Re: Desuperheating by liquid injection
Tycho.
I think that you have one misconception. You assumed that plant which has discharge pressure regulating valve between compressor and condenser is normal. This is not normal. Actually this is wasting of energy to have condensing temperature 20C and discharge 30C. Definitely, oil carry over can be issue, but increased size of oil separator will solve the issue.
Look at operation of normal refrigeration plant that have condensing pressure close to discharge pressure. Assume that suction pressure is 0 bars, economizer port 2 bars, LRI port 5 bars and discharge 10 bars. Condensing pressure slightly lower than discharge due to pressure drop in discharge line. LI in economize port should be compressed from 2 to 10 bars. LI in LRI port should be compressed from 5 to 10 bars. LRI injection is more efficient. However, water cooling of oil is the most efficient, because no additional work should be done by compressor. Thermosyphon cooling don't require compressor work as well just small additional load to the condensers. It is better than LRI as well.
Another issue with LRI port is that at lower condensing pressure liquid injection will be undersupplied. If condensing pressure will drop to 8 bars liquid supply pressure difference will be 8-5=3 bars instead of 10-5=5 bars that was before.
Typical refrigerant vapor injected in economized port was created by useful work of cooling. It means by subcooling liquid refrigerant or by high temperature refrigeration load. LI don't do any useful cooling. I mean cooling effect for refrigeration plant. Oil can be cooled by water or TS without work done by compressor.
Re: Desuperheating by liquid injection
Quote:
Originally Posted by
Segei
Tycho.
I think that you have one misconception. You assumed that plant which has discharge pressure regulating valve between compressor and condenser is normal. This is not normal. Actually this is wasting of energy to have condensing temperature 20C and discharge 30C. Definitely, oil carry over can be issue, but increased size of oil separator will solve the issue.
Look at operation of normal refrigeration plant that have condensing pressure close to discharge pressure. Assume that suction pressure is 0 bars, economizer port 2 bars, LRI port 5 bars and discharge 10 bars. Condensing pressure slightly lower than discharge due to pressure drop in discharge line. LI in economize port should be compressed from 2 to 10 bars. LI in LRI port should be compressed from 5 to 10 bars. LRI injection is more efficient. However, water cooling of oil is the most efficient, because no additional work should be done by compressor. Thermosyphon cooling don't require compressor work as well just small additional load to the condensers. It is better than LRI as well.
Another issue with LRI port is that at lower condensing pressure liquid injection will be undersupplied. If condensing pressure will drop to 8 bars liquid supply pressure difference will be 8-5=3 bars instead of 10-5=5 bars that was before.
Typical refrigerant vapor injected in economized port was created by useful work of cooling. It means by subcooling liquid refrigerant or by high temperature refrigeration load. LI don't do any useful cooling. I mean cooling effect for refrigeration plant. Oil can be cooled by water or TS without work done by compressor.
With seawater temp at 4-15 degrees, it's necessary with a PM valve to keep the discharge pressure up, even with VFD controlled condenser pumps.
Maybe a PM valve is a waste to energy, but it save you lot's of headaches... in case of a sudden shutdown of the compressor when it's at full capacity and full speed, you don't loose 3/4 of the oil in the oil separator.
The oil separator is more than large enough, but it won't stop the oil from going out into the system if it get's a sudden pressuredrop.
I agree that an oil cooler is the best solution, but with the pressure on prices, it's cheaper to install an LRI than to install an oil cooler, a 3 way valve and an extra sea water pump (need an extra pump because we run VFD on the condenser pump)
Re: Desuperheating by liquid injection
In North America typical ammonia refrigeration plant has minimum condensing temperature is 20C or 110 -115 psig. This is just typical plant operation. I know a few plants that have condensing temperature of 10C. Suction pressure of these plants around 0 bars. For higher suction pressure we need oversized oil separator. Cost of the compressor with this oil separator will increase by 5%. I talked to several compressor manufacturers and everybody claim that their compressors can operate at condensing pressure 15C even with 2 bars suction pressure, but oil separator should have right size.
Re: Desuperheating by liquid injection
Quote:
Originally Posted by
Segei
In North America typical ammonia refrigeration plant has minimum condensing temperature is 20C or 110 -115 psig. This is just typical plant operation. I know a few plants that have condensing temperature of 10C. Suction pressure of these plants around 0 bars. For higher suction pressure we need oversized oil separator. Cost of the compressor with this oil separator will increase by 5%. I talked to several compressor manufacturers and everybody claim that their compressors can operate at condensing pressure 15C even with 2 bars suction pressure, but oil separator should have right size.
