Economizer effects on system pressures

[RANGER1]

Have a large chilled water plant with economizer ,6MW

When economizer is turned on, compressor suction pressure increases by 20 kpa?

Anybody with any theories as to why suction pressure actually increases when economizer is turned on.

I suspect rotors not sealing very well possibly due to wear, as well as liquid injection compounding problem

Compressor is 100% loaded
NH3
Suction 400kpa 4deg C
Discharge 1250 kpa 35degC
Economizer port pressure 800kpa
Liquid injection oil cooling
Liquid line subcooling 33to23deg c = 10deg C liquid line subcooling

[NoNickName]

Because the heat rejection increases as a result of the economised operation, and therefore the suction pressure increases as a result of increased net mass flow.

[josef]

http://www.irc.wisc.edu/ASHRAE-TC101/programs/Annual%202001%20Pillis.pdf

[RANGER1]

Because the heat rejection increases as a result of the economised operation, and therefore the suction pressure increases as a result of increased net mass flow.

NoNickName thanks for reply but can you explain in more detail.

To me evaporator load reduces due to less flash gas, so suction pressure may reduce a little.

Or are you saying that the now subcooled liquid can absorb more heat so suction pressure increases?

Compressor mass flow rate definately increases, as extra vapour through economizer port& in this case because suction pressure is higher.

Liquid injection for oil cooling is slightly reduced due to cooling effect of economizer suction gas.

[RANGER1]

Thanks josef its a good basic article which explains effects of liquid injection well.

[NoNickName]

To me evaporator load reduces due to less flash gas, so suction pressure may reduce a little.


More the opposite. Less flash gas means better feeding, which in turn results in higher pressure and lower suction superheat.

[RANGER1]

NoNickName,
Agree for TX Valve but this is a flooded vessel with PHE, same as economizer.
Vessel makeup is controlled by probe & modulating valve, so we are not directly controlling superheat.

[mad fridgie]

Would it be fair to say you control your economiser on load, therefore greater load higher suction pressure.

[NoNickName]

How do you see flash gas in a flooded evaporator, then?

[RANGER1]

Would it be fair to say you control your economiser on load, therefore greater load higher suction pressure.

Economizer is only usefull on this application when machine is at full load only.

Its installed for getting a small boost in capacity due to plant modifications.

If suction pressure is constant with economizer turned off say 400kpa,
If economizer is turned on its effect is 10 C subcooling of liquid line making up level in surge drum/PHE
Suction pressure increases by 20 kpa when eco is turned on & continues to run at this slightly elevated suction pressure.

[RANGER1]

How do you see flash gas in a flooded evaporator, then?

To me the flash gas in this system is high pressure liquid makeup to surge drum/PHE.

The surge drum itself would have the flash gas in it which immediately goes to compressor
The PHE always stays 100% flooded.

It obviously is metered into surge drum via level control , with some liquid flashing off.

When economizer is on we subcool high pressure liquid by 10 deg C so there would be less flash gas
on entry to surge drum/PHE when making up

[NoNickName]

So, when economised the compressor is fed from three sources: the suction, the surge drum and the economizer line. Is that correct? I'm not sure why a surge drum is installed here, since the evaporator is flooded ....

[RANGER1]

Its all terminology
The surge drum is mounted above the PHE.
liquid level is maintained in surge drum so PHE is always full
& liquid is circulated naturally by thermosyphon (pretty standard practice).
On other side of PHE is chilled water.

Compressor draws gas off surge drum
Liquid injection for oil cooling & economizer are connected in common point, which is economizer port on screw compressor
Economizer is also a PHE with a surge drum mounted on top exactly same as evaporator.
Main high pressure liquid line that feeds evaporator to maintain liquid level first goes through economizer to get subcooled .
before

[mad fridgie]

When your economiser is on, do you get the correct amount of increased cooling duty.
Is the load flow through the evap PHE constant.
Could it be that you are getting some form of bubble entrapment in the surge drum(evap) when the economiser is off, which is slightly reducing the flow to the the evap PHE, reducing slightly the load profile, when you engage the economiser, the bubble entrapment is reduced better performing PHE higher suction pressure.
Your liquid/vapour injection into the surge drum, is this above the level of the liquid level (sprayed in) or is it piped into the liquid itself (the vapour bubbles through the liquid) causing some form of movement of the liquid itself, reducing flow to the evap.

