Controlled Pressure Receiver Operation

Hi Guys. I have some questions regarding the design of a Controlled Pressure Receiver system using R-22. If the CPR were kept at approximately 65psig (45deg.F) can this same vessel feed both a +15degF load and a
-15degF load (with 2 seperate suction group compressors of course)?
I know that the liquid overfeed rate of a CPR system is usually around 1.5:1 and use hand expansion valves for liquid feed to evaps (air units).
Could, however, electronic type expansion valves be used to feed evapoators, programmed to allow a fully flooded coil but only to the the point of exit, into the coil's suction line (before joining the main header)in order to reduce even further the amount of liquid overfeed?
And finally, instead of controlling the pressure of the CPR through a back pressure regulator tied to the +15degF load, could a seperate compressor dedicated to the CPR be used?
Thanks, I appreciate the feedback. And if there are any publications or articles on-line on this subject please let me know.

Great questions!

Sure, you can feed two different evaporating temperatures with a CPR liquid feed as long as they are separate suction pressures. You have to review how much pressure differential is available between the liquid feed and evaporating pressure. That delta P will determine the valve sizing and adjustment.

What you are describing is a hybrid DX system with the electronic expansion valves. The issue is; how do you control the valves? If you have superheat, you don't have a fully wet coil (inside). If you don't have superheat, how will you control the valve?

And yes, A separate compressor can be used for compressing the flash gas off of the CPR... However, you need to be careful. That operation will have a very high suction pressure so you may not be able to reduce the discharge pressure of the system. Compressors have lower limits on compression ratio, so you have to watch that!

Mike.
Did you mentioned about screw or recip. compressor?

Hi Segei.

I'm not quite sure what you are mentioning. Can you elaborate please? Thanks.

I mentioned about low limits on compression ratio. Are these limits apply to screw or recip. compressors? Why?

Thanks
Sergei

Each compressor has different issues. For most recip.'s and screws the minimum compression ratio is 2:1. So, if the suction pressure is high like on a CPR system, the minimum discharge pressure may be close to summer design conditions.

Screws you have to worry about oil pressure if they do not have a full time oil pump.

On recip.'s if the compression ratio is too low the explanation I have heard is; the loads on the wrist pin and valve springs can cause accelerated wear.

For screw compressors without oil pump, pressure difference between discharge pressure and suction pressure should be above the minimum limit. Some Frick compressors have 55psig.

Yes, which would mean the minimum discharge pressure would be at least 65 psig + 55 psig = 120 psig (using the CPR pressure provided by kooltek). This also means the compression ratio is approx. 1.7 at this condition.

Each application and compressor type may be slightly different. All I'm saying is to check the minimum requirements for operation.;)

Boy do I dislike CPr systems. Enough said.

Ken

Quote:

---

Originally Posted by TXiceman

Boy do I dislike CPR systems. Enough said.

---

And that, I also agree with. I am not a fan of these or gas-powered pumper drums either.

two words "pumped liquid" nuff said.

Quote:

---

Originally Posted by chilldis

two words "pumped liquid" nuff said.

---

Yeah, that is my preference. This makes the system a lot more flexible from an energy conservation standpoint IMO.

I think the CPR systems was figured out by a frustrated person with more time on his hands than he knew waht to do with.

I prefer to just pump the liquid...simple, straightforward and guess what....IT WORKS.

Ken

Quote:

---

Originally Posted by TXiceman

I think the CPR systems was figured out by a frustrated person with more time on his hands than he knew what to do with.
Ken

---

And I think that sums it up very well.:D

I am a big fan of the KISS principle. CPR systems & gas powered pumper drums do NOT fit this principle any way, shape, or form!

Just pump the liquid and be done with it....;)

CPR systems had a time when refrigerant pumps were leaking faster than the compressor shaft seals...and they still can make an evaporator that won't work on a pump system start to work pretty good, as in a 55-deg Air Handler for say a bakery. Pump it 20 deg refrigerant from a vessel and it will frost hard from the liquid header over, during a day's running time it will plug solid. Feed it 35 Deg. off a CPR and its very happy.

CPR's can also eliminate a small flock of regulators, cause they can be used to maintain holdback pressure or a 4-pipe hot gas defrost setup and eliminate the penalty of re-expanding the defrost condensate. Similarly for controlled dew point reheat. If the return from a reheater or an ice machine harvest is employed for vessel pressurization its also more energy efficient than flash gas or HP gas for pushing the liquid....

BUT...modern pumps, electrical and pressure controls: Give me the pump, the rest of it I can figure out.

