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    Question Refrigerant pressure in an idle unit? (compressor not running)



    Hello,

    I am new to this forum so sorry for any mistakes

    I'm now designing my first refrigeration unit and I have some problems, could you please check my questions? (Also, I checked your forum, went through lots of Google browsing + books about refrigeration but no luck for anwsers).

    I am now designing a reversible heat-pump with air and water heat exchangers. It has a Carel E2V EEV and a Danfoss Saginomya 4-way reversing valve. For refrigerant we chose R410a. Evaporating is at 7,5 C (+5K), condensing at 45 C (+2K subcooling). We already completed a prototype but I'm holding back to run it for the first time since I want to clear questions below.

    My questions are: I've learend as a rule of thumb, a way to estimate refrigerant charge. I used a formula: 1/2 of evaporator volume + 1/2 of condenser volume + volume of liquid lines (neglible in my project) + 20% safety factor. The density of R410a in the liquid state is around 940 kg/m3. I calculated that I need to charge with ~540g of refrigerant. Is this method any good?
    Could you share some other methods of calculating refrigerant charge?

    Other question: after charging the unit with 540 grams of R410a, the pressure equalized at around 6 bar (external temperature is about +15 C). I've set in the electronic control the cut off pressure on the suction side to be at around 8 bar (0,0 C for R410a) to prevent frost accumulation or freezing water.
    So the problem is: will the low side (suction) pressure rise to the 9,94 bars (for 7,2 C evaporationg temp) from 6 bars in idle?

    My question is:
    Is it normal for refrigeration units to have lower pressure in whole system when they compressor is not working, compared to the suction pressure when compressor is running?

    Thanks for any input, all and any help will be appreciated !
    Last edited by Chris3000; 25-02-2015 at 01:59 PM.



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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    No it is not normal to have a lower pressure in the whole system. Something does not add up! The standing pressure for the system should correspond to the ambient temperature around the system.

    Is the pressure the same in all the system? Or is the bulk of the refrigerant stuck in the high side due to the EEV being closed?

    Is your pressure measurement accurate?

    If the system is very large, it is possible that 540g is not enough to bring the system pressure up to 11.54bar to correspond with the stated 15C ambient.

    May be worth checking the pressure in your R410A cylinder incase it is low also, maybe wrong refrigerant, 6.5bar would be correct for R407C for example.
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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Quote Originally Posted by Chris3000 View Post
    Hello,

    I am new to this forum so sorry for any mistakes

    I'm now designing my first refrigeration unit and I have some problems, could you please check my questions? (Also, I checked your forum, went through lots of Google browsing + books about refrigeration but no luck for anwsers).

    I am now designing a reversible heat-pump with air and water heat exchangers. It has a Carel E2V EEV and a Danfoss Saginomya 4-way reversing valve. For refrigerant we chose R410a. Evaporating is at 7,5 C (+5K), condensing at 45 C (+2K subcooling). We already completed a prototype but I'm holding back to run it for the first time since I want to clear questions below.

    My questions are: I've learend as a rule of thumb, a way to estimate refrigerant charge. I used a formula: 1/2 of evaporator volume + 1/2 of condenser volume + volume of liquid lines (neglible in my project) + 20% safety factor. The density of R410a in the liquid state is around 940 kg/m3. I calculated that I need to charge with ~540g of refrigerant. Is this method any good?
    Could you share some other methods of calculating refrigerant charge?
    I work mostly in the field and don't get to calculate much, I do however see that the calculations for the required refrigerant charge can be off as much as 20% or more on smaller systems (total charge less than 50 kg of refrigerant mind you).

    For me, if I am told I am going to need 20 kg of refrigerant, I'll bring 40 kg to be sure that I will have enough

    So my methods are more hands on

    and remember "Theory is when you know everything but nothing works practice is when everything works but no one knows why"

    For both methods, use a calibrated refrigerant scale

    Depending on if the unit has a sight glass on the liquid line or not.
    if there is a sight glass on the liquid line I would just continue charging refrigerant until the the sight glass was clear with liquid when the system is operating at design temperatures.
    This takes time though, since you have to wait for the cold side to reach design temperature.

