subcooling

[cool king2007]

Hi everyone,
what is the advantage and disadvantge of subcooling? Thanks

[Sandro Baptista]

advantages: more guarantees of no flash gas at the expansion valves (» better operation) due to friction on the pipes and hydrostatic penalization due to rising of the liquid. Also less mass flow of refrigerant (if the subcooling is significant » smaller pipes to and from the evaporator and also smaller volumetric displacement needed at the LP)

[Brian_UK]

Advantage: System works correctly
Disadvantage: System does not correctly.

[desA]

advantages: more guarantees of no flash gas at the expansion valves (» better operation) due to friction on the pipes and hydrostatic penalization due to rising of the liquid. Also less mass flow of refrigerant (if the subcooling is significant » smaller pipes to and from the evaporator and also smaller volumetric displacement needed at the LP)

Some good points, Sandro.

In my view, ensuring sufficient sub-cooling in order to minimise, or prevent flash-gas, is a real bonus. It allows the TXV, throttle device, to operate correctly, but, more importantly lowers the liquid line pressure drop.

Lowering the liquid line pressure drop, in turn, reduces the high-side pressure required to push the liquid along the pipework. In other words, it can assist to reduce condenser pressure a few degrees. It should not be overlooked.

[nando]

hello evryone,
with R404A, when i have a condening temp under the 40°C, then you see that the gas will stay in the condensor, if the recever has a glas, you can see the gas staying.
so i think the best is to stay with a pressure high enough, pe 40°C, and after the condensor a evaporator to cool the liquid in temp.
So we have a presure 19 bar (40°C) but a real temp of pe 25°C.
i think correct?

[Sandro Baptista]

The refrigerant in liquid state should be well drained from the condenser. So lower condensing pressure you can achieved » lower liquid saturated temperature and better volumetric efficiency of the compressor (» higher cooling capacity) also long life for the compressor...and better COP (the capacity will be higher and the power consumption will be lower).

[nando]

yes i know this, but my problem is that when the condensor pressure is low, the gas will not flow to the evaporator.

[Sandro Baptista]

But if it keeps retained on the condenser that means something wrong with condenser«»receiver. Double the DN of the discharge liquid pipe from the condenser to the receiver or mount another liquid line in parallel with the existing one.
Also If the place where is the liquid receiver is warmer than the condensing temperature the problem can be reveled by what are you saying. Do you have an equalizing line between the inlet of the condenser and the liquid receiver?

[nando]

no, this happens whole the time with standard units, pre fabricated. bitzer has on the receiver a eyeglass where you can see the level. the level stay high, but the eyeglass on the liquidline has flashgas, evaporator pressue lower.
just make the pressure in the condensor higher and you see the gas flowing.

[Camille]

LIQUID SUB-COOLING : is the reduction of refrigerant temperature in the liquid line below the saturation temperature
BENEFIT
- decrease electricity by lowering HP [by increasing refrigeration effect "RE"]
- reduces maintenance/compressor run easier
- reduces pull-down time after defrost

example : compressor working at -45F suction, liquid supplied to the recirculator, will compare between +95F and +25F
RE= 200/(595.6-70.2)= .38 LBS/MIN/TR [25F LIQUID] RE: 525 BTU/LB
RF= 200(595.6-149.2)= .44 LBS/MINTR [95F LIQUID]
RE: 446 BTU/LB

17% IMPROVEMENT

Regards
Camille Zabbal

[Sandro Baptista]

Camille,

Pay attention to this example: condensing temperature +45ºC with 5K subcooling is different of condensing temperature +40ºC with 0K subcooling. In this last case the compressor will give more capacity, less power consumption and less maintenance.

[That's too cold]

Subcooling will increase the capacity in BTU's of the TXV. Which means you can design a smaller TXV for a evap coil. Example: No subcooling, you would have 5 ton TXV on a 5 ton coil. With subcooling you might use a 4.5 ton TXV on the same 5 ton coil. The 4.5 ton TXV will use a higher percent valve capacity and be more efficient. Sporlan has correction factors to calculate for subcooling. On a existing system, if you increase the subcooling temperature too low you will cause flood back to the compressor. Subcooling is highly recommend for freezers.