Desuperheating by liquid injection

[Tycho]

First. Liquid pump for LPI into discharge side. It can be done and it can be helpful to reduce condensing pressure and a lot of energy can be saved.
Second. Use economize port for the suction of vapor from economizer. This improve energy efficiency as well, because in this case vapor will be compressed from intermediate pressure to the condensing pressure instead of from suction pressure to the condensing pressure.
Third. Liquid injection into economize port is not efficient, because cooling refrigerant will evaporate and additional compressor work should be done to compress this vapor to condensing pressure. Better solutions. Inject refrigerant in LRI port. Every compressor manufacturer located this ports at later stage of compression to minimize work of the compressor that required to compress vapor to the condensing pressure.
Larger oil cooler. Usually, thermosyphon oil cooler have overfeed 3:1 and it will be able to handle load from superheated refrigerant.

First: if you want LRi on the LRI port of the compressor, wich is at the end of the compression, you NEED TO HAVE A PUMP, to supply the refrigerant to the TEAT valve, because the pressure at the end of the compressor compression is higher than the condenser pressure if you have a "normal system with a PM valve between the compressor and the condenser... you can have a discharge pressure of 30C, and a condensing pressure of 20C on a system with a VFD controlled condenser pump...
Second: I don't get this... if I am supplying "evaporated liquid" from the condenser as compared to gas from the economizer tank, what is the difference? there will be extra work on the compressor no matter what... So normally the economizer supplies the economizer port with gas at around -10C... There is extra load on the compressor in either case, how is the load from the LRI introduced in the economizer port different from the gas introduced to the LRI port?
I have seen a howden 255 with an economizer fitted, where the economizer valve opened when the compressor was at 100% and it was injected with -10C gas where the compressor almost took a bow and called it a night... so we had to introduce the economizer at -25 and slowly raise it to -10.
Third: The LRI port only works if you have a pump feeding refrigerant to the TEAT
if you want an economically working LRI, you have to use the economizer port... because the condensing pressure is lower than the pressure at the compressors "LRI stage"

[RANGER1]

First: if you want LRi on the LRI port of the compressor, wich is at the end of the compression, you NEED TO HAVE A PUMP, to supply the refrigerant to the TEAT valve, because the pressure at the end of the compressor compression is higher than the condenser pressure if you have a "normal system with a PM valve between the compressor and the condenser... you can have a discharge pressure of 30C, and a condensing pressure of 20C on a system with a VFD controlled condenser pump...
Second: I don't get this... if I am supplying "evaporated liquid" from the condenser as compared to gas from the economizer tank, what is the difference? there will be extra work on the compressor no matter what... So normally the economizer supplies the economizer port with gas at around -10C... There is extra load on the compressor in either case, how is the load from the LRI introduced in the economizer port different from the gas introduced to the LRI port?
I have seen a howden 255 with an economizer fitted, where the economizer valve opened when the compressor was at 100% and it was injected with -10C gas where the compressor almost took a bow and called it a night... so we had to introduce the economizer at -25 and slowly raise it to -10.
Third: The LRI port only works if you have a pump feeding refrigerant to the TEAT
if you want an economically working LRI, you have to use the economizer port... because the condensing pressure is lower than the pressure at the compressors "LRI stage"

Tyco can you remind me of what LRi & LRI is?

I feel a lot of your statements are a bit of a set up but here goes.

Liquid refrigerant injection into liquid injection port does not need a pump, it goes inbecause pressure is lower.
To the best of my knowledge economizer port is positioned to open to rotor thread almost immeidiately
after male rotor closes of female rotor vane. At this point as rotor continues to rotate it starts to reduce volume in female rotor thread(compress).
At this point economizer port is open to this vane in female rotor, which is slightly higher than suction pressure(asssuming compressor is over80% loaded).So this allows economizer suction to flow into rotor vane.
After it passes that port, then liquid injection port is open to metered liquid refrigerant to control discharge/oil temp.
So not sure why you need a pump.
If wrong selection of Vi is selected maybe, but over compression is possibly the last thing you want(draws more power consumption.
You also mention "normal system" having PM valve in discharge line raising pressure upstream of valve.
Why?
To me its another waste of power. Everything points to having as lower condensing pressure as possible.

If you inject gas from economizer you are increasing nett refigerant effect byu subcooling liquid refrigerant feeding evaporator. Your compressor basically increase in capacity, using a bit more power though.

If you inject liquid from condenser you get cooler discharge temp but thats all, not increased refrigeration effect.
I however agree that injecting liquid for oil cooling & economizer will still work but possibly a little shy of using correct ports.
Only Howden could tell us that!

[Tycho]

Tyco can you remind me of what LRi & LRI is?

I feel a lot of your statements are a bit of a set up but here goes.

Liquid refrigerant injection into liquid injection port does not need a pump, it goes inbecause pressure is lower.
To the best of my knowledge economizer port is positioned to open to rotor thread almost immeidiately
after male rotor closes of female rotor vane. At this point as rotor continues to rotate it starts to reduce volume in female rotor thread(compress).
At this point economizer port is open to this vane in female rotor, which is slightly higher than suction pressure(asssuming compressor is over80% loaded).So this allows economizer suction to flow into rotor vane.
After it passes that port, then liquid injection port is open to metered liquid refrigerant to control discharge/oil temp.
So not sure why you need a pump.
If wrong selection of Vi is selected maybe, but over compression is possibly the last thing you want(draws more power consumption.
You also mention "normal system" having PM valve in discharge line raising pressure upstream of valve.
Why?
To me its another waste of power. Everything points to having as lower condensing pressure as possible.

If you inject gas from economizer you are increasing nett refigerant effect byu subcooling liquid refrigerant feeding evaporator. Your compressor basically increase in capacity, using a bit more power though.

If you inject liquid from condenser you get cooler discharge temp but thats all, not increased refrigeration effect.
I however agree that injecting liquid for oil cooling & economizer will still work but possibly a little shy of using correct ports.
Only Howden could tell us that!

Finally, there it is

With LRI from the condenser, you take liquid directly from the condenser without any sub cooling effect.
You add load to the compressor and condenser without any gain other than cooling the discharge and oil in the compressor system

The PM valve is between the compressor and condenser to keep a stable discharge pressure of 30C and prevent oil carryover, so with 30C in the discharge you may end up with 10-20C in the condenser, depending on seawater temp.

When I talk about needing a pump to feed it into the LRI port, I see that I have forgotten to mention that it is a cooling system, not a freezing system. suction of -8C.
I am speaking from experience, that when the system starts with a suction pressure of around 5-6C you will need a condenser pressure of minimum 17-19C before it starts to feed into the economizer port.

on the Howden XRV's we use, I have never been able to feed the LRI into the LRI port, the LRI port is all the way at the end of the compression, right before the rotors opens to the discharge.

Was fun playing a little devils advocate here

[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.

[Tycho]

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)