Quote:

---

Originally Posted by sterl

Pump it 20 deg refrigerant from a vessel and it will frost hard from the liquid header over, during a day's running time it will plug solid. Feed it 35 Deg. off a CPR and its very happy.

---

One has nothing to do with the other. If you pump 20°F liquid to an evaporator it won't freeze up any faster or slower than if the liquid feed is from a CPR @ 20°F.

If you feed a coil with 35°F liquid from either a recirc. pump or a CPR system, you get the same effect; a wet coil.

You can't compare a recirc. system with a CPR system at two different temperatures and say one is better than the other.:eek:

Quote:

---

Originally Posted by US Iceman

On recip.'s if the compression ratio is too low the explanation I have heard is; the loads on the wrist pin and valve springs can cause accelerated wear.

---

Can you elaborate or speculate on that a bit?

Iceman:
Your statement is correct: but not addressing the point. In a typical plant targeting process space at 55 to 60 for bakery or 45 to 50 for protein, both will have coolers for ingredients or short term hold or tempering that will target something like 31 to 35 space temp; and often an ingredient water chiller or similar.

For a central arrangement, installing 2- pump recirculators, one for 33 +/- SST and another for 18 to 20 SST is not often done: with the additional compressor controls and low side piping, the laods have to get very large before it makes economic sense. And, of course, normal arrangement and normal control on a PRU intended for 33-deg SST will loose a little ground in terms of the Coil TD compared to an EPR arrangement at 31 or 32.

If an air handler targets a space temp of 55 and is equipped with an EPR to hold the coil at 31 or 32 ET but the recirculator vessel is held at 18 or 20 deg SST, the first few feet of the coil is going to be re-heating the liquid to evaporating temp. On R-717 its about 11% of total heat at 4:1 recirc rate.

The first couple of feet of the HtExch will be hard frozen within an hour; and the frost front will progress through the running period until the whole liquid feed face of the coil is blocked. Some coils are designed to be more tolerable of the effect but all will encounter it...And its actually worse for a protein operation targeting 45-deg process space temperatures.

Same thing can occur on batch type batter mixers and a variety of other equipment, particularly if the process load tends to go away for short intervals of time...

SO...

Feed the same coil from a CPR or a PRU held at Satn > 35: And the coil won't block up. Using the CPR cuts the sophistication down to an additional small vessel, maybe a side port load, and possibly a parallel liquid line to the Air Handler...Parallel to the recirc liquid from the 20-deg SST recirculator.

This effect has been addressed in a number of different ways..the elevated pressure CPR is just one way and only makes any sense in certain cases. Gravity flooded can do the same basic thing, though in cold climates, a surge drum installed outdoors can be an operational headache.

Quote:

---

Originally Posted by sterl

Iceman:
Your statement is correct: but not addressing the point. In a typical plant targeting process space at 55 to 60 for bakery or 45 to 50 for protein, both will have coolers for ingredients or short term hold or tempering that will target something like 31 to 35 space temp; and often an ingredient water chiller or similar.

For a central arrangement, installing 2- pump recirculators, one for 33 +/- SST and another for 18 to 20 SST is not often done: with the additional compressor controls and low side piping, the laods have to get very large before it makes economic sense. And, of course, normal arrangement and normal control on a PRU intended for 33-deg SST will loose a little ground in terms of the Coil TD compared to an EPR arrangement at 31 or 32.

If an air handler targets a space temp of 55 and is equipped with an EPR to hold the coil at 31 or 32 ET but the recirculator vessel is held at 18 or 20 deg SST, the first few feet of the coil is going to be re-heating the liquid to evaporating temp. On R-717 its about 11% of total heat at 4:1 recirc rate.

The first couple of feet of the HtExch will be hard frozen within an hour; and the frost front will progress through the running period until the whole liquid feed face of the coil is blocked. Some coils are designed to be more tolerable of the effect but all will encounter it...And its actually worse for a protein operation targeting 45-deg process space temperatures.

Same thing can occur on batch type batter mixers and a variety of other equipment, particularly if the process load tends to go away for short intervals of time...

SO...

Feed the same coil from a CPR or a PRU held at Satn > 35: And the coil won't block up. Using the CPR cuts the sophistication down to an additional small vessel, maybe a side port load, and possibly a parallel liquid line to the Air Handler...Parallel to the recirc liquid from the 20-deg SST recirculator.

This effect has been addressed in a number of different ways..the elevated pressure CPR is just one way and only makes any sense in certain cases. Gravity flooded can do the same basic thing, though in cold climates, a surge drum installed outdoors can be an operational headache.

---

I think that simple heat exchanger will solve this issue. Liquid from HP receiver to int. receiver on one side and liquid from int. receiver to the evap. coil on another side.