[Segei]

What do you mean about eco turn on? Is portside solenoid open or you open manual valve?

[RANGER1]

When your economiser is on, do you get the correct amount of increased cooling duty.yes as per design
Is the load flow through the evap PHE constant.yes
Could it be that you are getting some form of bubble entrapment in the surge drum(evap) when the economiser is off, which is slightly reducing the flow to the the evap PHE, reducing slightly the load profile, when you engage the economiser, the bubble entrapment is reduced better performing PHE higher suction pressure.No real trouble at all as performs in both with & without economizer as specified
Your liquid/vapour injection into the surge drum, is this above the level of the liquid level (sprayed in) or is it piped into the liquid itself (the vapour bubbles through the liquid) causing some form of movement of the liquid itself, reducing flow to the evap.Pretty sure it enters above liquid level in surge drum.

Everything is performing but when economizer is on I would have thought suction pressure should
actually decrease slightly instead of increase slightly. Thats why I think it must be gas & liquid injection internal leakage back to suction of compressor.
Compressor is about 10yrs old

[RANGER1]

What do you mean about eco turn on? Is portside solenoid open or you open manual valve?
Economizer is turned on when above 80% load by solonoid in economizer suction line

[Josip]

Hi, RANGER1 :)


Pretty sure it enters above liquid level in surge drum.

Everything is performing but when economizer is on I would have thought suction pressure should
actually decrease slightly instead of increase slightly. Thats why I think it must be gas & liquid injection internal leakage back to suction of compressor.
Compressor is about 10yrs old

I've been following this thread from beginning ... :confused: a little, like you ;)

.. as I know those two systems are working completely separated - eco pressure is self determining and there is no mixing of pressure or flow ... similar to have two stage plant efficiency within one stage compressor ...

maybe you are right there can be some leakage ... compressor brand? ... is this ammonia system with closed eco circuit or open flash eco system...

do you have some pressure equalizing pipe or similar ... maybe oil return from eco vessel to suction port working via small solenoid in parallel with eco vessel, where you get some liquid which evaporate at suction pressure thus causing rise of suction pressure for 0,2 bar/20kPa

having a scheme of your plant will be easier to discuss ....

.... just thinking out loud;)

Best regards, Josip :)

[Segei]

If your suction pressure increase by 20 kPa, compressor should load from 80% and up. First I thought that you compare suction pressure at 100%(with eco and without eco). What is the capacity of the compressor when pressure increase by 20 kPa?

[Sandro Baptista]

Just a questions:

1) The economizer should be closed, because you talked about subcooled liquid, right? (The open ECO doesn't give subcooled liquid).


2) "Economizer is turned on when above 80% load by solonoid in economizer suction line" » If it is a closed ECO what's the problem if is always open the solenoid suction valve? At 80% certainly that the ECO suction is at a pressure higher that the main suction...What is the disadvantage of keeping the ECO suction valve always open, even if it could be at the main suction pressure?

[Magoo]

Hi Ranger1.
very interesting problem.
Is economizer open or closed flash, possible bypass vent to suction if open flash.

[mad fridgie]

If you have a problem with some form of leakage raising the suction pressure, would you not see a reduction in cooling capacity (from design), TD across the flooded evap will reduce, What is your water flow and what split between flow and return. You should be able to see from changes in the flow temps if this is a technical issue or a load profile issue. What loads the compressor, suction pressure? Water inlet temp? Water outlet temp?

[Josip]

Just a questions:

1) The economizer should be closed, because you talked about subcooled liquid, right? (The open ECO doesn't give subcooled liquid).


Why we cannot obtain a sub-cooled liquid with open ECO?:confused:



2) "Economizer is turned on when above 80% load by solonoid in economizer suction line" » If it is a closed ECO what's the problem if is always open the solenoid suction valve? At 80% certainly that the ECO suction is at a pressure higher that the main suction...What is the disadvantage of keeping the ECO suction valve always open, even if it could be at the main suction pressure?

.... then that is not ECO system ... we gain nothing .... main suction capacity is reduced by volume of gas coming from ECO vessel


Best regards, Josip :)

[Sandro Baptista]

Why we cannot obtain a sub-cooled liquid with open ECO?:confused:


.... then that is not ECO system ... we gain nothing .... main suction capacity is reduced by volume of gas coming from ECO vessel


Best regards, Josip :)


Josip, outlet liquid (feed the liquid separator) from ECO is saturated (at the intermediate pressure)

Josip, while you have the ECO pressure above the LP you will always gain. Only on the limit you will not gain nothing but also nothing to lose.

