Sub-Cooled Liquid Refrigerant - 2013-01

Sub-Cooled Liquid Refrigerant    

The sub-cooler system is an integral part of the freeze dryer refrigeration loop, and the correct amount and temperature of sub-cooled liquid refrigerant is vital to the proper operation of the lyophilizer.

Sub-cooling is a state where the liquid refrigerant is colder than the saturation temperature, the minimum temperature required to keep it from boiling and changing from a liquid to a gas. The amount of sub-cooling is the difference between its saturation temperature and the actual liquid refrigeration temperature.

Sub-cooling is desirable for two reasons:

1.     Sub-cooling increases the efficiency of the system since the amount of heat being removed per pound of refrigerant circulated is greater.

In other words, less refrigerant is pumped through the system to maintain the desired temperature. This reduces the amount of time that the compressor must run to maintain the temperature which reduces wear and energy consumption. The amount of capacity boost achieved with each degree of sub-cooling varies with the refrigerant being used.

2.     Sub-cooling prevents the liquid refrigerant from changing to a gas before it reaches the evaporator (heat exchanger).

Pressure drops in the liquid piping and any vertical risers can reduce the refrigerant pressure to the point where it will boil or "flash" in the liquid line. This change of phase causes the refrigerant to absorb heat before it reaches the heat exchanger, reducing its capacity to do useful work. Inadequate sub-cooling also prevents the expansion valve from properly metering liquid refrigerant into the evaporator, also resulting in decreased heat absorption and poor system performance.

Generally, the temperature of the sub-cooled liquid in a freeze dryer refrigeration loop should be between -15°C and -25°C (+5°F and -13°F) in a no load condition. If the sub-cooled liquid is too warm under no load conditions, the other expansion valves in the system become inefficient. This inefficiency can cause the lyophilizer's refrigeration system to fail to reach the necessary blank-off temperature and fail validation. Even if the system is robust enough to reach the required blank-off temperature with inadequate sub-cooling, it will be working much harder than necessary and have less free capacity.

In two-stage compressor systems, the sub-cooled liquid heat exchanger also performs another function: it desuperheats the interstage refrigeration line between the low stage discharge and the high stage suction of the two stage compressor. In this instance the sub-cooler expansion valve influences the crankcase pressure, which is the interstage pressure in most cases. The interstage pressure in turn influences the compressor compression ratio and thus compressor performance.

Caution

Too Much Sub-cooling:
  • Can result in an accumulation of liquid refrigerant in the compressor crankcase.
  • Can result in liquid refrigerant attacking the compressor motor windings resulting in an electrical short.
  • And other...

Too Little Sub-cooling:

  • Can result in a low refrigerant charge.
  • Can result in loss of compressor motor cooling.
  • Can result in reduced inter-stage pressure and lubrication problems.
  • And other...
These problems and more will be addressed in a future Tech Tip, as well as how to insure the proper adjustment of the sub-cooler.

 

 

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These technical tips are for use by qualified maintenance personnel who are familiar with their specific freeze drying equipment. These tips are intended only as general guidelines. Lyophilization equipment is frequently custom configured and some tips may not be appropriate for all freeze dryers. Always read and follow the directions of your equipment's maintenance manual. If you would like to discuss one of our tech tips, please contact Dave Clayton at 215-672-7800 ext-1376.


 

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