Peak temperature occur at the discharge valves and normally the temperature of discharge line will be from 50° F. to 100° F. below the temperature at the valve plate. therefore the maximum allowable discharge line temperature are from 225° F. to 250° F
Suction gas entering the compressor should be no higher than 65° F. under low temperature load conditions, or 90° F. under high temperature load conditions, and must never exceed 100° F. On some abnormally critical low temperature applications it may be desirable to insulate the suction lines and return the suction gas to the compressor at lower than normal temperature to prevent the discharge temperatures from exceeding safe limits, but this is not normally necessary on commercial applications where the saturated evaporating temperature is -40° F. or above. The low discharge temperature characteristics of R-502 have made possible much more trouble free operation in single stage low temperature applications.
Air cooled motor-compressors must have a sufficient quantity of air impinging directly on the compressor body for motor cooling. Refrigerant cooled motor compressors are cooled adequately by the refrigerant vapor at evaporating temperatures above 0° F. but at evaporating temperature 0° F. additional motor cooling by means of air flow is necessary.
On air cooled condensing units, adequate cooling can normally be accomplished by locating the compressor in the discharge air blast from the condenser fan. For proper cooling, the fan must discharge air directly against the compressor, since the compressor usually cannot be adequately cooled by air pulled through a compartment in which the compressor is located. if the compressor is not located in the condenser discharge air stream,cooling must be provided by means of auxiliary fan discharging air directly against the compressor body. On compressor with multiple heads such as the Copeland 4R and 6R models, auxiliary horizontal air flow may not provide satisfactory cooling, and vertical cooling fan are required
Water cooled compressors are provided with a water jacket or are wrapped with a copper water coil, and water must be circulate through the compressor cooling circuit before entering the condenser.
Two-stage compressors are equipped with a desuperheating expansion valve for interstage cooling, and no auxiliary. cooling is required.
If compressors or condensing units are located in a machine room, adequate ventilation air must be provided to avoid an excessive temperature rise in the room. To allow for peak summer temperatures a 10° F. temperature rise is recommended, although a 15° F. rise in cooler ambients might be acceptable.
The most accurate calculation is to determine the total heat to be rejected by adding the compressor refrigerating capacity at the compressor refrigerating capacity at the design operating input. The CFM can then be calculated by the formula... CMF= BTU/HR
° T D
For example, determine the machine room ventilation for an air cooled condensing unit operating at -25° F. evaporator, 120° F. condensing with a net refrigeration capacity of 23,000 BTU/HR, 6,400 watts input to the compressor motor, and a 1 H.P. condenser fan motor.
Compressor capacity 23,000 BTU/HR
Heat equivalent 6,400 watts x 3.413 21,843 BTU/HR
Heat equivalent 1 H.P. fan motor 3,700 BTU/HR
Total Heat to be Rejected 48,543 BTU/HR
48,543 BTU/HR = 48,543 CFM
10° TD
With remote condensers, approximately 10% of the rejected is given the compressor casting and the discharge tubing, and the ventilation can be calculated accordingly.
For convenience, table 20A gives a quick estimate of the ventilation air requirement if only the compressor capacity is known.
TABLE 20A
Ventilation Air Requirements For Machine Rooms CFM/1000/ BTU/HR at 10° F. Air Temperature Rise
When a gas is compressed, its temperature increases. Conversely, when it is expanded, its temperature decreases. This is one of the ramifications of the First Law of Thermodynamics.
The temperature of a pure liquid remains constant as it boils or condenses. We have all witnessed this phenomenon in our kitchens. If you measure the temperature of water as it boils, the temperature remains constant at 212°F or 100°C as long as liquid water is present. Also, when gases are condensed, the system temperature remains constant until all the gas converts to a liquid.
No comments:
Post a Comment