Exhaust Heat Transfer 101

How Industrial Exhaust Heat Recovery Works

How a Boiler Economizer Works
How a Boiler Economizer Works

Definitions of Fundamental Terms

U Factor is the Coefficient of Heat Transfer or the rate of flow in BTU/HR through one square foot of surface area per degree difference in temperature.

British Thermal Unit (BTU) is the quantity of heat required to raise the temperature of one lb. of water one degree F. It is the standard unit in all heating calculations. The specific heat of air is 0.241 and standard air weighs 0.075 lb/ft3, one BTU will raise 55.2 ft3 of standard air one degree, or one ft3 of air 55.2° F.

Coefficient of Conductivity (K) is the rate of heat transfer through a material expressed in BTU/HR transmitted through 1 ft2 of surface per degree difference in temperature across the material. This is per thickness and expressed as "K". Where a wall or area with perhaps several materials is rated as a whole, it is given the symbol "U" and called the coefficient of over all transmission.

Boiler Horsepower (BHP) is a common rating for steam boilers: one boiler horsepower is the power to evaporate 34.5 lb. of water, at a temperature of 212°F. and atmospheric (sea level) pressure, into dry steam, per hour. Since the latent heat of evaporation at these conditions is 970.3 BTU/lb, one boiler horsepower equals 34.5 X 970.3, or 33,500 BTU/HR.

Cubic Feet per Minute (CFM) is a self explanatory term commonly used to describe the volume of air a fan moves. Fans are rated in CFM.

Decibel (DB) is a measure of the pressure energy of sound in the air. It is a measure of energy and not of loudness, and gives no indication of tone or quality of noise.

Latent Heat is:
  1. In heating systems it is the number of BTU required to change water, at boiling temperature, into steam at the same temperature. The latent heat is expressed in BTU/LB and will vary for different steam pressures. The same amount of heat is released when the steam is condensed to water. For example, the latent heat of steam at 2 PSI is 966 BTU/LB. This means 966 BTU of heat must be supplied to convert each pound of water, at 2 PSI and 219°F., to steam, at 2 PSI and 219°F. The same 966 BTU is released for heating when the pound of steam is condensed to water, at 2 PSI and 219°F.
  2. In heating or cooling load calculations latent heat is the number of BTU required to evaporate moisture. The most important consideration of latent heat is the evaporation of human perspiration. In air conditioning this can sometimes be the greatest single cooling requirement.

Specific Heat (SP H) is the heat required to warm a body 1°F., divided by the quantity of heat required to warm an equal weight of water 1°F. The specific heat of standard air is 0.241 and water is 1.00.

Sensible Heat is the heat of air as indicated by dry bulb temperature. Most heating load calculations are sensible heat calculations. The air is considered as dry air and is a close approximation of the moist air to be heated. Cooling calculations usually make use of both sensible and latent heat which, together, are total heat.

Standard Air weighs 0.075 lb/cu. ft. This weight corresponds approximately to dry air at a barometric pressure of 29.92 in. of mercury and a dry-bulb temperature of 70°F. There are 13.5 ft3 of standard air to a pound.

Equivalent Direct Radiation (EDR): Years ago, cast iron radiators were rated as being equivalent to a number of feet of radiant pipe surface. One edr is equal to 240 BTU. The capacities of many small heating boilers are based on edr.

Glossary of Fundamental Terms

"W.C. - Inches of Water Column
PSIG - Pounds per Square Inch Gauge
TDH - Total Dynamic Head
SCFM - Standard Cubic Feet per Minute, (exhaust flow rate @ 68°F)
ACFM - Actual Cubic Feet per Minute, (exhaust flow rate @ its temp.°F)
CFM - Cubic Feet per Minute, (exhaust flow rate)
Lbs/hr. - Pounds per Hour (flue gas or water)
GPM - Gallons per Minute
KW - Kilowatt
In3 - Cubic Inch Piston Displacement
RPM - Revolutions per Minute
FPM - Feet per Minute
Q - Heat Load or BTU/HR transfer
U - Heat Coefficient or "U factor"
A - Heating Surface (ft2)
Cp - Specific Heat
BHP - Boiler Horsepower
TGI - Temperature Gas In
TGO - Temperature Gas Out
FPM - Feet Per Minute

Q = U x A x ΔT
BTU/HR. Recovered = (TGI - TGO) x SCFM x 1.0845
BTU/HR. Saved = BTU recovered ÷ thermal efficiency of combustion source
BTU/HR. Recovered (water) = (TLO - TLI) x GPM x 500
BTU/HR. = ΔT x GPM x Density @°F x Cp @°F x 60 min.
GPM = BHP ÷ 14.5
TDH = PSIG x 2.31Ft
Velocity of Water = (GPM x .4085) ÷ (Inside Dia. of Pipe)2
FPM = 1096.7 x sqrt (Velocity Pressure "W.C.) ÷ (Density in Lbs/Ft3 @ °F)
Face Velocity = ACFM ÷ Ft2 face area of exchanger
CFM = FPM x Duct Area Ft2
ACFM = Ft2 of duct x Air Velocity Ft/min.
ACFM = Lbs/HR Exhaust ÷ (60 Min x Lbs/Ft2 @TGI)
SCFM = Lbs/HR flue gas ÷ 4.5
SCFM = ACFM x [528 ÷ (460 + TGI)]
SCFM = [(BTU/HR.input) ÷ (BTU/Ft3) x 9.6 x (% Excess air ÷ 100 + 1) + (BTU/HR. input) ÷ (BTU/Ft3)] ÷ 60 Min.
Lbs/HR Flue Gas = SCFM x 4.5

1Ft3 = 62.4 lbs @ 50°F
BHP = 33500 BTU/HR (output)
BHP = 34.5 lb. steam/hr.
GPM (gallon per minute) water flow = 500 lb/hr
1 gallon of water = 8.3lbs @ 68
1 ft3 of natural gas = 1000 BTU
1 Therm = 100,000 BTU
1 gallon of No. 2 oil = 139,000 BTU
1 gallon of No. 4 oil = 147,000 BTU
1 gallon of No. 6 oil = 150,000 BTU
10KW = 1 BHP output