Selection Of Compressor And Accessory - Bitzer KT-600-4 Manual Del Usuario

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Selection of compressor and accessory

Compressor selection and capacity grading
For the design of parallel compounding, the required power must be exactly evaluated:
• Power demand at maximum load (design conditions)
• Power demand at minimum load (operation at night, operation outside opening hours, for example in supermar-
kets, reduced cooling demand and low condensing temperatures at low outdoor temperatures, ...)
• Number of simultaneously operated evaporators
Since each evaporator can have a different effect on the overall load, it may be necessary to weight the individual
loads as to how much they contribute to the overall load during a certain operating period. Intelligent control can,
however, distribute the load such that the cooling demand does not undergo a drastic change.
The best control accuracy is achieved by having the compound cover the cooling demand by means of a quasi-
stepless variation of the cooling capacity between minimum and maximum. A low control range and significant
changes in load or capacity result in instability of the overall system. Compressors with variable speed or finely
graduated mechanical capacity control (e.g. CRII) are a good option for achieving stable process control, if the con-
trol range of at least one compressor can cover the capacity gaps caused by other compressors during switch-on
and switch-off.
CF = /b = 150% a
Fig. 2: Example of the control accuracy of parallel compounding with 2 compressors (source: ASERCOM)
VsC: Compressor with variable speed
FsC: Compressor with fixed speed
CF: Control accuracy in %
For further information on the optimized selection of compressors and their capacity control, see "ASERCOM
Guidelines for the design of multiple compressor racks using frequency inverters" (chapter 2).
Multiple stage systems
Capacity regulation is essential for modern refrigeration systems, especially for refrigerants with high suction dens-
ities and volumetric cooling capacities. The targets are:
• covering low minimum load, preferably without on-off cycling
• a high control accuracy (control factor CF) with minimum changes in capacity per step
• cost reduction, e.g. by higher capacity with a lower number of compressors
• minimized diversity of used compressor types
• operational safety
The opposing demands sometimes lead to load conditions with many on-off cycles and unstable (fluctuating) oper-
ating conditions by pour control factors. This may lead to reduced efficiency, wet operation, oscillating control cir-
cuit, unfavourable operating conditions for compressors, poor temperature control and product quality.
Supplementing the above example of a single-stage refrigeration system, it may therefore be helpful to calculate
the control accuracy of a multi-stage refrigeration system (medium and low temperature application) using the fol-
lowing method. It takes into account the industry trend of using fewer and fewer compressors per suction group or
temperature stage, and it is intended to combine the opposing demands: high control factor, covering minimum
KT-600-4 // PDF Download
a
VsC
VsC FsC
CF = /b = 80% a
VsC
b
VsC
a
b
FsC
30