[Sandro Baptista]

Why we cannot obtain a sub-cooled liquid with open ECO?:confused:




.... then that is not ECO system ... we gain nothing .... main suction capacity is reduced by volume of gas coming from ECO vessel


Best regards, Josip :)

Josip, the liquid that feed the liquid separator is saturated temperature from the open economizer (this one is at intermediate pressure)

Josip, while you have the ECO pressure above the LP you will always gain. Only on the limit you will not gain nothing but also nothing to lose.

[RANGER1]

Hi, RANGER1 :)



I've been following this thread from beginning ... :confused: a little, like you ;)

.. as I know those two systems are working completely separated - eco pressure is self determining and there is no mixing of pressure or flow ... similar to have two stage plant efficiency within one stage compressor ...

maybe you are right there can be some leakage ... compressor brand? ... is this ammonia system with closed eco circuit or open flash eco system...
Its a WRV Howden & closed circuit H/E ie PHE with HP liquid from reciever one side & flooded PHE with surge drum mounted above.
do you have some pressure equalizing pipe or similar ... maybe oil return from eco vessel to suction port working via small solenoid in parallel with eco vessel, where you get some liquid which evaporate at suction pressure thus causing rise of suction pressure for 0,2 bar/20kPa will check it out as site is remote

having a scheme of your plant will be easier to discuss ....

.... just thinking out loud;)

Best regards, Josip :)
Thanks Ranger

[RANGER1]

If your suction pressure increase by 20 kPa, compressor should load from 80% and up. First I thought that you compare suction pressure at 100%(with eco and without eco). What is the capacity of the compressor when pressure increase by 20 kPa?

Sorry I have given incorrect information.
Economizer is on all the time with a pressure regulator in suction line set at I think ( will check) 620 kpa.

If economizer is switched off for a comparison test & compressor is at 100% load, all pressures + temps stable suction pressureis 400kpa.
If economizer is turned on every thing stable etc suction pressure will increase by 20kpa , not the reverse
as anticipated.

[Sandro Baptista]

Sorry I have given incorrect information.
Economizer is on all the time with a pressure regulator in suction line set at I think ( will check) 620 kpa.

If economizer is switched off for a comparison test & compressor is at 100% load, all pressures + temps stable suction pressureis 400kpa.
If economizer is turned on every thing stable etc suction pressure will increase by 20kpa , not the reverse
as anticipated.


Ranger, since the economizer is closed type why do you need the pressure regulator on the ECO port?

[Josip]

Hi, Sandro :)

I'm lost again, sorry, maybe my English is not that good -that is reason to use a plain English ... also I feel there is a little problem with terminology and maybe personal opinion .... let's try to clear that ;)


Josip, the liquid that feed the liquid separator is saturated temperature from the open economizer (this one is at intermediate pressure)

Josip, while you have the ECO pressure above the LP you will always gain. Only on the limit you will not gain nothing but also nothing to lose.


I think for further discussion we need to agree about working condition i.e. compressor capacity for example is bellow 65% thus ECO port is at suction pressure (example 1)... or for example at compressor capacity above 65% til 100% (example 2) thus ECO port is at intermediate self determining pressure in case without any regulating valve on that line (pipe connected to ECO port at compressor housing) or some other higher then suction pressure obtain via pressure regulating valve installed on suction pipe from ECO vessel to ECO port at compressor housing ....


example 1 ... all the liquid within ECO vessel is at suction temperature i.e. sub cooled in comparison with liquid temperature coming from condenser (only this liquid we assume to be sub cooled, at other points within refrigeration cycle we are speaking about saturated liquid), valid both for open and closed ECO system. Practical problem with open system is, we do not have pressure difference to fed main separator/PHE. In closed system we have enough pressure (condensing pressure), but anyhow in this example we do not have ECO system in function .... in my own opinion it is better to have installed solenoid/servo stop valve if we are not using ECO vessel, because any volume of gas entering from ECO vessel reduce the volume of gas coming from main separator thus reduce main suction capacity for me this means lost ... (especially when compressor is running for days at capacity below 65% due to capacity request)....