On freezing plants with -0.3 bar in suction, we still run with a PM valve on the discharge to keep the discharge at 30C, if we do this, we can use the same oil separators on -0.3 bar suction, or at 3.5 bar suction to prevent any accidental oil loss in case of a "alarm stop" on the compressor... we have a "fineoil separator" incorporated in our design, so we can run on any pressures you say and still get 99% of the oil loss back into the compressor suction... we design our freezing systems with two PM valves, so when you need a defrost, we shut of all hotgas to the condenser because the condenser PM is on 0.1 bar higher pressure. and lead it all to the plate freezer or evaporator that needs the hotgas... sure we are like the most expensive air--air-split unit
but after awhile, we will give you more back for your buck...
Of course you could add a full speed condenser pump, but with a regulating valve at the outlet of the condenser... where is the gain, with the full speed pump with the trothling valve, or with the VFD pump with a full open outlet?
Re: Desuperheating by liquid injection
Quote:
Originally Posted by
Segei
In North America typical ammonia refrigeration plant has minimum condensing temperature is 20C or 110 -115 psig. This is just typical plant operation. I know a few plants that have condensing temperature of 10C. Suction pressure of these plants around 0 bars. For higher suction pressure we need oversized oil separator. Cost of the compressor with this oil separator will increase by 5%. I talked to several compressor manufacturers and everybody claim that their compressors can operate at condensing pressure 15C even with 2 bars suction pressure, but oil separator should have right size.
On freezing plants with -0.3 bar in suction, we still run with a PM valve on the discharge to keep the discharge at 30C, if we do this, we can use the same oil separators on -0.3 bar suction, or at 3.5 bar suction to prevent any accidental oil loss in case of a "alarm stop" on the compressor... we have a "fineoil separator" incorporated in our design, so we can run on any pressures you say and still get 99% of the oil loss back into the compressor suction... we design our freezing systems with two PM valves, so when you need a defrost, we shut of all hotgas to the condenser because the condenser PM is on 0.1 bar higher pressure. and lead it all to the plate freezer or evaporator that needs the hotgas... sure we are like the most expensive air--air-split unit
but after awhile, we will give you more back for your buck...
Of course you could add a full speed condenser pump, but with a regulating valve at the outlet of the condenser... where is the gain, with the full speed pump with the trothling valve, or with the VFD pump with a full open outlet??? I ask you???
This point could be discussed to death....
I am sure...
But of course I am right, if we use an oversized condenser, and use a VFD pump, to pump the minimum required through the condenser, is better than a full speed pump blowing crazy amounts of water though the condenser, throttled by a vale at the outlet, causing a higher pressure, and more cavitation on the pipe out of the condenser
Re: Desuperheating by liquid injection
Quote:
Originally Posted by
Segei
In North America typical ammonia refrigeration plant has minimum condensing temperature is 20C or 110 -115 psig. This is just typical plant operation. I know a few plants that have condensing temperature of 10C. Suction pressure of these plants around 0 bars. For higher suction pressure we need oversized oil separator. Cost of the compressor with this oil separator will increase by 5%. I talked to several compressor manufacturers and everybody claim that their compressors can operate at condensing pressure 15C even with 2 bars suction pressure, but oil separator should have right size.
Maybe in plants running in some allaskan fjords will be running at +20 C
I'm talking about seawater cooled plants running in the waters off norway that can be between 4-15 degrees
Yeah well, with 20C on a freezing system, that is normal operating temperatures... if you have 0 bars of suction pressure, that is all and good, everything should be working well :)
thanks to you :)
You know a few plants that run on 10C condensing, I can tell you, they are not using LRI for oil coolong... if they do, I would like to see first hand...
You know... without being hard headed, I thnk that if I had the chance to install one of our systems on a ship fishing in Alaska, I could have imroved that ships catch of 10% in a season
Re: Desuperheating by liquid injection
I have nothing against VFD pump this is good idea.;)
About alarm stops of the compressors. I had them several times and no problem with oil. When compressor stop check valves on suction and on discharge will close and pressure in the compressor will something between discharge pressure and suction pressure. For the compressor with suction 0 bars and discharge 8 bars, when compressor stop pressure will be 5-6 bars.
Re: Desuperheating by liquid injection
Plants with 10C condensing have thermosyphon oil cooling. For LRI cooling several options are available: electronic expansion valves, liquid pump or injection in port side.