ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND_UTILITY_SYSTEMS_(Chapter 2:ENERGY PERFORMANCE ASSESSMENT OF FURNACES)

ENERGY_PERFORMANCE_ASSESSMENT_FOR_EQUIPMENT_AND UTILITY SYSTEMS

(Chapter 2:ENERGY PERFORMANCE ASSESSMENT OF FURNACES)

Industrial Heating Furnaces

A heating furnace is by definition a device for heating materials and therefore a user of energy. Heating furnaces can be divided into batch-type (Job at stationary position) and continuous type (large volume of work output at regular intervals). The types of batch furnace include box, bogie, cover, etc. For mass production, continuous furnaces are used in general. The types of continuous furnaces include pusher-type furnace (Figure 2.1), walking hearth-type furnace, rotary hearth and walking beam-type furnace. (Figure 2.2)

The primary energy required for reheating / heat treatment (say annealing) furnaces are in the form of Furnace oil, LSHS, LDO or electricity

Purpose of the Performance Test

* To make a heat balance of the furnace

*To find out the efficiency of the furnace

* To find out the specific energy consumption

The purpose of the performance test is to determine various losses, efficiency of the furnace and specific energy consumption for comparing with design values or best practice norms. There are many factors affecting furnace performance such as capacity utilization of furnaces, excess air ratio, final heating temperature etc. Performance test is the key for assessing current level of performances and finding the scope for improvements and productivity.

Reference Standards

In addition to conventional methods, Japanese Industrial Standard (JIS) GO702 “Method of heat balance for continuous furnaces for steel” is used for the purpose of establishing the heat losses and efficiency of reheating furnaces. The standard has been simplified to suit practical calculation in the industry.

Performance Terms and Definitions

Furnace Heat Balance Method

Heat balance helps us to numerically understand the present heat loss and efficiency and improve the

furnace operation using these data. Thus, preparation of heat balance is a pre-requirement for assessing

energy conservation potential. The methodology ofa typical furnace heat balance is given simultaneously along with an example 2.6.

The total heat input is provided in the form of fuel or power. The desired output is the heat supplied

for heating the material or process. Other heat outputs in the furnaces are undesirable heat losses. The

inputs and outputs are calculated on the basis of per tonne of stock/charge.

The major losses that occur in the fuel fired furnace (Figure 2.3) are listed below.

1. Heat lost through exhaust gases either as sensible heat or as incomplete combustion

2. Heat loss through furnace walls and hearth

3.Heat loss to the surroundings by radiation and convection from the outer surface of the walls

4.Heat loss through gases leaking through cracks, openings and doors.

imilar to the method of evaluating boiler efficiency by indirect method, furnace efficiency can also be calculated from heat balance. Furnace efficiency is calculated after subtracting sensible heat loss in flue gas, loss due to moisture in flue gas, heat loss due to openings in furnace, heat loss through furnace skin and other unaccounted losses from the heat input to the furnace.

In order to carry out a heat balance, various parameters that are required are hourly oil consumption, material output, excess air quantity, temperature of flue gas, temperature of furnace at various zones,skin temperature and hot combustion air

temperature.

The energy absorbed by the material requires the use of specific heat which can be obtained from reference manual/data book.

If the process requires a change in state, from solid to liquid, or liquid to gas, then in addition to sensible heat, an additional quantity of energy is required called the latent heat of fusion or latent heat of evaporation and this quantity of energy needs to be added to the total energy requirement.

Measurement Parameters

The following are some of the key measurements to be made for working out the energy balance in

oil fired reheating furnaces.

i) Weight of stock / Number of billets heated

ii) Temperature of the stocks/billets

iii) Temperature of furnace walls, roof etc

iv) Area of furnace walls, roof etc

v) Flue gas temperature

vi) Flue gas analysis

vii) Fuel oil consumption

Instruments like infrared thermometer, fuel consumption monitor, surface thermocouple and other measuring devices are required to measure the above parameters. Reference manual should be referred

for data like specific heat, humidity etc.