... this can be discussed also in some other relation, because, we can use ECO vessel for some other cooling process and if that process cooling is not necessary or can be dangerous if we are cooling water or some other sensible medium .... this can be disadvantage on system with suction valve always open i.e. without pressure regulator ....


example 2 ... now within ECO vessel we have some intermediate pressure and belonging temperature (saturated) both in open and closed ECO system. Even now we can have practical problem with small pressure difference with open system working with ammonia (with R22 open flash is common - because open flash ECO vessel is cheaper in both cases ;) what can lead into problems tomorrow) and for that reason it is better to use closed system (PHE, Shell&Tube) especially with ammonia ...

Moreover, I cannot speak about ECO system whenever compressor is running below some capacity 60%-70%-80% ("suction cut-off" depends on physical construction and pressure ratio of compressor) and then begin intermediate pressure i.e. opening of ECO port), because there we gain nothing what is in conjunction with term ECO .... increase of specific capacity i.e. increase capacity as a function of input power vs conventional application of compressor ....

I do believe this helps to put some more light in this discussion on my question/s and my understanding ....

Of course this is still an open discussion trying to help (and learn something too) to one of our valuable members.

Best regards, Josip :)

[Josip]

Hi, RANGER_1 :)


Its all terminology
The surge drum is mounted above the PHE.
liquid level is maintained in surge drum so PHE is always full
& liquid is circulated naturally by thermosyphon (pretty standard practice).
On other side of PHE is chilled water.

Compressor draws gas off surge drum
Liquid injection for oil cooling & economizer are connected in common point, which is economizer port on screw compressor
Economizer is also a PHE with a surge drum mounted on top exactly same as evaporator.
Main high pressure liquid line that feeds evaporator to maintain liquid level first goes through economizer to get subcooled .
before

Principle your system is similar to attached photo ...

6385

Recently on site in Qatar I was speaking with Howden service engineer (he came to help regarding proximity probe) and he told me something about reduced capacity when using ECO port for direct liquid injection for oil cooling, but I do not remember side effect/s, sorry .... when inject liquid for oil cooling directly into discharge pipe it is without side effect/s ....


All our units are almost the same as your, mostly ECO vessel is S&T with pressure regulating valve at ECO suction port ... sometimes ECO is PHE with TXV

Best regards, Josip :)

[Sandro Baptista]

Hi, Sandro :)

I'm lost again, sorry, maybe my English is not that good -that is reason to use a plain English ... also I feel there is a little problem with terminology and maybe personal opinion .... let's try to clear that ;)

Your English is better than mine...so don't be modest ;)

I think for further discussion we need to agree about working condition i.e. compressor capacity for example is bellow 65% thus ECO port is at suction pressure (example 1)... or for example at compressor capacity above 65% til 100% (example 2) thus ECO port is at intermediate self determining pressure in case without any regulating valve on that line (pipe connected to ECO port at compressor housing) or some other higher then suction pressure obtain via pressure regulating valve installed on suction pipe from ECO vessel to ECO port at compressor housing ....


example 1 ... all the liquid within ECO vessel is at suction temperature i.e. sub cooled in comparison with liquid temperature coming from condenser (only this liquid we assume to be sub cooled, at other points within refrigeration cycle we are speaking about saturated liquid), valid both for open and closed ECO system. Practical problem with open system is, we do not have pressure difference to fed main separator/PHE (if the compressor is running near 100%, even at -40ºC NH3, and if the liquid separator is near the ECO we still have sufficient pressure difference...must pay attention to the pressure loss head of the valves and others)
. In closed system we have enough pressure (condensing pressure), but anyhow in this example we do not have ECO system in function (but also you are not "spending" refrigerant effect. If you are using the closed ECO without pressure regulator the liquid temperature will be just a little higher temperature than the liquid separator, 10K app, depending on the design of the closed ECO).... in my own opinion it is better to have installed solenoid/servo stop valve if we are not using ECO vessel (I agree and if the compressor stops the ECO port should be closed), because any volume of gas entering from ECO vessel reduce the volume of gas coming from main separator thus reduce main suction capacity for me this means lost ... (especially when compressor is running for days at capacity below 65% due to capacity request)....

