TECHNICAL |
gear when a compressor is operated with a |
cycle rates per day of between 120 and 160 |
||
|
frequency inverter . The danger of a lack of
oil arises when a compressor is operated
exclusively at a low frequency and with
periodic on-off cycles . To ensure that the drive
gear is sufficiently lubricated , the compressors
should be operated at a frequency of ≥ 40
Hz for ≥ 10 s in the starting phase , in order to
lubricate the drive gear properly before the
control is released . When evaluating systems
in operation , it is necessary to take into
account that the operation with higher cycle rates is largely focused on operation at night or outside of opening or operating hours .
Generally speaking , cycle rates of > 120 per day can be considered critical and cycle rates of > 160 per day can be considered inadmissible .
Additionally , operation that is as stepless as possible with low compressor cycle times promotes a good oil return from the system . For example , this prevents cold oil mixed with refrigerant from suddenly being returned from the evaporators to the compressors after defrosting phases or under high refrigerating capacity . Moreover , stable operating conditions allow the oil barrier in the oil level switches to settle and align with the level in the drive gear .
The following measurements demonstrate how favourable stepless minimum load coverage during compressor operation is . Before it was converted to stepless capacity control with VARISTEP and IQ MODULE , the lead compressor was operated with a frequency inverter . In this case , the necessary refrigerating capacity was far lower than expected . The consequences for the lead compressor were
|
in winter operation , long shut-off periods
and operation with cyclical pressure and
temperature fluctuations . The visualisation of
the operating conditions in the application limit
illustrate this . The VARISTEP capacity control
with IQ MODULE made it possible to stabilise
the operation and largely avoid the start-stop
cycles of the lead compressor . This is also
shown by the time-weighted mean operating
condition values . For example , the average
evaporation temperature is 2K higher and the
average high pressure is 3 bar lower – with a
positive impact on energy requirements .
RETROFITTING REFRIGERATED DISPLAY CABINETS IN SUPERMARKETS
Exceptional care is needed when refrigerated
display cabinets are to be retrofitted with
glass doors . Most systems are ordered and
delivered with considerable capacity reserves .
Retrofitting glass doors lowers the necessary
refrigerating capacity by an additional 40 to
50 %, depending on the temperature class and
evaporation temperature . This exacerbates
the deviation between the installed and
necessary refrigerating capacity , which has a
significant impact on the part load behaviour
of the systems . Cycle rates will rise rapidly . If a
system ’ s lead compressor has high cycle rates ,
it is necessary to check whether the additional
installation of glass doors can be combined
with a conversion of the compound system .
Suitable measures would include : Select a
lead compressor with a smaller displacement
|
Image 4 : BITZER ECOLINE + 4-cylinder CO2 reciprocating compressor with IQ MODULE and VARISTEP mechanical capacity control
volume and install a lag compressor with stepped capacity control .
A LOOK AHEAD
Compound systems with CO 2 as a refrigerant have become commonplace in supermarkets in recent years . For one , the practical experience supported by extensive data analyses shows that some system concepts are only partially able to meet the different load requirements for various reasons . The goal must be to reconcile high efficiency , low complexity and low investment costs . The control accuracy of the system and the coverage of minimum load conditions play an important role in this context . It is demonstrably
|
possible to improve both significantly with mechanical capacity control systems in the compressors . We can expect the systems to be optimised further in the future . As a specialist in refrigeration and air conditioning technology , BITZER makes its contribution by continuously developing its products and solutions . CLA
Exceptional care is needed when refrigerated display cabinets are to be retrofitted with glass doors .
|