Building Design and Construction Handbook - Merritt - Ventech!

13.74 SECTION THIRTEEN
unit, one must first determine design conditions for which this unit is to operate;
for example:
Suction temperature—whether at 35�F, 40�F, 45�F, etc.
Head pressure—whether serviced by a water-cooled or air-cooled condenser.
Manufacturers of compressor-motor units rate their equipment for Btu per hour
output and kilowatt and amperage draw at various conditions of suction and discharge
pressure. These data are available from manufacturers, and should be
checked before a compressor-motor unit is ordered.
Although capacity of a compressor decreases with the suction pressure (or back
pressure), it is usual practice to rate compressors in tons or Btu per hour at various
suction temperatures.
For installations in which the latent load is high, such as restaurants, bars, and
dance halls, where a large number of persons congregate, the coil temperature will
have to be brought low enough to condense out large amounts of moisture. Thus,
we find that a suction temperature of about 40�F will be required. In offices, homes,
etc., where the latent load is low, 45�F suction will be satisfactory. Therefore,
choose a compressor with capacity not less than the total cooling load and rated at
104�F head temperature, and a suction temperature between 40 and 45�F, depending
on the nature of the load.
Obtain the brake horsepower of the compressor at these conditions, and make
sure that a motor is provided with horsepower not less than that required by the
compressor.
A standard NEMA motor can be loaded about 15 to 20% above normal. Do not
depend on this safety factor, for it will come in handy during initial pull-down
periods, excess occupancy, periods of low-voltage conditions, etc.
Compressor manufacturers have all the above data available for the asking so
that one need not guess.
Sealed compressors used in most air-conditioning systems consist of a compressor
and a motor coupled together in a single housing. These units are assembled,
dehydrated, and sealed at the factory. Purchasers cannot change the capacity
of the compressor by changing its speed or connect it to motors with different
horsepower. It therefore is necessary to specify required tonnage, suction temperature,
and discharge temperature so that the manufacturer can supply the correct,
balanced, sealed compressor-motor unit.
13.25 COOLING EQUIPMENT—CENTRAL PLANT
PACKAGED UNITS
For an economical installation, the air-handling units should be chosen to handle
the least amount of air without danger of sweating ducts and grilles. In most comfort
cooling, an 18�F discharge temperature below room temperature will be satisfactory.
If a fresh-air duct is used, the required fresh air is mixed with the return air, and
the mix is sent through the cooling coil; i.e., the fresh-air load is taken care of in
the coil, and discharge air must take care of the internal load only. Then, from Eq.
(13.30), the amount of air to be handled is
q s
Q � (13.35)
1.08 �T