of at least 50 percent of rated capacity. Capacity reduction may be
accomplished by cylinder unloading, use of multi- or variable speed
compressors, use of multiple, but not more than four compressors, or a
combination of the two methods. Units with cylinder unloading shall start
with capacity reduction devices in the unloaded position. Units with
multiple compressors shall have means to sequence starting of compressors.
Provide compressors with devices to prevent short cycling when shut down by
safety controls. Device shall delay operation of compressor motor for at
least 3 minutes but not more than 6 minutes. Provide a pumpdown cycle for
units 70 kW 20 tons and over. Provide reciprocating compressors with
crankcase heaters in accordance with the manufacturer's recommendations.
If compressors are paralleled, provide not less than two independent
circuits.
2.5.7
Coils
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NOTE: Research project location conditions to
determine the environmental effects on finned tube
coils. The research should include a survey of
existing similar equipment. If needed, rewrite the
specifications based on the conclusions of the
research. Consideration should be given to the
following combinations based on past experience of
these materials in dealing with the local conditions.
1.
Copper tube and aluminum fins, coated;
2.
Copper tube and copper fins, coated;
3.
Aluminum tube and aluminum fins, coated;
4.
Aluminum tube and aluminum fins, uncoated;
5.
Copper tube and copper fins, uncoated; and
6. Copper tube and aluminum fins, uncoated.
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On coils with all-aluminum construction, provide tubes of aluminum alloy
1100, 1200, or 3102; provide fins of aluminum alloy 7072; and provide tube
sheets of aluminum alloy 7072 or 5052. Provide a separate air cooled
condenser circuit for each compressor or parallel compressor installation.
[Provide a coating on [condenser] [and] [evaporator] coils and fins as
specified in the paragraph entitled "Coatings for Finned Tube Coils." Coils
to be coated shall be part of manufacturer's standard product for
capacities and ratings indicated and specified. Provide plate type fins.]
2.5.8
Condenser Controls
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NOTE: Insert minimum temperature at which the
mechanical cooling equipment will be required to
operate.
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Provide start-up and head pressure controls to allow for system operation
at ambient temperatures down to [_____] degrees C degrees F.
SECTION 15730N
Page 19
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