TM-5-855-4
2-6. Internal loads.
machine rated at K brake horsepower (bhp) dissipated heat at the rate qe in Btuh.
(eq 2-3)
This power, delivered by the motor, is a fraction of the motor input. This fraction is by definition the full
load efficiency E of the motor and the motor input equivalent qi in Btuh is then
(eq 2-4)
The heat dissipated by the motor alone is the balance between motor and shaft input
Btuh.
(eq 2-5)
The efficiency of fractional horsepower motors increases with the rated power from a low 35 percent to a
--
maximum of 76 percent. For estimating purposes the heat emitted by fractional horsepower motors is:
(eq 2-6)
From 1 to 250 hp, the efficiency continues to improve to reach a maximum of 94 percent. The approximate
heat emitted by motors in that range is:
The heat emitted by the motor or the driven machine must be allocated to the spaces or air stream where
they are respectively located. If the shaft goes through a partition, these spaces may not be the same. The
preceding calculations are valid for continuous operation. For intermittent operation an appropriate
usage factor should be used and preferable measured.
b. Lights. Energy from electric lights is converted into heat. The heat equivalent of a watt (W) is
3.41 Btuh. The instantaneous lamp heat emission is the product of the heat equivalent of the total lamp
wattage and a use factor which is the ratio of the wattage in use to the wattage installed. To this must be
added the heat radiated by the ballast, which is usually 20 percent of the lamp heat emission. It bears
repeating that the heat emitted by the fixture and the ballast must be allocated to the respective space or air
stream where the heat is actually radiated. These spaces are not the same in the case of a false ceiling or
when the fixtures are recessed or used for air return, and only a portion of the lamp heat reached the room
it lights. This information should be supplied by the fixture manufacturer.
c. Occupant. Personnel emit sensible and latent (moisture) heat in the room. The individual
emission rates depend on clothing, activity level, sex, age, room temperature T, and other factors
influencing the person's metabolism. On the average the sum total qt of the sensible and latent heat
emission rate per capita is 400 Btuh for sedentary activities and 660 Btuh for light work. For design
purposes the sensible heat rate qs is 320 Btuh up to 68 F ambient, zero above 100 F, and between these limits
computed according to equation 2-8.
28
Previous Page