Furnace Efficiency

The efficiency of a furnace is the ratio of useful heat output to heat input. The direct determination of

furnace efficiency is carried out as follows.

Heat inthe stock

Heat inthe fuel consumed

Thermal efficiency of the furnace =

The quantity of heat to be imparted (Q) to the stock can be found from the formula

Where,

Q = Quantity of heat in kcal

m = Weight of the material in kg

C, = Mean specific heat, kcal/kg °C

t, = Final temperature desired, °C

t, _ Initial temperature of the charge before it enters the furnace, °C

Example: Heat Balance of Furnace

An oil-fired reheating furnace has an operating temperature of around 1340°C. Average fuel consumption

is 400 litres/hour. The flue gas exit temperature after air preheater is 655 °C. Air is preheated from ambient temperature of 40°C to 190°C through an air pre-heater. The furnace has 460 mm thick wall (x) on the billet extraction outlet side, which is | m high (D) and | m wide. Draw a heat balance to identify heat losses, efficiency and specific fuel consumption. The other data are as follows.

Flue gas temperature after air preheater = 655 °C

Ambient temperature = 40 °C

Abs. humidity = 0.03437 kg/kg dry air

Preheated air temperature = 190°C

Specific gravity of oil = 0.92

Average fuel oil consumption = 400 Litres / hr

= 400 x 0.92 =368 kg/hr

O, in flue gas = 12%

CO, in flue gas = 6.5%

CO in flue gas = 50 ppm

Weight of stock = 6000 kg/hr

Specific heat of Billet = 0.12 kcal/kg °C

Surface temperature of ceiling = 85 °C

Surface temperature of side walls = 100 °C

Surface temperature of flue duct = 64 °C

Area of ceiling = 15 m2

Area of side walls = 36 m2

Area of flue duct = 10.3 m2

Diameter of flue duct = 0.4m

Furnace oil constituents (% by weight)

Carbon - 85.9 %, Hydrogen -12%, Oxygen - 0.7%, Nitrogen - 0.5%, Sulphur - 0.5%, H,O - 0.35%,Ash - 0.05%, GCV- 10,000 kcal/kg

1) Calculation of air quantity and specific fuel consumption

5) Efficiency of furnace

=(1,56,000 / 6,13,300) x 100

=25.4 %

Factors Affecting Furnace Performance

The important factors, which affect the efficiency, are listed below for critical analysis.

« Under loading due to poor hearth loading and improper production scheduling

« Improper design

« Use of inefficient burner

« Insufficient draft/chimney

« Absence of waste heat recovery

« Absence of instruments/controls

« Improper operation/maintenance

« High stack loss

« Improper insulation /refractories

Useful Information

Radiation Heat Transfer

Heat transfer by radiation is proportional to the fourth power of absolute temperature. Consequently

the radiation losses increase considerably as temperature increases.

In practical terms this means the radiation losses from an open furnace door at 1500°C are 11 times

greater than the same furnace at 700°C. A good incentive for the iron and steel melters is to keep the

furnace lid closed at all times and maintaining a continuous feed of cold charge onto the molten bath.

Furnace Utilization Factor

Utilization has a critical effect on furnace efficiency and is a factor that is often ignored or underestimated. If the furnace is at temperature then standby losses of a furnace occur whether or not a

product is in the furnace.

Standby Losses

Energy is lost from the charge or its enclosure in the way of heat: (a) conduction, (b) convection; or/ and (c) radiation

Furnace Draft Control

Furnace pressure control has a major effect on fuel fired furnace efficiency. Running a furnace at a slight positive pressure reduces air ingress and can increase the efficiency.

Scale

Scale is the flaky material formed on the surface due to oxidation of iron/steel at high temperature in the presence of excess air. This results in loss of material and hence useful output.

Data Tables

Table — I: Co-efficient based on the profile of furnace openings (φ)