Optimal IP

[Superfridge]

NH3 system, single stage, pumped liquid with open flash economiser (no subcooler) at IP going to screw compressor economiser port.
LP at -18degC/105kpa,
HP at +30degC/1065kpa.

How do i determine what is the optimal IP?

I have seen this equation Pip=sq root(Php x Plp) psia and that would equate to 390kpa and the site manual states 190kpa.

Thanks in advance.

[Segei]

NH3 system, two stage, pumped liquid with flash economiser at IP going to screw compressor economiser port.
LP at -18degC/105kpa,
HP at +30degC/1065kpa.

How do i determine what is the optimal IP?

I have seen this equation Pip=sq root(Php x Plp) psia and that would equate to 390kpa and the site manual states 190kpa.

Thanks in advance.

You are right. Optimum IP will be 390 kpa, but this is gauge pressure. Psia is absolute pressure. Variation of the IP pressure from 300 kpa to 500 kpa will not significantly change efficiency of the plant.

[Superfridge]

Cheers Segei
I can fathom the above equation when the system is compound ie: A low + high stage comps or a twin impellor comp as I have read about the geometric mean and keeping compression/volume ratios the same between the two stages but with one screw comp, I would have thought a lower economiser pressure (190 kpa as per site manual) would mean less flash gas losses in the LP and greater flash gas entering the compressor at IP. Or would the compressor IP port not cope with the extra load? Or am I looking at this the wrong way?

[Segei]

Cheers Segei
I can fathom the above equation when the system is compound ie: A low + high stage comps or a twin impellor comp as I have read about the geometric mean and keeping compression/volume ratios the same between the two stages but with one screw comp, I would have thought a lower economiser pressure (190 kpa as per site manual) would mean less flash gas losses in the LP and greater flash gas entering the compressor at IP. Or would the compressor IP port not cope with the extra load? Or am I looking at this the wrong way?

You can have 190 kpa port pressure if no load. If load increase, this pressure will increase either. This initial pressure will change if suction pressure change.
Typical compressor has inlet pressure regulator and check valve. Close stop valve before pressure regulator and manually open this regulator. Regulator gauge will show port pressure. Try gradually open stop valve. You will see how port pressure change.

[Superfridge]

RANGER1 i have always looked at this system as two stage but your desription is correct I guess, single stage economised. I will ammend.

The economiser is open, no subcooler.
HP liquid from HP reciver enters IP pot, flash gas goes to comp economiser port, IP liquid enters LP separator, LP liquid is pumped to evaps.

I have closed down ecomomiser isolation valve and found comp port pressure to be 200kpa at 100% slide and at design pressures. Comp is on VSD so nearly always at 100% slide. Below 75% economiser BPRV closes/de-energises and economiser flash gas enters suction via BPRV set slightly higher than econ BPRV.

Would it be just a matter of setting IP BPRV to say 50kpa or so above maximin compressor economiser port pressure? As long as IP to LP soleniod and hand expansion valves can cope with the low pressure drop and maybe two phase liquid.(not 100% saturated)
I would hope there is a more scientific means than this!
Or stick with geometric mean?

Cheers

[Segei]

RANGER1 i have always looked at this system as two stage but your desription is correct I guess, single stage economised. I will ammend.

The economiser is open, no subcooler.
HP liquid from HP reciver enters IP pot, flash gas goes to comp economiser port, IP liquid enters LP separator, LP liquid is pumped to evaps.

I have closed down ecomomiser isolation valve and found comp port pressure to be 200kpa at 100% slide and at design pressures. Comp is on VSD so nearly always at 100% slide. Below 75% economiser BPRV closes/de-energises and economiser flash gas enters suction via BPV set slightly higher than econ BPRV.

Would it be just a matter of setting IP BPRV to say 50kpa or so above maximin compressor economiser port pressure? As long as IP to LP soleniod and hand expansion valves can cope with the low pressure drop and maybe two phase liquid.(not 100% saturated)
I would hope there is a more scientific means than this!
Or stick with geometric mean?

Cheers

For me, it is not clear about BPV and BPRV. At slide valve position below 75% , port will be connected to the suction and efficiency will be lost. Unless, other compressors can handle IP load, the gas at IP pressure will go to the suction(again) through BPV.
What are you going to achieve? Do you have solenoids and hand expansion valves for LP and IP receivers?

