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Thread: Optimal IP

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    Optimal IP



    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.
    Last edited by Superfridge; 08-07-2010 at 10:03 AM.



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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    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.

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    Re: Optimal IP

    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?
    Last edited by Superfridge; 07-07-2010 at 08:25 AM. Reason: More info added

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    Re: Optimal IP

    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 .

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    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.

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    Re: Optimal IP

    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
    Last edited by Superfridge; 09-07-2010 at 11:56 PM.

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    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?

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    Re: Optimal IP

    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 ?

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    Re: Optimal IP

    Quote Originally Posted by Segei View Post
    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.

    Quote Originally Posted by RANGER1 View Post
    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?

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    Re: Optimal IP

    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?

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    Re: Optimal IP

    Thanks for your replies and interest

    Quote Originally Posted by Segei View Post
    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.


    Quote Originally Posted by Segei View Post
    You mentioned about design pressures. Do you operate plant at 1065 kpa condensing pressure?
    Plant keeps to above conditions well.

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    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.

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    Re: Optimal IP

    Quote Originally Posted by Segei View Post
    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.
    Thats an interesting point, every industrial plant I have ever worked on has had a fixed condensing pressure in the range of 1000kpa and yes it wastes energy.

    It is best practice in this neck of the woods to keep condensing pressure up when ambient is down.

    Would the hand expansion valves from HP to IP need to be opened if the condensing pressure was lowered from 1000kpa to 600kpa to compensate the lower deltaP and lower liquid flow? As the IP pressure is fixed, the liquid flow rate to LP would stay the same.

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    Re: Optimal IP

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

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    Thats an interesting point, every industrial plant I have ever worked on has had a fixed condensing pressure in the range of 1000kpa and yes it wastes energy.

    It is best practice in this neck of the woods to keep condensing pressure up when ambient is down.

    Would the hand expansion valves from HP to IP need to be opened if the condensing pressure was lowered from 1000kpa to 600kpa to compensate the lower deltaP and lower liquid flow? As the IP pressure is fixed, the liquid flow rate to LP would stay the same.
    Condensing pressure should "float" based on wet bulb temperature. Hand expansion valve setting is one of many issues that should be solved. Recently, I've posted several newsletters related to floating condensing pressure and energy savings. You can read them.

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    Re: Optimal IP

    Quote Originally Posted by RANGER1 View Post
    Superfridge ,
    what type of oil cooling do you have thermosyphon , water cooled ?
    Do you have VSD fan on condenser or on/off ?
    Thermosyphon and VSD, why do you ask?

    Quote Originally Posted by Segei View Post
    Condensing pressure should "float" based on wet bulb temperature. Hand expansion valve setting is one of many issues that should be solved. Recently, I've posted several newsletters related to floating condensing pressure and energy savings. You can read them.
    Many of the compressor skids I work on do not have an oil pump and require pressure differential for lubrication. If the system ran at 500kpa differential (100kpa LP & 600kpa HP) that would be very close to minimum lube requirements. Have you experienced issues with this situation?

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    Re: Optimal IP

    For Frick compressors minimum pressure differential should be 55 psig(370 kpa). You don't need to go below 470 kpa. Compressors with high suction pressure should have full lube oil pump. This pump will run at low condensing pressure and it will be off at high condensing pressure. Typically, hot gas defrost and liquid injection oil cooling are first barriers to low condensing pressure.

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    Re: Optimal IP

    Superfridge.

    Unfortunately, many people believe that energy in industrial refrigeration should be saved by VFDs(VSDs). This is significant misconception. I found that VFDs can save 20-30% of total energy savings and they have several years payback. Optimization of refrigeration plant operation can save 70-80% and payback a few months. Look at your plant. Compressors have VSDs. They are very expensive. Reduction of condensing pressure from 1000kpa to 800kpa doesn't require capital investment and it will save more energy then VFDs. I saw several plants where people installed VFDs for compressors, condenser fans and evaporator fans, but they were operated at 150 psig of condensing pressure all year around. This is clear example that these people have little information about energy savings in industrial refrigeration.

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    Re: Optimal IP

    With low condensing, do you tend to have the associated low condensing issues? Like liquid hang-up in the condensers? Some plant I deal with suffer from this when at low load. I know it comes down to poor design and poor plant management and these are generally out of the average fridgie's control.

    Is it a bit of "trial and error" when undertaking the optimization process? as some plant fall over when outside of design conditions or unforseen issues arrise.

    How do you get over the lack of heat available for hot gas defrost?

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    With low condensing, do you tend to have the associated low condensing issues? Like liquid hang-up in the condensers? Some plant I deal with suffer from this when at low load. I know it comes down to poor design and poor plant management and these are generally out of the average fridgie's control.

    Is it a bit of "trial and error" when undertaking the optimization process? as some plant fall over when outside of design conditions or unforseen issues arrise.

    How do you get over the lack of heat available for hot gas defrost?
    If you have enough liquid, liquid hang-up won't affect operation. Sometimes during cold weather plant will have very low condensing pressure. This happen when cold wind blow through the condensers. In this case I suggest to install shields to prevent direct wind.
    Unfortunately, we have many misconception in industrial refrigeration. One misconception is that pressure drop in hot gas line is major reason of poor hot gas defrosting at low head pressure. This is wrong. I think that misbalancing of hot gas supply, ammonia condensation and ammonia condensate draining is major reason of poor hot gas defrosting. Read couple newsletters about defrosting.

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    Re: Optimal IP

    Quote Originally Posted by Superfridge View Post
    With low condensing, do you tend to have the associated low condensing issues? Like liquid hang-up in the condensers? Some plant I deal with suffer from this when at low load. I know it comes down to poor design and poor plant management and these are generally out of the average fridgie's control.

    Is it a bit of "trial and error" when undertaking the optimization process? as some plant fall over when outside of design conditions or unforseen issues arrise.

    How do you get over the lack of heat available for hot gas defrost?
    Typically, to optimize operation of the refrigeration plant 3 steps should be done. Assume that every step will save $50,000 in energy costs.
    1. Initial optimization. Usually, done by the operators. Cost of implementation is from $ 0 to $10,000. This is the step of "trial and error".
    2. Final optimization. Usually done by consultant. Cost of implementation is $10,000-20,000.
    3. Investment in energy saving equipment(VSDs, PLCs...). Cost of implementation is $100,000 - 200,000.
    Probably, your company doesn't have information about optimization and they invested in step 3. This is the most expensive investment.
    In North America, many companies are focused on steps 1 and 3, but they miss step 2. Final optimization doesn't require significant investment, but it can save additional 30-40% of total energy savings.

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