One of the limiting characteristics of a refrigerant is its ability to be able to achieve the least temperature and maintain the same without going into negative pressure. From the PT chart Figure 1, we can notice that various refrigerants cease to be at positive pressure and go into vacuum conditions at various temperatures. For example: R22 (HCFC) starts to go into vacuum at below -40 degrees C, R134a (HFC) starts to go into vacuum at below -25.6 degrees C. R 410a (Blended HFC) starts to go into vacuum at below -55 degrees C .
Figure 1: Refrigerant PT Chart
What happens if these
refrigerants attain temperatures below these threshold limits? The pressure
will go into vacuum i.e., there is negative pressure created in the system.
What harm is there if there is negative pressure in the system? There is always
a possibility of air ingress due to collapse failure of the gaskets at joints.
Along with air there will be ingress of moisture. And as everybody would admit,
air and moisture are the two biggest enemies of refrigeration. We have to keep
air and moisture out of the system at any cost. We must prevent occurrence of
negative pressure in the system.
How do we do it? By stopping the
compressor by cutting off through the operation of low pressure switch. In
fact, that is the primary purpose of L P cutout – to switch off the compressor
motor electrically at a specified low pressure, when the load is minimal. That
means it helps us to maintain a minimum positive pressure in the system,
especially on the compressor suction side.
Some of the system manufacturers
are promising minimum temperatures lower than the above mentioned threshold
limits of temperatures using various refrigerants.
Carrier is able to promise
temperature of upto -29 degrees C using R134a as refrigerant for its latest
Thinline Container Units, even though the lowest temperature possible with R
134a working under positive pressure is -25.56 degrees C.
How are they able to do it? – By using
Figure 2: Performance Specs of Carrier ThinLINE Container Units
HERMETICALLY SEALED UNITS!
Till recently, we only heard
about Hermetic Expansion valves and Hermetic Compressors.
Hermetic basically means fully
sealed or air tight. Conventionally, the compressor, receiver, filter drier and
valves used to be fitted to the pipelines through flanged nut and bolt
connections or threaded connections. With these units, there was always a
limitation on the low pressure that the unit could work on. One had to be
doubly cautious not to allow the compressor to run in vacuum conditions,
especially, during maintenance work. There was a limitation of the minimum
temperature which could be reached using a particular refrigerant.
Hermetically Sealed Compressors do not have any access to maintenance of internal parts. Repair is not possible, only replacement is possible. They have one suction pipe connection going into the compressor and one discharge pipe connection coming out from the compressor. There is one electrical terminal box. Even the suction and discharge pipes themselves have brazed connections. So, there is no chance of any air ingress into the compressor through the inlet or outlet connections.
Hermetically Sealed Expansion Valves do not permit access to the internal parts of the Expansion Valve. They have brazed connection for the inlet, outlet and the external equalizing pipes. No adjustment of the internal setting of the expansion valve is possible or permitted. These are more commonly used in refrigerated containers and reefer cargo ships. If found defective, they need only to be replaced by de-brazing the three connections and re-brazing them.
Hermetically Sealed Compressors
Hermetically Sealed Expansion Valves
Hermetically Sealed Refrigeration Units
The EU F Gas Regulations provides a precise definition of the term ‘hermetically sealed’ as quoted below.
“Hermetically sealed system means a system in which all refrigerant containing parts are made tight by welding, brazing or a similar permanent connection which may include capped valves and capped service ports that allow proper repair or disposal and which have a tested leakage rate of less than three grams per year under a pressure of atleast a quarter of the maximum allowable pressure;”
In other words, Hermetically Sealed Refrigeration Units do not have any joints or flanges. The entire pipeline system is in single piece and all the individual component and pipes have only brazed connections. So, there is no chance of any ingress through the connections. The compressor can run in vacuum, but still there is no chance of air ingress.
Figure 5: Hermetically Sealed Refrigeration System
Advantages
1. No chance of air ingress under any circumstances.
2. More reliable operation.
3. Units are designed to run under low and vacuum suction pressure, hence lower temperatures can be achieved.
The new generations of container refrigeration units are hermetic or completely sealed.
AUTHORS CREDIT & PHOTOGRAPH
Chilukuri Maheshwar
Faculty Member,
Anglo Eastern
Maritime Academy,
Karjat, Mumbai.