[RANGER1]

Superfridge ,
You originally stated 2 stage but seems its single stage economized !
The economizer port is at different pressures at different conditions ie pressures & slide position.
This being the case economizer is designed & sized to suit ,trying to utilize as much economizer capacity as possible or to suit system load balance .
The more subcooling from economizer the better efficiency all round .
If its an open economizer it also must maintain a pressure above suction pressure so it can feed into lowside vessel ( if compressor is u/loaded economizer gets closer to suction pressure so BPR is installed ) .
Hope I'm on the right track for you .

[RANGER1]

Does the site manual explain function of valves , or just settings .
Sometimes someone has a bright idea in design but no one can understand it without explanation .
Where was system designed , NZ ?

[Superfridge]

For me, it is not clear about BPV and BPRV. At slide valve position below 75% , port will be connected to the suction and efficiency will be lost. Unless, other compressors can handle IP load, the gas at IP pressure will go to the suction(again) through BPV.
What are you going to achieve? Do you have solenoids and hand expansion valves for LP and IP receivers?

I need to re-read my post before posting
I missed the R out of the BPV. I will ammend that too.
Yes, hand expansion + soleniod valves for both receivers and you are right, at less than 75% the econ flash gas goes to suction.

Does the site manual explain function of valves , or just settings .
Sometimes someone has a bright idea in design but no one can understand it without explanation .
Where was system designed , NZ ?

The site manual states the BPRV between economiser and suction is there to provide a pressure difference at all times ie: when low load and compressor unloads and economiser to compressor BPRV is de-energised. The only settings in the manual is this 190kpa economiser pressure. I think the econ to suction BPRV was put in later as it's not on the drawings.
It's a NZ company installation but the design?

[Segei]

Pressure regulator can be wide open manually. Your minimum IP pressure is 200 kpa. If solenoid to IP vessel open, high pressure liquid will go to IP vessel and a lot of flash gas will be produced. This flash gas is the load to the port. Pressure in the port will increased to (assume) 400 kpa or to the pressure when bypass PR will open. Some plants have BPR to keep certain pressure in IP vessel, because this is requirement for additional refrigeration load. Dock cooling can be connected to IP vessel and pressure in IP should be 300 kpa. Lowering set point of BPR slightly increase capacity of the plant, but it will not improve efficiency. What are you going to achieve by lowering BPR setting?
You mentioned about design pressures. Do you operate plant at 1065 kpa condensing pressure?

[Superfridge]

Thanks for your replies and interest

What are you going to achieve by lowering BPR setting?

Thats the reason for the post, i'm not sure. Less LP flash gas but will the associated valves work with roughly 90kpa deltaP? I will have to check supplier.

As stated above, the manual lists LP=100kpa, IP=190kpa and HP=1065kpa but I think the IP is too low. I have set the IP to comp BPRV to 390kpa and should the comp go below 75%, the IP to comp BPRV de-energises and the IP to LP BPRV will then keep IP to 420kpa (this valve has no soleniod control so that is why it is set higher) and will see how the plant runs.

You mentioned about design pressures. Do you operate plant at 1065 kpa condensing pressure?

Plant keeps to above conditions well.

[Segei]

Thanks for your replies and interest



Thats the reason for the post, i'm not sure. Less LP flash gas but will the associated valves work with roughly 90kpa deltaP? I will have to check supplier.

As stated above, the manual lists LP=100kpa, IP=190kpa and HP=1065kpa but I think the IP is too low. I have set the IP to comp BPRV to 390kpa and should the comp go below 75%, the IP to comp BPRV de-energises and the IP to LP BPRV will then keep IP to 420kpa (this valve has no soleniod control so that is why it is set higher) and will see how the plant runs.




Plant keeps to above conditions well.

Probably, you provide required cooling. However, a lot of energy is wasted.
I looked at current weather in Welligton(I don't know where is your plant located). Temp. 6 degC, humidity 81%. I assume that your plant has evaporative condensers. Current optimum(the best) condensing pressure is in the range of 600 kpa. This is huge energy savings by changing condensing pressure from 1000 kpa to 600 kpa.

[RANGER1]

Superfridge ,
what type of oil cooling do you have thermosyphon , water cooled ?
Do you have VSD fan on condenser or on/off ?