    Another way is to charge by suction pressure, this takes more time, R410a has a very small glide, but personally I would have charged in liquid state.
    This means you need to have someone with a deft hand on the valve and charge into the compressor suction. Going by the charge it's an hermetic compressor, so the suction goes into the crankcase and there won't be any problems with giving it tiny bursts of liquid as long as you take your time and let the system stabilize between each tiny burst.

    Keep charging until the system is stable in the conditions you require it to work in, then record the amount of refrigerant put into the system.

    And again, remember as I said in the beginning "Theory is when you know everything but nothing works practice is when everything works but no one knows why, in refrigeration those two are sometimes combined, and everything works and nobody knows why.
    and in some cases "Nothing works and everybody knows why"


    Other question: after charging the unit with 540 grams of R410a, the pressure equalized at around 6 bar (external temperature is about +15 C). I've set in the electronic control the cut off pressure on the suction side to be at around 8 bar (0,0 C for R410a) to prevent frost accumulation or freezing water.
    So the problem is: will the low side (suction) pressure rise to the 9,94 bars (for 7,2 C evaporationg temp) from 6 bars in idle?
    As Monkey Spanners said, a system containing refrigerant should always correspond to the ambient pressure, unless the refrigerant charge is so small that all the refrigerant has evaporated before the ambient pressure/temperature has been reached.

    Unless refrigerant is trapped somewhere in your system, this seems to confirm my thoughts that your refrigerant charge is too small.

    My question is:
    Is it normal for refrigeration units to have lower pressure in whole system when they compressor is not working, compared to the suction pressure when compressor is running?

    Thanks for any input, all and any help will be appreciated !
    No that is not normal

    However, when the compressor is running you are using force to move the refrigerant around and the pressures/temperatures will be entirely different from standstill.
    I understand partially how your system is designed, and I would like to say that with a too small refrigerant charge I would like to see lower suction pressure than normal, but I don't know if you are using the 4-way valve to bleed hotgas/discharge pressure into the suction line to keep stable running conditions for the compressor.

    My conclusion however is that the refrigerant charge is too small
    If you don't get a pressure corresponding with the ambient temperature after the unit has been at a standstill for example overnight, it means that all the refrigerant has evaporated into gaseous state and that the amount of gas created is not enough to reach ambient pressure/temperature.


    My conclusion, charge more refrigerant

    I'm a field technician, so many people say "what do you know", but these are my observations


    Hope it works out for you, what are the heating/cooling capacities on this unit?
    -Cheers-

    Tycho

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Hi,
    Many thanks fo the input
    I will go ahead with the test today and just see what happens when we run the compressor. I dont have ability to charge more R410a into the system till tomorrow so it will have to do for today.

    Hope it works out for you, what are the heating/cooling capacities on this unit?
    Cooling 1,8kW, heating 2,3kW, compressor power cosumption 0,55kW, COP=3,3. Its purely theoretical , I'm waiting for a heat meter to be able to check the water flow and temperatures and calculater power extracted form or given into the water circuit.

    but I don't know if you are using the 4-way valve to bleed hotgas/discharge pressure into the suction line to keep stable running conditions for the compressor.
    I use 4-way valve to reverse the refrigerant direction flow - one time the air coil will be an evaporator and the water coil will be condenser and when the 4-way valve coil is energized it will be reversed. So you can use the heat pump to cool or heat air stream ventilating the room.

    I'm a field technician, so many people say "what do you know", but these are my observations[quote
    Thanks for your anwsers, nothing can replace years of experience in the field


    maybe wrong refrigerant, 6.5bar would be correct for R407C for example.
    I will double check that.

    Is the pressure the same in all the system? Or is the bulk of the refrigerant stuck in the high side due to the EEV being closed?

    Is your pressure measurement accurate?
    We were able to open the EEV and close it reptetedly (simulating normal operation) - before we did that the pressure was 4,3 bars, after opening the valve it rised up to 6 bars so the was some refrigerant trapped in the high side. But thats not enough for 11,43 bars in ambient temp. Measurement was done by Carel SPKT pressure sensor connected to the controller.

    Thanks for anwsers, I will keep you updated

    Diagram and state points form CoolPack:

    cycle diagram.jpgstate points.jpg
    Last edited by Chris3000; 26-02-2015 at 07:57 AM. Reason: adding diagram

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Do not trust the SPKT sensor until you have confirmed it's reading with an actual pressure gauge.
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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Ok, I had a test run yesterday and got some more surprising results.

