Engineering Encyclopedia

Saudi Aramco DeskTop Standards

Boilers And Furnaces Performance And Efficiency

Note: The source of the technical material in this volume is the Professional
Engineering Development Program (PEDP) of Engineering Services.
Warning: The material contained in this document was developed for Saudi
Aramco and is intended for the exclusive use of Saudi Aramco’s employees.
Any material contained in this document which is not already in the public
domain may not be copied, reproduced, sold, given, or disclosed to third
parties, or otherwise used in whole, or in part, without the written permission
of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Vessels For additional information on this subject, contact

File Reference: MEX30211 M.Y. Naffa’a
Engineering Encyclopedia Vessels
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Contents Pages

INTRODUCTION................................................................................................................ 1

CALCULATING THERMAL EFFICIENCY ....................................................................... 2

Heat Balance (Input/Output) Method ........................................................................ 2
Stack Loss Method.................................................................................................... 5
Calculation Procedures.............................................................................................. 5
Simple Efficiency Equation ............................................................................ 5
API RP 532 Procedure................................................................................... 8
ASME Abbreviated Efficiency Test ..........................................................................11
EFFICIENCY IMPROVEMENTS ......................................................................................18
Excess Air................................................................................................................18
Reduce Stack Temperatures .....................................................................................18
Reduce Other Losses................................................................................................19
FUTURE IMPROVEMENTS..............................................................................................20
Excess Air Reduction ...............................................................................................20
Stack Temperature Reduction ..................................................................................20
Combustion Improvements.......................................................................................24
Work Aid 1 Rp 532 Thermal Efficiency Calculation Procedure .....................25
Work Aid 2 Vapor Pressure Of Water...........................................................32
Work Aid 3 Enthalpy Of Flue Gas Components ............................................33
Work Aid 4 Asme Test Form For Abbreviated Efficiency Test ......................35
GLOSSARY........................................................................................................................37

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INTRODUCTION

This module contains information on procedures for calculating the efficiency of boilers and
furnaces. Means of improving efficiency are also presented. This information will assist the
participant in completing typical tasks, such as:

• Calculating the efficiency of boilers and furnaces.

• Determining the increase in efficiency resulting from changes in operating conditions or

equipment additions.

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CALCULATING THERMAL EFFICIENCY

The thermal efficiency of a boiler or furnace is used to monitor its operation and determine the
rate of energy usage. Changes in efficiency may indicate a deteriorating condition of the
equipment, or the need to change operating conditions.

Thermal efficiency is defined as the total heat absorbed by the boiler or furnace, divided by the
total heat input. Efficiency can be related to either the higher heating value (HHV) or the lower
heating value (LHV) of the fuel, and it is important to state which value is being used. The
difference between the two values is that the higher heating value includes the heat of evaporation
of the water vapor formed in the combustion of the fuel (1059.7 Btu/lb of water). This heat is
almost never recovered in boilers or furnaces. Therefore, LHV is a better measure of achievable
thermal efficiency. The HHV efficiency is several percentage points lower than the LHV
efficiency. It is common practice in the furnace industry to use the lower heating value in
calculations, while the boiler industry uses the higher heating value. Lower heating value is used
in the following calculations.

There are two basic methods to determine thermal efficiency, the heat balance method and the
stack loss method. Both of these methods are described below.

Heat Balance (Input/Output) Method

In this method, efficiency is determined by the following equation:

Efficiency = Heat Absorbed

Heat Input

The following data are required to determine efficiency using this method:

• Heat Absorbed:
- Process flow rate (typical meter accuracy + 3%).
- Process inlet enthalpy.
- Process outlet enthalpy.

+ For process furnaces, it is necessary to know the feed composition and the
percent vaporization at the outlet, as well as the temperatures, to determine
enthalpy conditions. It may be very difficult to determine these data accurately.

+ For boilers, this is not as great a problem, as long as the steam at the outlet is
fully saturated or superheated.

