14 BULLETIN 1335, U. S. DEPARTMENT OF AGRICULTURE 
In the United States the unit of heat customarily used is the British 
thermal unit (B. t.u.), which for practical purposes is defined as the 
heat required to raise the temperature of a pound of water 1° F. 
Heat is most commonly produced through the combustion of oil, 
coal, wood, or gas. Heating by electricity is seldom practicable 
because of its greater cost, but where cheap rates prevail it is one of the 
safest and most efficient, convenient, and easily regulated methods. 
The average numbers of heat units (B. t. u.) furnished by electricity 
and various fuels are given in Table 4. With the exception of those 
for electricity, these average values will vary with the origin and 
character of the fuel. 
TaBLE 4.—Heat units furnished by various fuels and electricity 
Fuel 
Heat required 
Fuel units fur- | to furnish 
nished 750,006 
| Babu 
Bate: 
per pound| Pounds 
Buelow soe 2a. es Pepe eee cy Speed Nn as ae Ren ete ia ni le TEE A ia hee 8 18, 500 40 
COs Bese eee a ae ne Rte gre Wore ie ENT Cerys EG So a Sat ees Nee 12, 500 60 
Woods (@in dred; 10:3 percent moisture) sss a ee eee 7, 589 100 
CS Ee EEE ED TaD AR SES ens ape Oe re eg = Oe a ee 1750 21, 0C0 
HDI CEYCLB LL GId FN forges tc ee ON Vl anes eR Snel ioe Cy ae Ie Ray Erb ee A Be ah 3 3, 415 4 200 
1 Per cubic foot. 2 Cubie feet. 3 Per kilowatt hour. 4 Kilowatt hours. 
Direct heat, direct radiation, and indirect radiation are the types 
of heat generally employed. 
Direct-heating systems have the highest fuel or thermal efficiency. 
The mixture of fuel gases and air in the combustion chamber passes 
directly into the air used for drying. This method requires the use. 
of special burners and a fuel, such as distillate or gas, which burns 
rapidly and completely, without producing soot or noxious fumes. 
The heater consists simply of a bare open firebox, equipped with 
one or more burners, an emergency flue to discharge the smoke 
incidental to lighting, and a main flue, through which the gases of 
combustion are discharged into the air duct leading to the drying 
chamber. 
Direct-radiation systems also are relatively simple and inexpensive 
and have a fairly high thermal efficiency. A typical installation 
consists of a brick combustion chamber with multiple flues, which 
carry the hot gases of combustion back and forth across an air-heating 
chamber and out to a stack. The air is circulated over these flues 
and heated by radiation from them. The flues are made of hght 
east iron or sheet iron. The air-heating chamber should be con- 
structed of fireproof materials. The efficiency of the installation 
depends upon proper provision for radiation. This is attained by 
using flues of such length and diameter that the stack temperatures 
will be as low as is consistent with adequate draft. 
Heating the air by boiler and steam coils or radiators is an indirect- 
radiation system, as the heat is transferred from the fuel to the air 
through the intermediate agency of steam. Such a system costs 
more to install and has a lower thermal efficiency than either of the 
other two systems. It is principally adapted to large plants oper- 
