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Thread: Help theary

  1. #1
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    Help theary



    Hi,

    I am new here and Hello to everyone. I have a question. If we have simple machine (compressor, evaporator, air cooled condenser and thermal expansion valve) what happens when load decrease? I now that TEV will close a little, evaporation pressure will decrease and condensing pressure will also decrease. But if when condensing pressure drops, fan of condenser will stop working and condensing temperature will rise, also rising evaporating temperature. So we are back at start. I dont understand what is the problem here, but I now that somewhere I am wrong.

    Thanks



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    Re: Help theary

    Who says there is a problem?
    Because the load has reduced so has the amount of heat needed to be rejected. It's the natural point of equilibrium.
    When the condenser fans slow down or stop, then the condensers internal temperature increases and the SCT has to rise to reject the heat.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Marcs got the "magic"!

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    Re: Help theary

    It took a while though!! Where's my rep points
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Quote Originally Posted by marc5180 View Post
    It took a while though!! Where's my rep points
    For the rep points,

    The only law I learnt at college.

    Balls Law.

    The angle of the dangle is directly proportional to the the thrust
    "???????????"

    complete

    "and they shall be yours"

  6. #6
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    Re: Help theary

    Of the bust....


    Presssure=thrust/area =>P x A =T =>T (DP)A As Thrust.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Thanks. The problem was that I though that TEV is used to reduce the capacity. But this is not true, so with constant condensation pressure machine has more or less the some cooling capacity.

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    Re: Help theary

    The thermostatic expansion valve allows the flow of the refrigerant as per the cooling load inside it. At higher load the flow of the refrigerant is increased and at the lower loads the flow is reduced.
    So with a constant condensing pressure the cooling capacity will increase if the evaporation temperature (SST) increases and the cooling capacity will decrease if the evaporation temperature decreases.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Yes, but if you only have this way of capacity reduction (only by TEV, and condensation pressure is kept constant) machine practically works with constant capacity. This is because when SST drops also will drop condensation pressure and fans will stop working increasing the condensation pressure that will result in SST increase. So cooling capacity is first decreased and then increased, so practically it is constant.

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    Re: Help theary

    Quote Originally Posted by mad fridgie View Post
    For the rep points,

    The only law I learnt at college.

    Balls Law.

    The angle of the dangle is directly proportional to the the thrust
    "???????????"

    complete

    "and they shall be yours"
    Or........as long as the urge stays constant
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Quote Originally Posted by marc5180 View Post
    Or........as long as the urge stays constant
    correct, points given.
    Last edited by mad fridgie; 08-09-2013 at 11:50 AM.

  12. #12
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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    Yes, but if you only have this way of capacity reduction (only by TEV, and condensation pressure is kept constant) machine practically works with constant capacity. This is because when SST drops also will drop condensation pressure and fans will stop working increasing the condensation pressure that will result in SST increase. So cooling capacity is first decreased and then increased, so practically it is constant.
    That isn't what happens. When the condensing pressure is increased by lowering the condenser fan speed, or by cycling the condenser fan off, then the head pressure will rise and along with it the approach. In other words the liquid line temperature will also increase. This hotter liquid causes an additional amount of flash gas at the TEV outlet, increasing the suction pressure but lowering the refrigeration effect. Capacity is reduced.The TEV doesn't control capacity, it only maintains a constant superheat as the capacity varies due to changing indoor/outdoor loads or staging.
    Last edited by hvacrmedic; 08-09-2013 at 03:41 PM.

  13. #13
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    Re: Help theary

    hi lambo I think that you want to know why fans stop the reason is to ensure solid liquid feeding tev and to prevent flash gas situation mostly in cold weather

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    Re: Help theary

    Quote Originally Posted by hvacrmedic View Post
    That isn't what happens. When the condensing pressure is increased by lowering the condenser fan speed, or by cycling the condenser fan off, then the head pressure will rise and along with it the approach. In other words the liquid line temperature will also increase. This hotter liquid causes an additional amount of flash gas at the TEV outlet, increasing the suction pressure but lowering the refrigeration effect. Capacity is reduced.The TEV doesn't control capacity, it only maintains a constant superheat as the capacity varies due to changing indoor/outdoor loads or staging.
    So, if I understand you this is how it goes: Load start to decrease, TEV decrease evaporation pressure and condensation pressure is decreased. One fan is turned off (for example), so less liquid is passed to evaporator, superheat is increased, TEV opens increasing evaporation and condensation pressure returning thing to starting point. But now one fan less is working so less liquid is passed to evaporator so capacity is reduced.

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    Re: Help theary

    In this way system is always in equilibrium, so how it is possible that load is decreased and LWT (leaving water temperature) is decreased. For example in project condition temperature regime of chilled water is 7/12, but when load decreases and returning water is drop to 9 how leaving temperature is not 7 but 6 for example when everything is at equilibrium.

