do not result in energy or water losses, but can result in significantly reduced
heating capacity and/or damage to steam heating equipment.
Visual Testing
Visual testing includes traps with open discharge, sight
glasses (Figure 3.19), sight checks, test tees and three way
test valves. In every case, the flow or variation of flow is
visually observed. This method works well with traps that
cycle on/off, or dribbles on light load. On high flow or
process, due to the volume of water and flash steam, this
method becomes less viable. If condensate can be diverted
ahead of the trap or a secondary flow can be turned off, the load on the trap will drop to zero or a very minimal amount so the visual test will allow in determining the leakage.
Sound testing includes ultrasonic leak detectors (Figure 3.20), mechanics stethoscopes, screwdriver
or metal rod with a human ear against it. All these use the sound created by flow to determine the trap
function like the visual method. This method works best with traps that cycle on/off or dribbles on
light load. Traps which have modulating type discharge patterns are hard to check on high flows.
(Examples are processes, heat exchangers, air handling coils, etc). Again by diverting condensate flow
ahead of the trap or shutting off a secondary flow as mentioned under visual testing, the noise level
will drop to zero or a very low level if the trap is operating correctly. If the trap continues to flow
heavily after diversion it would be leaking or blowing through.
Temperature testing includes infrared guns (Figure 3.21), surface pyrometers, temperature tapes, and
temperature crayons. Typically they are used to gauge the discharge temperature on the outlet side of
the trap. In the case of temperature tapes or crayon, they are set for a predetermined temperature and
they indicate when temperature exceeds that level. Infrared guns and surface pyrometer can detect
temperatures on both sides of the trap. Both the infrared and surface pyrometers require bare pipe and a clean surface to achieve a reasonable reading. The
temperature reading will typically be lower than actual
internal pipe temperature due to the fact that steel does have
some heat flow resistance. Scale on the inside of the pipe
can also affect the heat transfer. Some of the more expensive
infrared guns can compensate for wall thickness and
material differences. Blocked or turned off traps can easily
be detected by infrared guns and surface pyrometers, as
they will show low or cold temperatures. They could also
pick up traps which may be undersized or backing up large
amounts of condensate by detecting low temperature readings.
1. Monitoring Steam Traps
For testing a steam trap, there should be an isolating valve provided in the downstream of the trap and
a test valve shall be provided in the trap discharge. When the test valve is opened, the following points
have to be observed:
Condensate discharge-
Inverted bucket and thermodynamic disc traps should have intermittent
condensate discharge. Float and thermostatic traps should have a continuous condensate discharge.
Thermostatic traps can have either continuous or intermittent discharge depending upon the load. If
inverted bucket traps are used for extremely small load, it will have a continuous condensate discharge.
Flash steam-
This shall not be mistaken for a steam leak through the trap. The users sometimes get
confused between a flash steam and leaking steam. The flash steam and the leaking steam can be
approximately identified as follows:
¢ Ifsteam blows out continuously in a blue stream, it is a leaking steam.
¢ Ifsteam blows out continuously in a blue stream, it is a leaking steam.
¢ Ifasteam floats out intermittently in a whitish cloud, it is a flash steam.
2. Continuous steam blow and no flow indicate, there is a problem in the trap.
Whenever a trap fails to operate and the reasons are not readily apparent, the discharge from the trap
should be observed. A step-by-step analysis has to be carried out mainly with reference to lack of
discharge from the trap, steam loss, continuous flow, sluggish heating, to find out whether it is a system
problem or the mechanical problem in the steam trap.
3. Avoiding Steam Leakages
Steam leakage is a visible indicator of waste and
must be avoided. It has been estimated that a 3 mm diameter hole on a pipeline carrying 7kg/cm2 steam would waste 33 KL of fuel oil per year. Steam leaks on high-pressure mains are prohibitively costlier than on low pressure mains. Any steam leakage must be quickly attended to. In fact, the plant should consider a regular surveillance programme for identifying leaks at pipelines, valves, flanges and identifying leaks at pipelines, valves, flanges and may reach up to 5% of the steam consumption in
a small or medium scale industry or even higher in installations having several process departments.
