Liquid subcooling theory

[kooltek]

I know there have been many threads concerning liquid subcooling, maybe these questions have been asked, and if so, please direct me.

1.) +45deg.F has been the magic number for liquid subcooling setpoint(approximately). What is the significance of that temperature and how is it figured?


The purpose of subcooling is to remove flash gas and provide a solid column of high quality liquid entering the expansion device. This increases the capacity and overall efficency of the system. The colder the liquid entering the expansion valve is to the evaporator temperature the more efficent the TXV becomes. According to Sporlan, keeping a 150psi drop through the system, at +10deg.F a 1 ton valve gains the capacity close to a 2 ton valve.

2.) Why then would it not be an advantage to further subcool the +45deg.F liquid to say +10deg.F by a another Heat X connectd to the meat case rack or to a small dedicated subcooling compressor?

3.) Why is it not common practice to subcool the liquid for the med. temp +15deg.F rack?
Thanks guys, I look forward to all the responses

[intellitech]

I know there have been many threads concerning liquid subcooling, maybe these questions have been asked, and if so, please direct me.

1.) +45deg.F has been the magic number for liquid subcooling setpoint(approximately). What is the significance of that temperature and how is it figured?


The purpose of subcooling is to remove flash gas and provide a solid column of high quality liquid entering the expansion device. This increases the capacity and overall efficency of the system. The colder the liquid entering the expansion valve is to the evaporator temperature the more efficent the TXV becomes. According to Sporlan, keeping a 150psi drop through the system, at +10deg.F a 1 ton valve gains the capacity close to a 2 ton valve.

2.) Why then would it not be an advantage to further subcool the +45deg.F liquid to say +10deg.F by a another Heat X connectd to the meat case rack or to a small dedicated subcooling compressor?

3.) Why is it not common practice to subcool the liquid for the med. temp +15deg.F rack?
Thanks guys, I look forward to all the responses


I am not a market tech but will say that to further subcool the liquid would require more energy use by running the mechanical subcooling compressor longer to achieve the lower desired temperature.

So I will venture to say that the efficiency gained by the lower temperature subcooled liquid is offset by the extended energy usage by the compressor being used in the subcooling circuit.

Just some thought........

[Joe Harper]

This would require a recirculator and refrigerant pumps. Its pretty common on ammonia systems and in large refrigeration plants.

[icemeister]

The concept of "Mechanical Subcooling" is really rather simple actually.

It's based on the fact 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. A lower compressor load equals lower required horsepower.

The savings comes in when you shift the subcooling load to another system operating at a higher evaporator temperature. 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.

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.

[man from trane]

Great info there! ^

I'm getting excited about this. :D I have a combo rack with subcooling on the low temp liquid only. There are 3 low temp comps and 3 medium temp comps, and a smaller "spare" that can be valved to either side. I have it running on low temp but I still need more capacity for the freezers. My medium temp side could lose a compressor and still be ok.

There's an ORI valve from the subcooling heat exchanger back to the medium temp suction that keeps the pressure from going too low. Since I have plenty of capacity on the meduim temp side, why not just crank that sucker open, lower the subcooler setpoint from 45° to 15°, and let it go as low as it will? Medium temp runs +18°, Low is -28°.

[kooltek]

I'm liking these responses. Maybe there is something to the lower subcooled temperature after the 45degF original temp. Ice, thanks for explaining the theory and how the load is shifted to the MT rack and how the savings could be greater using a seperate dedicated compressor.