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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    So, if I understand you this is how it goes: Load start to decrease, TEV decrease evaporation pressure and condensation pressure is decreased. One fan is turned off (for example), so less liquid is passed to evaporator, superheat is increased, TEV opens increasing evaporation and condensation pressure returning thing to starting point. But now one fan less is working so less liquid is passed to evaporator so capacity is reduced.
    Load starts to decrease, TEV starts to close to maintain evaporator superheat, the SST drops, the capacity drops the power input drops and because of the this low load the condenser doesn't have as much energy to reject and so the condensing temp may drop.

    The refrigerant cycle is like a hamster wheel going round and round. When one thing changes everything else has to change with it.....Each time the system reaching an equilibrium
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    But now one fan less is working so less liquid is passed to evaporator so capacity is reduced.
    The amount of liquid going through the evaporator is controlled by the TEV which controlls superheat.... If the load increases then the superheat will increase and so the TEV has to open to provide more refrigerant.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    So, if I understand you this is how it goes: Load start to decrease, TEV decrease evaporation pressure and condensation pressure is decreased. One fan is turned off (for example), so less liquid is passed to evaporator, superheat is increased, TEV opens increasing evaporation and condensation pressure returning thing to starting point. But now one fan less is working so less liquid is passed to evaporator so capacity is reduced.
    I didn't say there was less liquid. There will be more liquid, but the temperature of the liquid will be higher with one fan off, that's why capacity is reduced. Hot liquid has less capacity than cold liquid.

  19. #19
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    Re: Help theary

    Quote Originally Posted by hvacrmedic View Post
    I didn't say there was less liquid. There will be more liquid, but the temperature of the liquid will be higher with one fan off, that's why capacity is reduced. Hot liquid has less capacity than cold liquid.
    If temperature of liquid is higher than when it goes through TEV it will be less liquid and more flash. I think I understand you but where is the limit of this way of capacity reduction? Withthis way of capacity reduction how it is possible to subcool chilled water, that will result is chiller shut off (if set point is 7 LWT, how it is possible to drop to 6 LWT when everything is always in equilibrium).

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    Re: Help theary

    Tried to give you points Marc but the system won't let me...says I have to 'Spread the Love'
    I'm back on the Pale

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    Re: Help theary

    Quote Originally Posted by frank View Post
    Tried to give you points Marc but the system won't let me...says I have to 'Spread the Love'
    Damn!!! Save them for me
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    If temperature of liquid is higher than when it goes through TEV it will be less liquid and more flash.
    I think you are getting confused. This isn't a way of capacity reduction per se but an effect of increased head pressure caused by the fans shutting off due to low load.
    This isn't a constant because the system is always trying to balance


    Quote Originally Posted by Lambo View Post
    I think I understand you but where is the limit of this way of capacity reduction?
    When you say what is the limit to this capacity reduction do you mean how low can we drop the condensing temperature?



    Quote Originally Posted by Lambo View Post
    Withthis way of capacity reduction how it is possible to subcool chilled water, that will result is chiller shut off (if set point is 7 LWT, how it is possible to drop to 6 LWT when everything is always in equilibrium).
    As the LWT drops the SST drops and the capacity decreases until set point is reached.
    The difference between genius and stupidity is that genius has its limits.

    Marc

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    Re: Help theary

    OK. I still dont understand. Maube I did not explained well the problem. We have compressor, evaporator, TEV and condesator. Condensation pressure is kept constant. Machine works in design mode. Now, load is decreased, so superheat is decreased and TEV closes, that results SST to decrese. If load continues to decrease, SST will drop more and this will cause head pressure to drop. When head pressure drop, condensator fan will decrease speed, so more flash will be in evaporator, and this will increase superheat and this will result in TEV opening, that will result in SST increase. So now one fan work with decreased speed, matching the load. Where is the limit of this way of capacity reduction? And how it is posible to subcool the water when machine can match reduced load?

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    Re: Help theary

    Quote Originally Posted by Lambo View Post
    OK. I still dont understand. Maube I did not explained well the problem. We have compressor, evaporator, TEV and condesator. Condensation pressure is kept constant. Machine works in design mode. Now, load is decreased, so superheat is decreased and TEV closes, that results SST to decrese. If load continues to decrease, SST will drop more and this will cause head pressure to drop. When head pressure drop, condensator fan will decrease speed, so more flash will be in evaporator, and this will increase superheat and this will result in TEV opening, that will result in SST increase. So now one fan work with decreased speed, matching the load. Where is the limit of this way of capacity reduction? And how it is posible to subcool the water when machine can match reduced load?
    During pull-down the machine capacity will always be greater than the load. If a balance point is reached where capacity is equal to the load, then a steady state condition will occur where the temperature will not drop any further. The fan cycling isn't for "matching the load" it's to maintain head pressure during low ambient conditions, to allow proper feed to the TEV.
    Last edited by hvacrmedic; 10-09-2013 at 07:53 PM.

  25. #25
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    Re: Help theary

    Quote Originally Posted by hvacrmedic View Post
    During pull-down the machine capacity will always be greater than the load. If a balance point is reached where capacity is equal to the load, then a steady state condition will occur where the temperature will not drop any further. The fan cycling isn't for "matching the load" it's to maintain head pressure during low ambient conditions, to allow proper feed to the TEV.
    OK. I think I understand now. Thanks for effort.

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