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Heat Balance (Input/Output) Method (Cont’d)

• Heat Input:

- Fuel flow (typical meter accuracy + 3%).

- Fuel heating value (often varies with time, particularly when using plant gas as the
fuel).

Typical furnace instrumentation is shown in Figure 1. The required data can be obtained
using this instrumentation.

Because of the inaccuracies inherent in many of these measurements, it is very difficult to achieve
a reasonably accurate efficiency estimate using this method, particularly in process furnaces.
Consequently, this method is not used very much in the petroleum industry.

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Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 1 Instrument And Measurement Locations

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Stack Loss Method

In this method, efficiency is determined by the following equation:

Efficiency = Heat Input - Losses

Heat Input

In this equation, the losses can be expressed as a percentage of the heat input, so it is not
necessary to determine the actual rate of heat input.

The major heat loss from a boiler or furnace is the stack loss (the heat in the flue gas leaving the
boiler or furnace). Methods for calculating the stack losses are described below.

A smaller, but still significant heat loss is through the walls of the boiler or furnace to the
atmosphere. This is known as the radiation loss. This is difficult to calculate and is usually set as
a percentage of the heat fired. Radiation loss can also include an allowance to cover other
unidentified losses. The following are typical values used for radiation losses:

Heat Fired, MBtu/hr Qr, % of Net Heat Fired

<15 3
15 - 100 2
>100 1

Another heat loss from boilers is in the blowdown water. Assuming a 10% blowdown rate, this
heat loss is approximately 1.9% of heat fired. For detailed calculations, the heat loss can be
calculated using the blowdown flow rate and the enthalpy of the water being discharged.

Calculation Procedures

Several different calculation procedures can be used to calculate efficiency. Two procedures are
described below: a simple procedure that can be used to estimate efficiency, and a detailed
procedure.

Simple Efficiency Equation

e = 100 - (0.0235 + 0.0002EA)(Tst - 60) - Qr (Eqn.1)

where: e = LHV Efficiency, %.
EA = Excess Air, %.
Tst = Stack temperature, _F.
Qr = Radiation loss, % of net heat fired.

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This equation requires just two measurements: stack temperature and stack excess air. The
excess air and stack temperature should be measured at the same location. It is not necessary to
measure any flow rates. This equation is based on typical Saudi Aramco operating conditions and
assumes an average ambient air temperature of 90_F. Variations in ambient air temperature have
a minor effect on calculated efficiency.

Excess Air. If not specified, the excess air rate can be determined from the oxygen content in the
flue gas. The following equations can be used to estimate the excess air rate.

When the flue gas analysis is on a wet basis:

111 . 4 x % O 2
EA = (Eqn . 2)
20 . 95 - % O 2

where: %O2 = Percent oxygen in the flue gas.

When the flue gas analysis is on a dry basis:

91. 2 x % O 2
EA = (Eqn . 3)
20 .95 - % O 2

It is important to know which basis is being used. When the oxygen analyzer is located directly at
the stack, the flue gas analysis is usually on the wet basis. When the flue gas is extracted from the
stack and is transported to an analyzer that is located some distance away, the analysis is usually
on the dry basis.

The precise relationship between oxygen content and excess air is a function of the hydrogen-to-
carbon ratio of the fuel. However, there is very little change in this relationship over a wide range
of fuels at low excess air rates. For typical plant fuels, this calculated efficiency should be within
about 1% of that calculated by more precise methods. This should be adequate for most plant
applications. These results can be verified for specific operating conditions by comparing the
results with one of the detailed methods presented below.

Stack Temperature. Another potential source of error in all efficiency calculations is an error in
stack temperature measurements. Ordinary stack temperature thermocouples can read low by as
much as 100_F, depending upon their location and the flue gas temperature being measured. If
the thermocouple can "see" cold surroundings, such as the top of the convection section or the
sky, the indicator will likely read low. The higher the actual stack temperature, the higher the
radiation losses and thus, the higher the error.