To avoid leaks it may be worthwhile considering
replacement of the flanged joints which are rarely opened in old plants by welded joints. Figure 3.22
provides a quick estimate for steam leakage based on plume length.
Example
Plume Length = 700 mm
Steam loss = 10 kg/h
The following Table 3.4 highlights the significance of loss through steam leaks.
4. Providing Dry Steam for Process
The best steam for industrial process heating is the dry saturated steam. Wet steam reduces total heat
in the steam. Also water forms a wet film on heat transfer and overloads traps and condensate equipment.
Super heated steam is not desirable for process heating because it gives up heat at a rate slower than
the condensation heat transfer of saturated steam.
It must be remembered that a boiler without a super heater cannot deliver perfectly dry saturated steam.
At best, it can deliver only 95% dry steam. The dryness fraction of steam depends on various factors,
such as the level of water to be a part of the steam. Indeed, even as simple a thing as improper boiler
water treatment can become a cause for wet steam.
As steam flows through the pipelines, it undergoes progressive condensation due to the loss of heat to
the colder surroundings; the extent of the condensation depends on the effectiveness of the lagging.
For example, with poor lagging, the steam can become excessively wet.
Since dry saturated steam is required for process equipment, due attention must be paid to the boiler
operation and lagging of the pipelines.
Wet steam can reduce plant productivity and product quality, and can cause damage to most items of
plant and equipment. Whilst careful drainage and trapping can remove most of the water, it will not
deal with the water droplets suspended in the steam. To remove these suspended water droplets,
separators are installed in steam pipelines.
The steam produced in a boiler designed to generate saturated steam is inherently wet. Although the
dryness fraction will vary according to the type of boiler, most shell type steam boilers will produce
steam with a dryness fraction of between 95 and 98%. The water content of the steam produced by
the boiler is further increased if priming and carryover occur.
A steam separator (Refer Figure 3.23) may be installed on the steam main as well as on the branch
lines to reduce wetness in steam and improve the quality of the steam going to the units. By change
of direction of steam, steam separators causes the entrained water particles to be separated out and
delivered to a point where they can be drained away as condensate through a conventional steam trap.
A few types of separators are illustrated in the Figure 3.23 below
5. Utilising Steam at the Lowest Acceptable Pressure for the Process
A study of the steam tables would indicate that the latent heat in steam reduces as the steam pressure
increases. It is only the latent heat of steam, which takes part in the heating process when applied
to an indirect heating system. Thus, it is important that its value be kept as high as possible. This
can only be achieved if we go in for lower steam pressures. As a guide, the steam should always be
generated and distributed at the highest possible pressure, but utilized at as low a pressure as possible
since it then has higher latent heat.
However, it may also be seen from the steam tables that the lower the steam pressure, the lower will
be its temperature. Since temperature is the driving force for the transfer of heat at lower steam
pressures, the rate of heat transfer will be slower and the processing time greater. In equipment where
fixed losses are high (e.g. big drying cylinders), there may even be an increase in steam consumption
at lower pressures due to increased processing time.
There are, however, several equipments in
certain industries where one can profitably go in for lower pressures and realize economy in steam
consumption without materially affecting production time.
Therefore, there is a limit to the reduction of steam pressure. Depending on the equipment design,
the lowest possible steam pressure with which the equipment can work should be selected without
sacrificing either on production time or on steam consumption.
6. Proper Utilization of Directly Injected Steam
The heating of a liquid by direct injection of steam is
often desirable. The equipment required is relatively
simple, cheap and easy to maintain. No condensate
recovery system is necessary. The heating is quick,
and the sensible heat of the steam is also used up along
with the latent heat, making the process thermally
efficient. In processes where dilution is not a problem,
heating is done by blowing steam into the liquid (i.e) direct steam injection is applied. If the dilution of the tank contents and agitation are not acceptable in the process (i.e) direct steam agitation are not acceptable, indirect steam heating is the only answer.
