Bitzer KT-600-4 Manual Del Usuario página 31

loads and evaluation for the entire system. This is achieved by separately evaluating the low temperature (LT)
stage and the minimum load of the medium temperature (MT) stage.
Fig. 3: Calculation of the total control factor CF
Q : refrigerating capacity of the variable speed compressor at maximum speed
VsC max
Q : refrigerating capacity of the variable speed compressor at minimum speed
VsC min
Q : Minimum refrigerating capacity of the fixed speed compressor (taking capacity control into consideration if fitted)
FsC
nom: nominal capacity at design conditions
min: minimum capacity at design conditions
In a first step (formulas 1 and 3), the control factors are calculated for the MT and LT stage as described in the ASERCOM Guideline .
In addition, formula 2 calculates the ability to cover the minimum load of the MT stage. For this purpose, standard conditions without LT load are
assumed (i.e. only normal cooling, the compressor of the LT stage is mathematically switched off).
In the last step (formula 4), CF
The final evaluation distinguishes 5 categories of the control factor CF:
Control factor CF
CF ≥ 3
3 > CF ≥ 2,5
2,5 > CF ≥ 1,95
1,95 > CF ≥ 1,55
CF < 1,55
This calculation of CF is also applicable if none of the compressors is capacity controlled. In this case, the term in
question has a value of zero, which significantly reduces the overall CF
Tandem compressors
A tandem compressor is the simplest version of parallel compounding of two compressors. The common high-
volume suction chamber usually guarantees uniform oil distribution. However, for optimum oil balance and a long
lifetime, in general a control with automatic load sequence switching should be provided, which also guarantees a
sufficient minimum running time of the two compressors. Short-term operation increases oil carry over and reduces
the lifetime.
A finely graduated power adjustment can be obtained by switching the two compressors on and off in combination
with capacity regulators (blocked suction ‒ CRII). Speed control of (only) one compressor side is an alternative.
Here, too, load sequence switching is recommended, in which the frequency inverter should be assigned altern-
ately to the lead compressor by means of electrical control switch-over.
KT-600-4 // PDF Download
of a multiple stage refrigeration system:
System
is calculated as the sum of the individual CF.
System
Evaluation
excellent
good
acceptable
poor
unacceptable
.
system
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