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If precise stack temperature data are needed, such as for setting the basis for a waste heat
recovery project, then the data should be taken with an aspirating ("high-velocity") thermocouple,
as illustrated in Figure 2. The tip of this thermocouple is shielded, and flue gas is continually
pulled past the thermocouple.

Although it may produce inexact readings, an ordinary stack thermocouple should give
representative, consistent readings, which should be satisfactory for monitoring day-to-day
performance.

3
6
4
5
2 2
1
6
6

A A–A

2
2
1 2
2

A
7

1 = Thermocouple junction.
2 = Thermocouple wires to temperature-indicating instrument.
3 = Outer thin-wall 310 stainless steel tube.
4 = Middle thin-wall 310 stainless steel tube.
5 = Center thin-wall 310 stainless steel tube.
6 = Centering tripods.
7 = Air or steam at 10 lb/sq in. gage or more in
increments of 10 lb/sq in. until stable.
8 = Hot gas eductor.
From Furnace Operations, Third Edition by Robert Reed. Copyright © 1981 by Gulf Publishing Company, Houston, Texas.
Used with permission. All rights reserved.

FIGURE 2 Typical Aspirating (High-Velocity) Thermocouple

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API RP 532 Procedure

The RP 532 procedure is a detailed version of the stack loss method. In addition to the data
required by the Simple Efficiency Equation, an analysis of the fuel composition is required.

All sources of heat inputs and losses need to be included to make a precise efficiency calculation.
These sources are illustrated in Figure 3. This calculation requires the following additional data.

• Relative humidity of the air.

• Temperature and specific heat of the fuel.

• Temperature and rate of atomizing steam when liquid fuel is fired.

If not known, it is usually satisfactory to estimate these data, based on typical local conditions.

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Qs
Ts t

Qr

LHV+H +H
f m H a at T t = T a
Fuel Ambient
Air
Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition,
August 1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 3 Typical Heater Arrangement

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Work Aid 1 contains the work sheets required for the RP 532 procedure. An example of how it is
used to calculate the efficiency of a gas-fired furnace is shown in Figure 4.

This procedure consists of the following steps:

1. Using the Lower Heating Value Work Sheet, determine the lower heating value of liquid
fuel (if required). If the fuel is gas, or if typical liquid fuel properties are known, it is not
necessary to complete this work sheet.
2. Using the Combustion Work Sheet, determine flue gas properties for stoichiometric combustion
conditions.
3. Using the Excess Air and Relative Humidity Work Sheet, determine the amount of water
vapor in the flue gas.
The vapor pressure of water at the ambient temperature can be determined from Work
Aid 2.
4. Using the Stack Loss Work Sheet, determine the stack heat losses.
The enthalpy of the flue gas components can be determined from Work Aid 3.
5. The thermal efficiency can then be determined by the following equation:
100 ( Q s + Q r )
e = 100 − (Eqn. 4)
LHV + H a + H f + H m

where: e = Net thermal efficiency, % (LHV).

LHV = Lower heating value of the fuel, Btu/lb of fuel.
Ha = Air sensible heat correction, Btu/lb of fuel.
= Cp(air)(Ta - Td)(pounds of air per pound of fuel).
Hf = Fuel sensible heat correction, Btu/lb of fuel.
= Cp(fuel)(Tf - Td).
Hm = Atomizing medium (usually steam) sensible heat correction, Btu/lb of fuel.
= Cp(medium)(Tm - Td)(pounds of medium per pound of fuel).
If steam, Hm = (Enthalpy difference)(lb of steam/lb of fuel).
= (hs - 1087.7)(lb of steam/lb of fuel).
Qr = Radiation heat losses, Btu/lb of fuel.
Qs = Calculated stack heat losses (from Stack Loss Work Sheet), Btu/lb of fuel.
Ta = Ambient air temperature, _F.
Td = Reference (or datum) temperature, _F.
= 60_F (usually).
Cp = Specific heat, Btu/lb-_F.
Tf = Temperature of fuel, _F.
Tm = Temperature of atomizing medium, _F.
hs = Enthalpy of atomizing steam, Btu/lb.