... this can be discussed also in some other relation, because, we can use ECO vessel for some other cooling process and if that process cooling is not necessary or can be dangerous if we are cooling water or some other sensible medium ....(I agree) this can be disadvantage on system with suction valve always open i.e. without pressure regulator ....


example 2 ... now within ECO vessel we have some intermediate pressure and belonging temperature (saturated) both in open and closed ECO system. Even now we can have practical problem with small pressure difference with open system working with ammonia (with R22 open flash is common - because open flash ECO vessel is cheaper in both cases ;) what can lead into problems tomorrow) and for that reason it is better to use closed system (PHE, Shell&Tube) especially with ammonia ...Open economizer allows better COP (though be just a little higher) because you can get lower liquid temperature feeding the liquid separator.

Moreover, I cannot speak about ECO system whenever compressor is running below some capacity 60%-70%-80% ("suction cut-off" depends on physical construction and pressure ratio of compressor) and then begin intermediate pressure i.e. opening of ECO port), because there we gain nothing what is in conjunction with term ECO (but nothing to loose also).... increase of specific capacity i.e. increase capacity as a function of input power vs conventional application of compressor ....

I do believe this helps to put some more light in this discussion on my question/s and my understanding ....

Of course this is still an open discussion trying to help (and learn something too) to one of our valuable members.

Best regards, Josip :)

As you said, personal opinion may you agree with me or not ;)


Regards

Sandro

[Josip]

Open economizer allows better COP (though be just a little higher) because you can get lower liquid temperature feeding the liquid separator.
As you said, personal opinion may you agree with me or not ;)


Regards

Sandro

Definitely agree with you ;) open flash gives better capacity in comparison with closed system, ...... but, when using ammonia I'll go for closed system (personal decision) ... then, maybe, open flash is OK at load of 100%, but what will happen at part load .... ECO pressure is going down to suction pressure and we are loosing refrigerant flow into evaporator what is one of the reasons to use closed system ... otherwise we do not have any COP ;)

above are facts .... not personal opinion :cool:

Best regards, Josip :)

[RANGER1]

Thanks for everyones input but It all does'nt add up.
We need to take new readings & observe system results more closely.

The trouble was it was a long & difficult job (installation of new economizer on existing system) for various reasons & during commissioning it gave us the theoretical result we were after so we probably were happy
to get out of there.

Basically suction pressure increased when economizer was on & gave us the extra cooloing on chilled water
0.5 deg C lower than without econo
water 600 l/sec
10 deg C subcooling

Can't be right even if compressor is suspect.

Will let you know


Regards Ranger

After returning & looking at commissioning sheets things didn't add up.

[mad fridgie]

All is clear now.
You are starving the evap with economiser isolated. (no vapour flow from the intermidiate open flash vessel)
Your load figures indicate an icreased capacity of 1250kw (give or take) thats massive at your compression ratios (i do not have data on your compressor)
When the econ is isolated is there still a small pressure differnce between condensing pressure and intermidiate open flash vessel pressure, (and i suspect that there is, due to port sizing of the float) this being the case both liquid and vapour must be leaving the open flash vessel, this derates the port on the main surge drum, starving the evap and lowering the suction pressure.

[Josip]

Hi, mad fridgie :)


All is clear now.
You are starving the evap with economiser isolated. (no vapour flow from the intermidiate open flash vessel)
Your load figures indicate an icreased capacity of 1250kw (give or take) thats massive at your compression ratios (i do not have data on your compressor)
When the econ is isolated is there still a small pressure differnce between condensing pressure and intermidiate open flash vessel pressure, (and i suspect that there is, due to port sizing of the float) this being the case both liquid and vapour must be leaving the open flash vessel, this derates the port on the main surge drum, starving the evap and lowering the suction pressure.

good logical assumption, but in this case there is closed ECO system .... liquid is coming directly from condenser under condensing pressure ... but, I can be wrong too..

Best regards, Josip :)

[mad fridgie]

Hi, mad fridgie :)



good logical assumption, but in this case there is closed ECO system .... liquid is coming directly from condenser under condensing pressure ... but, I can be wrong too..

Best regards, Josip :)
You could be right, that being the case , i still believe the evap is being starved, if the econ is off, the high pressure liquid is not being sub cooled, but the high pressure liquid is still passing through the heat exchanger, there will be some form of pressure drop through the heat exchanger, this pressure drop could be causing flashing of the liquid prior to entering the evap float, derating the float, hence straving the evap. if the system does not have a high side reciever, you may also be getting some vapour entrapment in the liquid stream, which would normally be removed by the sub cooling, when the econ is off, theis vapour would further increase pressure drop across the heat exchanger, increasing the flash problem further, or could it be some thing as simple as the orifice in the evap float is so critically sized, that it simply can not allow for full mass flow at the required pressure drop with out the liquid being sub cooled, bigger pressure drop with no sub cooling to reach equalibrium, starving the evap.