Ideally, the injected steam should be condensed completely as the bubbles rise through the liquid.
This is possible only if the inlet steam pressures are kept very low—around 0.5 kg/cm2 —and certainly
not exceeding | kg/cm’. If pressures are high, the velocity of the steam bubbles will also be high
and they will not get sufficient time to condense before they reach the surface. Figure 3.24 shows a
recommended arrangement for direct injection of steam.
7. Minimising Heat Transfer Barriers
The metal wall may not be the only barrier in a heat transfer process. There is likely to be a film of
air, condensate and scale on the steam side. On the product side there may also be baked-on product
or scale, and a stagnant film of product.
Agitation of the product may eliminate the effect of the stagnant film, whilst regular cleaning on the
product side should reduce the scale.
Regular cleaning of the surface on the steam side may also increase the rate of heat transfer by reducing
the thickness of any layer of scale; however, this may not always be possible. This layer may also be
reduced by careful attention to the correct operation of the boiler, and the removal of water droplets
carrying impurities from the boiler.
The elimination of the condensate film is not quite as simple. As the steam condenses to give up its
enthalpy of evaporation, droplets of water may form on the heat transfer surface. These may merge
together to form a continuous film of condensate. The condensate film may be between 100 and 150
times more resistant to heat transfer than a steel heating surface, and 500 to 600 times more resistant
than copper.
As air is such a good insulator, it provides even more resistance to heat transfer. Air may be between
1500 and 3000 times more resistant to heat flow than steel, and 8000 to 16000 more resistant than
copper. This means that a film of air only 0.025 mm thick may resist as much heat transfer as a wall
of copper 400 mm thick. Of course all of these comparative relationships depend on the temperature Figure 3.25 illustrates the effect this combination of layers has on the heat transfer process. These
barriers to heat transfer not only increase the thickness of the entire conductive layer, but also greatly
reduce the mean thermal conductivity of the layer.
The more resistant the layer to heat flow, the larger the temperature gradient is likely to be. This means
that to achieve the same desired product temperature, the steam pressure may need to be significantly
higher.
The presence of air and water films on the heat transfer surfaces of either process or space heating
applications is not unusual. It occurs in all steam heated process units to some degree.
To achieve the desired product output and minimise the cost of process steam operations, a high heating
performance may be maintained by reducing the thickness of the films on the condensing surface. In
practice, air will usually have the most significant effect on heat transfer efficiency, and its removal
from the supply steam will increase heating performance. profiles across each layer.
8. Proper Air Venting
Proper air venting is required because of the effect air has upon heat transfer. A layer of air only 1 mm
thick can offer the same resistance to heat as a layer of water 25 um thick, a layer of iron 2 mm thick
or a layer of copper 15 mm thick. It is very important therefore to remove air from any steam system.
9. Condensate Recovery
The steam condenses after giving off its latent heat in the heating coil or the jacket of the process
equipment. A sizable portion (about 25%) of the total heat in the steam leaves the process equipment
as hot water. Figure 3.26 compares the amount of energy in a kilogram of steam and condensate at the
same pressure. The percentage of energy in condensate to that in steam can vary from 18% at | bar g
to 30% at 14 bar g; clearly the liquid condensate is worth reclaiming.
If this water is returned to the boiler house, it will reduce the fuel requirements of the boiler. For every
6 °C rise in the feed water temperature, there will be approximately 1% saving of fuel in the boiler.
Benefits of condensate recovery
Financial reasons
Condensate is a valuable resource and even the recovery of small quantities is often economically
justifiable. The discharge from a single steam trap is often worth recovering.