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6. The gross thermal efficiency can be determined by the following equation:

100 Q s + Q r + latent heat

e gross = 100 - (Eqn. 5)
HHV + H a + H f + H m

where: egross = Gross thermal efficiency, % (HHV).

Latent heat = (H2O formed by combustion of fuel) x1059.7.

7. The firing rate can be calculated, based on the heat absorbed in the boiler or furnace, as
follows:

Qa
Qf =
e/100 (Eqn. 6)

where: Qf = Heat fired, MBtu/hr (LHV).

Qa = Heat absorbed, MBtu/hr.
e = Net thermal efficiency, %.

ASME Abbreviated Efficiency Test

This procedure calculates the efficiency of boilers by both the Input/Output and Stack Loss
methods. It uses the HHV of the fuel and can be used for coal-fired boilers, as well as gas- and
oil-fired units. The forms for this procedure are included in Work Aid 4. Line items on these
forms that do not apply to Saudi Aramco boilers have been crossed out.

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Sample Calculation - RP 532 Procedure

The following sample calculation illustrates the use of the RP 532 calculation procedure to determine
thermal efficiency. (Based on Par. 3.2.2 of RP 532.)
Given:
Stack temperature Tst = 300 _F
Air temperature Ta = 28 _F
Specific heat of air Cp(air) = 0.24 Btu/lb- _F
Relative humidity = 50 %
Oxygen content of flue gas = 3.5 % (wet basis)
Radiation losses Qr = 2.5 % of lower heating value of fuel
Fuel temperature Tf = 100_F
Fuel specific heat Cp(fuel) = 0.525 Btu/lb- _F
Fuel composition:
Methane = 75.41 vol. %
Ethane = 2.33
Ethylene = 5.08
Propane = 1.54
Propylene = 1.86
Nitrogen = 9.96
Hydrogen = 3.82
Solution:
1. Complete the following work sheets from Work Aid 1 (completed copies attached).
Combustion Work Sheet.
Excess Air and Relative Humidity Work Sheet.
Stack Loss Work Sheet.
2. Determine Net Thermal Efficiency, as follows:
From Combustion Work Sheet, LHV = 18,120 Btu/lb
Radiation Loss Qr = 18,120 x 0.025
= 453.0 Btu/lb of fuel
From Stack Loss Work Sheet, Qs = 1162.1 Btu/lb of fuel

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4

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Sensible heat corrections:

Pounds of air/pound of fuel is obtained by adding the total from column 7 of the Combustion
Work Sheet with the pounds of dry excess air per pound of fuel from the Excess Air and Relative
Humidity Work Sheet.

Air: Ha = Cp(air) (Ta - Td)(pounds of air/pound of fuel)

= 0.24 (28 - 60)(14.322 + 3.191)
= -134.5 Btu/lb of fuel
Fuel: Hf = Cp(fuel) (Tf - Td)
= 0.525 (100 - 60)
= 21.0 Btu/lb of fuel
Atomizing medium Hm = 0 (no atomizing steam required)

100 ( Q s + Q r )
Using Eqn. 4: e = 100 −
LHV + H a + H f + H m

100 (1162 .1 + 453 . 0 )

e = 100 − = 91. 03 % ( LHV )
(18120 -134 . 5 + 21. 0)

3. Determine Gross thermal efficiency, as follows:

From Combustion Work Sheet, H2O formed = 1.784 lb/lb of fuel.

Latent heat = H2O formed x 1059.7

= 1.784 x 1059.7
= 1890.5 Btu/lb of fuel

HHV = LHV + latent heat

= 18120 + 1890.5 = 20010 Btu/lb.