[Sandro Baptista]

Definitely agree with you ;) open flash gives better capacity in comparison with closed system, ...... but, when using ammonia I'll go for closed system (personal decision) ... then, maybe, open flash is OK at load of 100%, but what will happen at part load .... ECO pressure is going down to suction pressure and we are loosing refrigerant flow into evaporator what is one of the reasons to use closed system ... otherwise we do not have any COP ;)

above are facts .... not personal opinion :cool:

Best regards, Josip :)

Because the closed ECO can have an approach temperature of about 5...10 K (depending on the design) you will compensate that effect and in a part load situation the open and closed ECOs will gave about the same COP.

Nevertheless as far I understand from you I felt the sensation that you also put a pressure regulator on the closed ECO or even you closed (though I don't agree to be necessary)...so at part load you will also penalize the COP of the plant.

[Sandro Baptista]

You could be right, that being the case , i still believe the evap is being starved, if the econ is off, the high pressure liquid is not being sub cooled, but the high pressure liquid is still passing through the heat exchanger, there will be some form of pressure drop through the heat exchanger, this pressure drop could be causing flashing of the liquid prior to entering the evap float, derating the float, hence straving the evap. if the system does not have a high side reciever, you may also be getting some vapour entrapment in the liquid stream, which would normally be removed by the sub cooling, when the econ is off, theis vapour would further increase pressure drop across the heat exchanger, increasing the flash problem further, or could it be some thing as simple as the orifice in the evap float is so critically sized, that it simply can not allow for full mass flow at the required pressure drop with out the liquid being sub cooled, bigger pressure drop with no sub cooling to reach equalibrium, starving the evap.

Mad fridgie, I also agree with you.

Ranger, when the ECO is off what happens with the liquid level of the surge drum? Does the liquid level keeps very low or it keeps at same as when you have the ECO on?

[Sandro Baptista]

...on the closed ECO or even you closed (though I don't agree to be necessary)...so at part load you will also penalize the COP of the plant.

Josip, I meant: "...on the closed ECO or even at part load you close the ECO (though I don't agree to be necessary)...so at part load you will also penalize the COP of the plant."

[RANGER1]

All is clear now.
You are starving the evap with economiser isolated. (no vapour flow from the intermidiate open flash vessel)
Your load figures indicate an icreased capacity of 1250kw (give or take) thats massive at your compression ratios (i do not have data on your compressor)
When the econ is isolated is there still a small pressure differnce between condensing pressure and intermidiate open flash vessel pressure, (and i suspect that there is, due to port sizing of the float) this being the case both liquid and vapour must be leaving the open flash vessel, this derates the port on the main surge drum, starving the evap and lowering the suction pressure.

Plant has 2 off 321 Howdens so a little means a lot in this case.
Economizer was a new add on for more capacity due to alterations of cooling water pipe run length.

Because plant is in isolated region I can't give any readings etc at the moment.

The main high pressure liquid feed was re-routed through PHE which on the other side is flooded
with surge drum . Vapour fron this is piped to economzer port with BPR set at 650kpa

If main surge drum on evaporator is starving I think we would know very quickly there is a problem.
Plant has operated for 10yrs without economizer with no problems at all.
If PHE loses its prime I think plant would fall over.
The liquid reciever has good level & will stop feeding if it reaches low level switch.

[RANGER1]

Ranger, since the economizer is closed type why do you need the pressure regulator on the ECO port?


Good question I will ask design engineer.

[mad fridgie]

Hi Ranger, check other thread (I had originally thought was open flash, teach me to read all the detail?), I think you mis understand me about starving the evap, i am comparing between when eco on or off. You have introduced a PHE into the high pressure liquid, "pressure drop" your increase in capacity is greater than what i would expect from economiser only. If you increase your evap temp in the main PHE you effectively reduce capacity, if the PHE is being underfed (not all plates are fully wetted) then evap pressure will be lower, by ensuring all plates are wetted greater heat transfer surface area, the td can drop for a fixed amount of cooling duty. Your issues relate to heat transfer of the main PHE, if was a fault you would not get the significant increase in net cooling duty.
Equalibrium will be reached in both cases

[Josip]

Hi, Sandro :)


Because the closed ECO can have an approach temperature of about 5...10 K (depending on the design) you will compensate that effect and in a part load situation the open and closed ECOs will gave about the same COP.