    Quote Originally Posted by Brian_UK View Post
    Do not trust the SPKT sensor until you have confirmed it's reading with an actual pressure gauge.
    We connected a pressure gauge to the low side and guess what - the pressure was correct, responding the the ambient temperature (11,5 bars, +15,0 C ambient). So you were right - the SPKT sensor had to be calibrated in the controller to give a proper reading.

    There is a whole new problem however:
    After starting the compressor.... nothing happened - the suction pressure and the discharge pressure was the same , no change (11 bars). Like there was no flow of refrigerant whatsoever. The compressor made a sound normal to a compressor, and vibrated but not really much. The thermal protection didn't kick in (we run it few times for 2-3 minutes each).
    We tried to fully close and open EEV but it didnt help - no response from either pressure gauge or electronic sensor. Still equal pressure on discharge and suction.

    Could compressor be damaged? It came fresh from the factory, took it from the box and brazed into the system.

    Could it be that it was connected to to the opposite voltage phases? Will a scroll compressor "pump" in reverse if the phases are not propelly connected? How will such compressor behave?

    The compressor is 1phase 230VAC Tecumseh HGA.
    Last edited by Chris3000; 27-02-2015 at 12:59 PM.

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    They won't pump if running backwards, check your wiring to rule that out. Hears one i had to swap the phases over to get it to run the correct way, it had been disconnected as it was redundant.

    https://www.youtube.com/watch?v=hLXDb8jA-_4
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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    the tecumsah hga range I think are rotary compressors

    tecumsah hga.jpg
    If the World did not Suck, We would all fall off !

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Chillerman2006 was right, Tecumseh HGA series is a rotary compressor.
    Quick update guys - the compressor was damaged - it didnt provide any pressure ratio. We de-brazed it from the unit, now its being sent do manufacturer to detremine whats wrong with it (it has to be cut open).

    I have a question: is there a possibility that we damaged the compressor by flooding it with liquid refrigerant?
    We charged the system via a Schrader valve on the suction side, and waited for about 48 hours until we run it for the first time. Could liquid phase accumulated in the compressor and damaged it on start-up?

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Could have done. Liquid refrigerant charging always needs doing with care.

    Did you power up the crankcase heater prior to starting the unit?
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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Rotary comps cant tolerate liquid at all as the suction is routed directly into the compression chamber, Most of them i have seen come with an accumulator already attached to the suction.I would image an accumulator would be needed even more so on a heat pump system with changeover from heating to cooling etc.
    Last edited by joe-ice; 05-03-2015 at 11:53 PM.

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Quote Originally Posted by Brian_UK View Post
    Could have done. Liquid refrigerant charging always needs doing with care.
    Did you power up the crankcase heater prior to starting the unit?
    I couldn't, this model does not have a crankcase heater.

    Most of them i have seen come with an accumulator already attached to the suction.I would image an accumulator would be needed even more so on a heat pump system with changeover from heating to cooling etc.
    There is a small accumulator on the suction, maybe it was not enough? We have 540g of R410a, this equals to approx. 0,57 L of volume. I don't know the capacity of our particular accumulator on the Tecumseh HGA but I guess it's smaller than 0,5L... Anyhow - ony part of the refrigerant should be in liquid state in unpowered system, I hope this will be enough.

    Anyway, I'm going to charge it first time with small amount of refrigerant and run the compressor simultaneusly adding more, up to the full calculated charge. I hope this will prevent liquid accumulating in the suction side.

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Good choice.

    Just a quick thought - you mentioned this is a reversible heat pump. Is there only one expansion device in the system or two? If there is only one - does the Carel EEV work in the reverse direction?

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    Re: Refrigerant pressure in an idle unit? (compressor not running)

    Quote Originally Posted by Scramjetman View Post
    Good choice.

    Just a quick thought - you mentioned this is a reversible heat pump. Is there only one expansion device in the system or two? If there is only one - does the Carel EEV work in the reverse direction?
    Yes, the EEV is two-way - Carel E2V series.|
    https://www.youtube.com/watch?v=0bjkahOwn7k

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