Using Eqn. 5:

100 Q s + Q r + latent heat

e gross = 100 -
HHV + H a + H f + H m

100 1162.1 + 453.0 + 1890.5

e gross = 100 - = 82.83% HHV
20010 - 134.5 + 21.0

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process
Heaters, 1st Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4 (Cont’d)

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Use photostat

Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August
1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4 COMBUSTION WORK SHEET(CONT'D)

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use photostat

Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August
1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4 EXCESS AND RELATIVE HUMIDITY WORK SHEET(CONT’D)

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use photostat

Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August
1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4 EXCESS AND RELATIVE HUMIDITY WORK SHEET(CONTíD)

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use photostat

Source: API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st Edition, August
1982. Reprinted courtesy of the American Petroleum Institute.

FIGURE 4 STACK LOSS WORK SHEET(CONT'D)

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EFFICIENCY IMPROVEMENTS

Excess air level and stack temperature are the two major parameters that affect boiler and furnace
efficiency.

Excess Air

All the air that enters a boiler or furnace is ultimately discharged to the atmosphere at the stack
temperature, and the energy it contains is lost. The primary objective of efficient boiler and
furnace operations is to minimize air flow beyond that required for good combustion. The air
required for combustion should enter only through the burners. The following steps can be taken
to reduce excess air:

1. Seal air leaks. This is particularly important in furnaces, which operate with a draft, or
negative pressure, inside and are susceptible to air infiltration. Leakage into the radiant section
has the worst effect, but all leaks are wasteful. Figure 5 shows typical sources of air leaks
into a furnace.

Since most boilers operate with a positive pressure inside, air leakage into boilers is usually
not a problem.

2. Fire all burners at the same rate.

3. Control furnace draft.

4. Determine excess air targets for each furnace through a series of plant tests. These targets
are the minimum excess air rates that have been found to be necessary for good combustion.
Since no two furnaces or boilers are exactly the same, there can be different targets for each
boiler and furnace in the plant.

Reduce Stack Temperatures

Fouling of the convection section tubes is the primary cause of stack temperatures that exceed
design. The extent of fouling can be determined by visual inspection of the tubes or by observing
an increase in stack temperature over time. A 40°F increase in stack temperature represents a 1%
efficiency loss.

Fouling can be reduced by operating sootblowers in boilers and furnaces. Sootblowers should be
provided for all boilers and furnaces where heavy liquid fuels are fired. Units without sootblowers
should be periodically cleaned during turnarounds.

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Inlet
Construction
Joint

Clearance
Around Tube
Penetration

Poor Seal on Casing

Access Door Corrosion

Leaky Covers
on Observation
Doors

Idle
Burner

Outlet

FIGURE 5 Furnace Air Leaks

Reduce Other Losses

Although less important than excess air and stack temperature, several other parameters
affect boiler and furnace efficiency:

• Boiler blowdown should be controlled to the rate needed to maintain boiler drum water
impurities at the specified concentration. Excess blowdown wastes heat and water. Heat
can be recovered from the blowdown stream.

• Insulation should be maintained to reduce heat losses.

• Steam leaks should be repaired.

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FUTURE IMPROVEMENTS

The following methods of improving the efficiency of existing boilers and furnaces require the
addition of new equipment.

Excess Air Reduction

• Add improved combustion control systems.

- Automatic draft control on furnaces.

- Closed loop oxygen and/or CO control.

• Replace oversized burners. It is difficult to operate burners efficiently at high turndown

rates.

Stack Temperature Reduction

• Reducing the stack temperature of a furnace or boiler that is operating satisfactorily usually
requires the addition of heat transfer surface.

• Additional heat transfer surface in convection section of furnaces.

• Economizers on boilers to preheat the boiler feedwater before entering the steam drum.

• Combustion air preheaters. Air preheaters can transfer heat from the flue gas leaving the
stack, to the air used for combustion. Depending upon the flue gas temperature, the
incoming air can be heated several hundred °F. The flue gas temperature should be kept
above about 300°F to prevent corrosion of the heat exchanger due to sulfuric acid in the flue
gas.

Combustion air can also be preheated using other waste heat, such as low-pressure
steam. While this does not recover heat from the stack gases, it does improve the
overall plant energy balance. Typical air preheater systems are shown in Figure 6.