Nevertheless as far I understand from you I felt the sensation that you also put a pressure regulator on the closed ECO or even you closed (though I don't agree to be necessary)...so at part load you will also penalize the COP of the plant.

In "normal" refrigeration plants, with ammonia where we use (always closed, usually shell&coil sometimes PHE) ECO to sub cool the liquid we do not install pressure regulating valve on pipe to ECO port so we gain what we gain ... at full load or at part load .... what is a must is a non return valve on suction pipe and of course all needed manual stop valves ....

In this thread (I assume) we are discussing some petrochemical plant and installation of full control of ECO vessel (level and pressure control devices) is something "normal" (even if all of us are thinking that is not necessary ..... they use to have SCADA system and that's all folks ;) everything must be under survey ... very simple philosophy :cool:

.... moreover opening and closing of ECO vessel is controlled via position transmitter, so it is on above 65% to 100% and off below 65% .... one thing I must say, all plants usually are designed for normal run close to 100% of compressor capacity

Best regards, Josip :)

[RANGER1]

Hi Ranger, check other thread (I had originally thought was open flash, teach me to read all the detail?), I think you mis understand me about starving the evap, i am comparing between when eco on or off. You have introduced a PHE into the high pressure liquid, "pressure drop" your increase in capacity is greater than what i would expect from economiser only. If you increase your evap temp in the main PHE you effectively reduce capacity, if the PHE is being underfed (not all plates are fully wetted) then evap pressure will be lower, by ensuring all plates are wetted greater heat transfer surface area, the td can drop for a fixed amount of cooling duty. Your issues relate to heat transfer of the main PHE, if was a fault you would not get the significant increase in net cooling duty.
Equalibrium will be reached in both cases

I get where you are coming from & anything is possible.

Without being able to look at it on scada screen can't be 100% sure but
believe level in main PHE is satisfied (modulating valve as main trim valve with 3 other solonoids all in parallel with manual expansion valves)If modulating valve is 100% open then another solonoid opens up
This line is from memory 150mm.
I will check theoretical capacity of economizer.

[mad fridgie]

If we go back one step, prior to install, hp liquid from reciever to surge drum, all worked well. "But need for more capacity"
Install enconomiser. Under normal operations would it be fair to say at 100% your econ. would always be in operation
"maximum liquid mass flow" sub cooling the liquid?
So when designing the econ PHE, we can assume normal operation. (looking at increasing profits) one of the determining issues is pressure drop through the heat exchanger (can we reduce the number of plates) If for example only your designer decided that 1bar pressure drop was OK on the main liquid flow side, would this be an issue if the liquid is being sub cooled, "no" the sub cooling would compensate for pressure reduction, but if the econ. is off at full load (max mass flow) then 1bar pressure drop would be an issue, flashing would occur. A small amount of vapour in your surge drum feed would derate the mass flow potential of your float/expansion devices.
When in normal operation, you unload the comp and the econ. becomes redundent, the liquid mass flow reduces along with the PHE pressure drop and is not an issue.
As you have stated you do not a problem in normal operation, the issue only arises when you take the plant out of normal operation (closing the econ.)
If you had a pressure drop free by-pass around the econ. PHE, (back effectively to original set up) you could prove the point.
Only you have seen the plant, so in our heads we are picturing what we think the system looks like.
The above figures are only examples, you need to check with your designer how he chose the PHE selection.

[mad fridgie]

One other point (again not know the plant lay out) I presume that you are a wetted condensor?, so depending upon location the liquid stream could be well below the dry bulb temp, the liquid could be pick up some sensible heat causing flashing (you do have more liquid pipe to pick up energy), which of course would be over come by the sub cooling

[Lodiev]

I had the same problem with two Compressors on a normal pumped liquid -40'c +32'c system.
After much speculation we found that the compressors rotors lip seals haze worn out at only one end, the slide valve haze worn the casing out and lifted into the rotors and grinned of the oil lip seals on the rotors.
We would not have noticed this has it not been for previous records of the motor amps at same percentage under the same conditions.
Ask the question has it always been like this or did the problem escalate over years?