The concept of “Mechanical Subcooling” is really rather simple actually.
It’s based on the idea that every Btu that you remove from the liquid refrigerant going to a system’s evaporator increases the refrigerant’s ability to remove heat by the same number of Btu’s….ie, one pound of liquid at 50ºF can remove over 25% more heat than liquid at 100ºF.For a given evaporator load, if you have 50F liquid, then you need less liquid flow. Less flow means the compressor will see a lower load coming from the evaporator. If the mechanical subcooling is being done with a heat exchanger connected to the same system, the total load on the compressor remains the same as before form the point of view on the compressor.
However, if the subcooling load is shifted to another system operating at a higher evaporator temperature, significant overall capacity and efficiency gains can be realized. Remember….at -25ºF Saturated Suction Temperature (SST) you need about 3 HP/Ton of refrigeration, at +10ºF SST it’s approximately 2 HP/Ton and at +40ºF SST you only need 1 HP/Ton. So it’s easy to see why supermarkets use a high temperature +40°F SST compressors to subcool the liquid of their -25°F SST systems.
Consider a basic 90 HP LT rack running at -25ºF SST with a 30 ton load. Now we add a subcooler tied to a MT rack running at +10ºF SST. If we assume the 50ºF liquid will shift 25% of the LT load to the MT rack, the LT compressor would then see a reduction of 7.5 tons to a 22.5 ton load and the horsepower would drop to 67.5 HP. The subcooler load would then be seen by the MT rack as a 7.5 ton increase requiring only 15 HP more.
That’s a 7.5 HP savings…or about 8% in overall system efficiency.
Now…if you shifted that same 7.5 ton subcooler load to a separate compressor running at 40ºF SST, the required horsepower would only be 7.5 HP. That’s now a 15 HP savings over the standard LT rack…or nearly 17%.
That’s it in a nutshell. You get some significant energy reduction with a relatively inexpensive addition of some basic hardware.
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