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a. Air Preheat System Using Regenerative,

Recuperative, or Heat Pipe Unit

b. External Heat Source for Air Preheating

Source: API Standard 560, Fired Heaters for General Refinery Services, 1st Edition, January 1986. Reprinted courtesy of the
American Petroleum Institute.

FIGURE 6 Air Preheat Systems

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Stack Temperature Reduction (Cont’d)

Several types of air preheaters can be used, with the most common shown in Figures 7, 8, and 9.
The rotary regenerative, or Ljungstrom type, preheater rotates a heat-absorbing mass from the hot
flue gas duct, where it picks up heat, to the incoming air duct, where the heat is released.
Another type is the tubular exchanger, in which the hot flue gas passes on one side of the tubes
and cold incoming air passer on the other side of the tubes.

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Basketed Heating
Surface

Shaft
Radial Seal
(Stationary)

Axial Seal
(Stationary)
Housing
Seal
Sector

Gas Out Air In

Plate Plate
Groups Groups

Gas In Air Out

Elements of a Rotary Regenerative Air Heater Diagrammatic illustration of

rotary regenerative air heater
with gas air counterflow.
With permission from Babcock & Wilcox. With permission from Babcock & Wilcox.

FIGURE 7 AIR PREHEATERS FIGURE 8

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Gas Inlet

Air Outlet

Air Inlet

Gas Outlet
Gas Downflow
Air Counterflow, Three-Pass

With permission from Babcock & Wilcox.

FIGURE 9 Typical Tubular Air Preheater

Combustion Improvements

• High-capacity, high-intensity, or axial flow forced-draft burners for improved, low excess air
combustion.

• Low NOx burners for reduced emissions.

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Work Aid 1 Rp 532 Thermal Efficiency Calculation Procedure

The following procedure can be used to calculate the thermal efficiency of a boiler or furnace,
using the Stack Loss Method, as described in API RP 532.

1. Complete the attached work sheets:

Lower Heating Value Work Sheet (if required for liquid fuels)
Combustion Work Sheet.
Excess Air and Relative Humidity Work Sheet.
Stack Loss Work Sheet.

2. Determine Net Thermal Efficiency, as follows:

From Combustion Work Sheet, LHV = ________________Btu/lb

Radiation Loss Qr = LHV x %Qr/100

= (__________)(_________) = ___________ Btu/lb of fuel

From Stack Loss Work Sheet, Qs = _________Btu/lb of fuel

From Combustion Work Sheet, air required = _____________(lb of air/lb of fuel)

From Excess Air Work Sheet, excess air = _____________(lb of air/lb of fuel)
Total air rate = _____________(lb of air/lb of fuel)

Sensible heat corrections:

Pounds of air/pound of fuel is obtained by adding the total from column 7 of the Combustion
Work Sheet with the pounds of dry excess air per pound of fuel from the Excess Air and Relative
Humidity Work Sheet.

Air: Ha = Cp(air)(Ta - Td)(total lb of air/lb of fuel)

= ___________(________ - 60)(___________)
= ___________Btu/lb of fuel

Fuel: Ha = Cp(fuel)(Tf - Td)

= ___________(___________ - 60)
= ___________Btu/lb of fuel

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Work Aid 1 (Cont'd)

RP 532 THERMAL EFFICIENCY CALCULATION PROCEDURE (CONT’D)

Atomizing medium Hm
= Cp(medium) (Tm - Td)(lb of medium/lb of fuel)
If steam is used: Hm
= (Enthalpy difference)(lb of steam/lb of fuel)
= (hs - 1087.7)(lb of steam/lb of fuel)
Atomizing steam temperature = ____________F
Steam enthalpy hs = ___________Btu/lb

Hm = (__________ - 1087.7)(___________)
= ___________Btu/lb of fuel

Using Eqn. 4:
100 ( Q s + Q r )
Thermal efficiency e = 100 −
LHV + H a + H f + H m

= 100 - 100 ( + )
(________ + _______ + _______ + _____)

e = ___________% (LHV)