Un-recovered condensate must be replaced in the boiler house by cold make-up water with additional
costs of water treatment and fuel to heat the water from a lower temperature.
Water charges
Any condensate not returned needs to be replaced by make-up water, incurring further water charges
from the local water supplier.
Effluent restrictions
High temperature of effluent is detrimental to the environment and may damage to pipes. Condensate
above this temperature must be cooled before it is discharged, which may incur extra energy costs.
Maximising boiler output
Colder boiler feed water will reduce the steaming rate of the boiler. The lower the feed water temperature,
the more heat, and thus fuel needed to heat the water.
Boiler feed water quality
Condensate is distilled water, which contains almost no total dissolved solids (TDS). Boilers need to be
blown down to reduce their concentration of dissolved solids in the boiler water. Returning more condensate
to the feed tank reduces the need for blow down and thus reduces the energy lost from the boiler.
Summary of reasons for condensate recovery:
o Water charges are reduced.
o Effluent charges and possible cooling costs are reduced.
o Fuel costs are reduced.
o More steam can be produced from the boiler.
o Boiler blow down is reduced - less energy is lost from the boiler.
o Chemical treatment of raw make-up water is reduced.
Condensate Pumping
The condensate generated from the steam consuming equipment or process widely distributed in the
plant, is required to be pumped back to the condensate tank in the boiler house. The limitation with
hot condensate, which is generally close to 100°C, is cavitation of the pump and pump impeller.
Centrifugal pumps are prone to cavitation problem, if Net Positive Suction Head (NPSH) available is
less than design NPSH required. To avoid the problem there are two alternative solutions:
¢Increase the NPSH available.
¢ Use a pressure powered (steam driven) pump instead of a centrifugal pump
Increase the NPSH available
This can be achieved by:
¢ increasing the difference in the pressure in the receiver and the condensate pressure,
and/or
¢ increasing the static head in suction side by lifting the receiver or lowering the pump, and/or
¢ increasing the piping dimensions for minimizing the friction loss in the suction pipe
Use a Steam Driven Pump
A pressure powered pump (Figure 3.27) uses steam
pressure to push the condensate from the receiver
back to the boiler house. In principle it consists of a
receiver which receives condensate from different
process/equipment. Once the condensate reaches a
set level, the steam valve is opened and the steam
pressure pushes the condensate to the boiler room.
The operation is cyclic in nature. The advantage is
pumping of condensate without losing much heat in
the form of flash steam without any cavitation problems.
10. Insulation of Steam Pipelines and Hot Process Equipments
Steam lines including flanges and valves should be insulated to prevent heat loss. The recommended
thickness of insulation will mainly depend on surface temperature desired after insulation. The energy
and cost savings will depend on the size of the pipe (diameter and length of run), the temperature of
steam and the surroundings, heat transfer co-efficient and the number of hours of operation of the
plant.
Note: Reference calculation is given in Chapter-5 “Insulation and Refractories”.
11. Flash Steam Recovery
Flash steam is produced when condensate at a high pressure is released to a lower pressure and can
be used for low pressure heating.
The higher the steam pressure and lower the flash steam pressure the greater the quantity of flash
steam that can be generated. In many cases, flash steam from high pressure equipments is made
use of directly on the low pressure equipments to reduce use of steam through pressure reducing
valves.
The flash steam quantity can be calculated by the following formula with the help of a steam
table:
Example:
Calculating the amount of flash steam from condensate
Hot condensate at 7 bar g has a heat content of about 721 kJ/kg. When it is released to atmospheric
pressure (0 bar g), each kilogram of water can only retain about 419 kJ of heat. The excess energy in
each kilogram of the condensate is therefore 721 — 419 = 302 kJ. This excess energy is available to
evaporate some of the condensate into steam, the amount evaporated being determined by the proportion
of excess heat to the amount of heat required to evaporate water at the lower pressure, which in this
example, is the enthalpy of evaporation at atmospheric pressure, 2258 kJ/kg.
