Pressure regular valve for wet return

[hphien]

Hi all,
Pls help me.
I am designing a freezing sytem ( NH3 overfeed pump)
It's include:
1 IQF(evaporator temp. = -45 oC,)
1 contact freezer(evaporator temp. = -40 oC)
1 air blast (evaporator temp. = -40 oC)
All wet return pipe of freezing system install in manifold at low receiver vesell.
Should I install a pressure regular valve for wet return ?

Thanks and best regard,
hphien

[Peter_1]

How will you ensure proper NH3 flow through each evaporator?

How will you manage 2 different evaporating temperatures in an overfeed systeem?

[US Iceman]

The back-pressure regulators would be installed on the the warmer, wet return lines (-40C). The -45C wet suction line would be the common low pressure piping.

In this application, the -40C systems would be regulated to maintain the pressure equivalent to the -40C temperatures. The -40C systems would then be piped into the -45C wet suction line.

The refrigeration system would be operated at -45C for the entire cooling load. The evaporators on the contact freezer and air blast would simply operate at -40C.

This will work.

You should be aware of the other requirements. The ammonia pumps will have to supply sufficient pressure to overcome the higher evaporating pressures of the -40C coils, if a common -45C liquid overfeed vessel is used.

Another method is to use two liquid overfeed vessels. One operating at -45C, and another operating at -40C. The -40C system could have a common back-pressure regulator attached in the dry suction line before the compressors. This method would not require higher pump discharge pressures.

Using two vessels can also reduce potential problems. If a common -45C system is used to supply all evaporators, the -40C evaporators will be supplied with -45C liquid. This can reduce the evaporator capacity of the -40C coils.

Or, you can use two evaporating temperatures; one at -40C and one at -45C. Each suction temperature would have it's own booster compressors. With this method you have no back-pressure regulators and no problems with subcooled liquid feeding a warmer evaporator.

The last method would be my choice as it is simpler to operate and could be much less expensive to operate if you have significant loads at the warmer temperatures.

Hope this helps.

[hphien]

Thanks your reply
In this application, i use one liquid overfeed vessel.
Please show me type back-presure regulator valve install on the warmer, wet return line (-40C) of contact freezer and air blast freezer.This valve will regular pressure in wet return line after or before it.
In this case, how many suction temperature of compressor for contact freezer and air blast freezer?-40C or -45C

[Peter_1]

You have to install it in the highest temperature lines.

Look around a little bit on http://www.hantech.com/products/press.asp to learn more about NH3 valves.

Compressors have to work on -45°C and you can dial in a higher temperature on the valves whatever you want, in your case -40°C but that's a temperature you have to chose yourselves.

Why not run everything on -45°C, a lot less complicated?
Compressors have to run anyway at -45°C in your system, so a COP increase due to a higher temperature isn't possible with the pressure regulating valves.

[TXiceman]

Check with either Hansen or Refrigerating Specalities (Parker). for back-pressure regulators/

ken