3. If gross thermal efficiency is needed, determine as follows:

From Combustion Work Sheet, H2O formed = ___________lb/lb of fuel

Latent heat = H2O formed x 1059.7

= (_________) x 1059.7
= __________Btu/lb of fuel
HHV = LHV + latent heat
= (_________) + (_________) = ____________Btu/lb
Using Eqn. 5:
egross = 100 - 100(Qs + Qr + latent heat)
HHV + Ha + Hf + Hm
= 100 - 100( + + )
(_______ + ______ + ______ + ______)
egross = ___________% (HHV)
Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Work Aid 1 (Cont'd)

EXCESS AIR AND RELATIVE HUMIDITY WORK SHEET

USE PHOTOSTAT

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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(Pg. 4 of 7)
Work Aid 1 (Cont'd)

EXCESS AIR AND RELATIVE HUMIDITY WORK SHEET (CONT'D)

USE PHOTOSTAT

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Boilers And Furnaces Performance And Efficiency

(Pg. 5 of 7)
Work Aid 1 (Cont'd)
STACK LOSS WORK SHEET

USE PHOTOSTAT

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

(Pg. 6 of 7)
Work Aid 1 (Cont'd)

LOWER HEATING VALUE (LIQUID FUELS) WORK SHEET

USE PHOTOSTAT

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

(Pg. 7 of 7)
Work Aid 1 (Cont'd)
COMBUSTION WORK SHEET

USE PHOTOSTAT

Data extracted from API Recommended Practice 532, Measurement of the Thermal Efficiency of Fired Process Heaters, 1st
Edition, August 1982. Reprinted courtesy of the American Petroleum Institute.

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

Work Aid 2 Vapor Pressure Of Water

Source: Data taken from Steam Tables

FIGURE 10

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

(Pg. 1 of 2)
Work Aid 3 Enthalpy Of Flue Gas Components

USE PHOTOSTAT

Figure 11

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Boilers And Furnaces Performance And Efficiency

(Pg. 2 of 2)
Work Aid 3 (Cont'd)

USE PHOTOSTAT

FIGURE 12 Enthalpy Of Flue Gas Components

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Boilers And Furnaces Performance And Efficiency

(Pg. 1 of 2)
Work Aid 4 Asme Test Form For Abbreviated Efficiency Test

USE PHOTOSTAT

With permission from the American Society of Mechanical Engineers.

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

(Pg. 2 of 2)
Work Aid 4 (Cont'd)

ASME TEST FORM FOR ABBREVIATED EFFICIENCY TEST (CONT'D)

USE PHOTOSTAT

With permission from the American Society of Mechanical Engineers.

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Engineering Encyclopedia Vessels
Boilers And Furnaces Performance And Efficiency

GLOSSARY

blowdown Water removed from the boiler to control the level

of dissolved impurities in the boiler water.

economizer A device for transferring heat from the flue gas to

the boiler feedwater (BFW) before the BFW enters
the boiler drum.

excess air The percentage of air in excess of the

stoichiometric amount required for combustion.

flue gas Gaseous products from the combustion of fuel.

higher heating value The amount of heat released during complete

(HHV) combustion of fuel when the water formed is
considered as a liquid (credit is taken for its heat of
condensation.) Also called gross heating value.
Usually expressed in Btu/lb.

lower heating value The amount of heat released during complete

(LHV) combustion of fuel when no credit is taken for heat
of condensation of water in the flue gas. Also
called net heating value. Usually expressed in
Btu/lb.

radiation heat loss A defined percentage of the net heat of combustion

of the fuel to account for heat losses through the
boiler or furnace walls to the atmosphere.

stack heat loss The total sensible heat of the flue gas components,
at the temperature of flue gas, when it leaves the
last heat exchange surface.

thermal efficiency The total heat absorbed divided by the total heat
input. Usually expressed in percent.

total heat absorbed The total heat input minus the total heat losses.

total heat losses The sum of the radiation heat loss and the stack
heat loss.

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