Flash steam can be used on low pressure applications like direct injection and can replace an equal
quantity of live steam that would be otherwise required. The demand for flash steam should exceed
its supply, so that there is no build up of pressure in the flash vessel and the consequent loss of steam
through the safety valve. Generally, the simplest method of using flash steam is to flash from a
machine/equipment at a higher pressure to a machine/equipment at a lower pressure, thereby
augmenting steam supply to the low pressure equipment.
In general, a flash system should run at the lowest possible pressure so that the maximum
amount of flash is available and the backpressure on the high pressure systems is kept as low as
possible.
Flash steam from the condensate can be separated in an equipment called the ‘flash vessel’. This is
a vertical vessel as shown in the Figure 3.29. The diameter of the vessel is such that a considerable
drop in velocity allows the condensate to fall to the bottom of the vessel from where it is drained
out by a steam trap preferably a float trap. Flash steam itself rises to leave the vessel at the top.
The height of the vessel should be sufficient enough to avoid water being carried over in the flash
steam.
The condensate from the traps (A) along with some flash steam generated passes through vessel (B).
The flash steam is let out through (C) and the residual condensate from (B) goes out through the steam
trap (D). The flash vessel is usually fitted with a “pressure gauge’ to know the quality of flash steam
leaving the vessel. A ‘safety valve’ is also provided to vent out the steam in case of high pressure build
up in the vessel.
12. Pipe Redundancy
All redundant (piping which are no longer needed) pipelines must be eliminated, which could be, at
times, up to 10-15 % of total length. This would reduce steam distribution losses significantly.
13. Reducing the Work to be done by Steam
The equipments should be supplied with steam as dry as possible. The plant should be made efficient.
For example, if any product is to be dried such as in a laundry, a press could be used to squeeze as
much water as possible before being heated up in a dryer using steam.
Therefore, to take care of the above factors, automatic draining is essential and can be achieved by
steam traps. The trap must drain condensate, to avoid water hammer, thermal shock and reduction in
heat transfer area. The trap should also evacuate air and other non-condensable gases, as they reduce
the heat transfer efficiency and also corrode the equipment. Thus, a steam trap is an automatic valve
that permits passage of condensate, air and other non-condensable gases from steam mains and steam
using equipment, while preventing the loss of steam in the distribution system or equipment.
The energy saving is affected by following measures:
o Reduction in operating hours
o Reduction in steam quantity required per hour
o Use of more efficient technology
o Minimizing wastage.
When the steam reaches the place where its heat is required, it must be ensured that the steam has no
more work to do than is absolutely necessary. Air-heater batteries, for example, which provide hot air
for drying, will use the same amount of steam whether the plant is fully or partly loaded. So, if the plant
is running only at 50 per cent load, it is wasting twice as much steam (or twice as much fuel) than
necessary.
Thus, the steam heated iron has to evaporate nearly 48 kg of water. This requires 62 kg of steam.
If, due to inefficient drying in the hydro-extractor, the steam arrive at the iron with 52% moisture
content i.e. 52 kg of water has to be evaporated, requiring about 67 kg of steam. So, for the same
quantity of finished product, the steam consumption increases by 8 per cent. This is illustrated in
Figure 3.30.
Solved Example:
In a crude distillation unit of a refinery, 50 Metric Tonne/hr of crude is heated using saturated steam in
a heat exchanger from 35°C to 85°C. Plant is operating for 8000 hrs/annum. Consider specific heat of
the crude as 0.631 kcal/kg°C. The plant has two steam headers operating at 3 bar and 8 bar respectively,
passing nearby the heat exchanger. Cost of steam is same for both 3 bar and 8 bar @ Rs.4.50/kg.
As an Energy Manager, which of the following options will you recommend to the unit based on the
annual cost of steam?
a) Utilising 3 bar steam
b) Utilising 8 bar steam
Given: Data from steam table:
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