Bally Refrigerated Boxes BVC Generation C – Direct Drive Air-Cooled – Vertical Air - Manual (Page 24)

CONDENSER SELECTION
PRELIMINARY DATA REQUIREMENTS
There are several factors that influence the size of an air
cooled condenser. Before a condenser can be properly
selected, this information must be obtained. It may be
convenient for you to refer to the calculation worksheets
(P. 26 and 27) as you read through the following
information.
1. What are the Desired Evaporating and
Condensing Temperatures?
The evaporating temperature is needed to determine
the THR (total heat of rejection) of the condenser. As the
evaporating temperature is lowered, the heat of
compression increases due to the higher compression
ratio. This affects THR.
The required condensing temperature (CT) must be known
before the temperature difference can be determined. This
is necessary since condenser capacity varies with
temperature difference. The required compressor capacity
will determine the maximum CT since the compressor can
only provide this capacity at certain operating conditions.
You could also refer to Table 1 for CT recommendations.
The heat of compression varies with compression ratio.
Both evaporating and condensing temperatures affect the
compression ratio.
Often customers may request a specified TD value (i.e 10,
15 etc.). The condensing temperature is then established
as being the sum of this TD value and the design ambient
temperature. (i.e 10 + 95 = 105
o F)
2. Compressor Capacity
Determine the capacity of the compressor at the desired
evaporating and condensing conditions. Remember,
tons
refrigeration does not necessarily equal horsepower. As the
evaporating temperature decreases and/or the condensing
temperature increases,
tons refrigeration per horsepower
decreases. One ton refrigeration equals 12000 Btuh.
3.Condenser Ambient Design Temperature
This will be the maximum design temperature of the air
entering the condenser. It is typical to add about 5
o F
to the maximum outdoor design temperature in some
instances to compensate for radiation from a dark
surface such as a black roof.
4. Type of Compressor
It is necessary to identify the type of compressor to be
utilized in the application so that accurate heat of
rejection information may be obtained. For example, open-
drive compressors can be belt driven or direct coupled to
the motor. Electrical energy from the motor is converted to
heat energy which is not transferred to the refrigerant as in
a refrigerant cooled compressor.
In a hermetic refrigerant cooled compressor, the cool
to the maximum outs
suction vapor picks up heat as it travels through the
warm motor windings. The condenser must be sized to
reject this heat along with any other heat absorbed by
the refrigerant. It can be observed in Table 2 that
hermetic refrigerant cooled compressors have higher
heat of rejection factors.
5. Heat of Compression
As the refrigerant is compressed in the compressor, its
heat content increases due to the physical and
thermodynamic properties of the refrigerant. Additional
heat from friction between moving parts in the compressor
also increases the heat content of the refrigerant. The
amount of heat added to the refrigerant is dependent on
the refrigerant type, the compression ratio and the type of
compressor.
Accurate THR or heat of compression factors may be
available from the compressor manufacturer. Always
attempt to access this information prior to using other
methods. If this information is not available, refer to the
heat of rejection factors in Table 2.
However, in situations where your application exceeds the
limits of this table, such as in compound compression and
cascade systems, one of the following calculations may be
performed.
For OPEN DRIVE COMPRESSORS:
Total heat of Rejection = Compressor Capacity (Btuh) + (2545 x BHP)
(BHP - Brake Horsepower of the motor)
For SUCTION COOLED COMPRESSORS:
Total heat Rejection = Compressor Capacity (Btuh) + (3413 x KW)
(KW may be obtained from the power input curve for that compressor)
6. What is the Refrigerant Type?
A condenser’s capacity can vary by 8 to 10% due to
differences in physical and thermodynamic properties.
Refer to the correct refrigerant capacity table or use factor
as indicated. (see P. 2)
7. Altitude
The volume of a given mass of air increases as it rises
above sea level. As its volume increases, its density
decreases. As the air becomes less dense, its heat
capacity decreases. Therefore, more air volume would
have to be forced through the condenser at 6,000 feet
above sea level than at sea level.
Since condenser capacities are based on operation at
sea level, an altitude correction factor must be applied to
the total heat of rejection. Basically, the load on the
condenser will be increased to a point which will compen
sate for the higher altitude.
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