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  1. #1
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    Question Hot gas bypass function



    Hey guys,

    I hope someone could provide any advise for me. I've having reversible air-to-air heatpump (R410A). For the winter mode as a defrost strategy I decided to test HGBP solution. As an activation factor for HGBP will be suction pressure (it means that I will measure suction pressure).

    In several pdf's it's written that bypassing quantity should not exceed 30%, because in other way I can face improper condenser work (cause too little quantity reaches it).
    During bypassing period, discharge pressure should be stable (moreless), compressor power input/speed and suction pressure should start to increase.

    In other similar applications with HGBP solution, for HGBP activation pressure difference between evaporator inlet and outlet is measured. Shortly, HGBP is activated when wet/freezed evaporator pressure difference becomes doubled compated to dry evaporator pressure difference. It makes sense to use such kind of HGBP start-finish strategy.

    What are other recomendations for HGBP activation start-finish strategy? Whether it is appropriate without measuring pressure difference according to evaporator to set start like 4 bars (R410A) and finish till somekind of limit? All ideas are welcome



  2. #2
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    Re: Hot gas bypass function

    ascorone,
    This valve is designed just for that & is adjustable with inbuilt differential.

    http://products.danfoss.com/productr...ion-with-lg/#/

  3. #3
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    Re: Hot gas bypass function

    Hi
    The required pressure has to be set on HGBP valve spring by turning the setting screw .The valve opens and by pass the discharge vapor once the suction pressure drops below the set pressure. You can choose the valve size based on the offset pressure and bypass load (Compressor capacity -Evaporator capacity )

  4. #4
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    Re: Hot gas bypass function

    Can't say I have seen or heard of a heat pump with HGBP.

    Gibbo

  5. #5
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    Re: Hot gas bypass function

    HGBP is a poor method of defrosting, as when using a valve such as the Danfoss CPCE hot gas dump valve it simply dumps hot gas into the distributor to circulate the hot gas mixed with the expanded refrigerant from the expansion valve to keep a constant suction pressure . So the hot gas becomes de superheated by the cold refrigerant as the hot gas starts to raise the superheat which the expansion valve bulb reacts to. It does this to protect the compressor from high superheat and hence overheating the compressor windings. So any HGBP METHOD would be very slow in carrying out a defrost.
    The heat pump has an inbuilt effective defrost by simply reversing from heating to cooling .
    I'm not sure what you are trying to achieve with the HGBP valve or why you deem it necessary on this system but if you can be more specific as to what you are trying to achieve maybe with a circuit diagram then maybe a better solution is possible

  6. #6
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    Re: Hot gas bypass function

    is it not to prevent low evap pressure/temp frosting in low ambient conditions ?

  7. #7
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    Re: Hot gas bypass function

    I will use two electronic expansion valves. One valve will be fitted for the whole refrigeration cycle (the whole control solution I mean main system controller and electronic expansion valve has own algorithms, and adjust valve openings - I mean it adjust opening automatically according to your set superheat temperature even the conditions changes).
    Second electronic expansion valve will be used for HGBP. Control solution manufacturer advised to use it.
    --
    The purpose of using HGBP is for tests. I mean I have tested by strategy by reversing cycle, but I'm not interested in that because I can't provide cold temperature even for 5 minutes. Also I have read that HGBP is inefficient in some cases, so I will combine it by reversing the cycle.

    Maybe someone could explain more, why it's inefficient and how to evaluate for air-to-air heatpump while using R410A refrigerant?
    I've also read that maximum bypass'ing quantity should not exceed 30%, because in other cases, condenser will start to starve. What are your opinions on that?

  8. #8
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    Re: Hot gas bypass function

    Ascorone,
    Is this just to heat a room, or are you custom building a specialised refrigeration system.
    If you can give more detail people can
    advise you more accurately.

  9. #9
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    Re: Hot gas bypass function

    Quote Originally Posted by RANGER1 View Post
    Ascorone,
    Is this just to heat a room, or are you custom building a specialised refrigeration system.
    If you can give more detail people can
    advise you more accurately.
    We are small air handling units manufacturer company. We are trying to integrate refrigeration systems into ventilation units. Refrigeration is quite new sector for us, but however we had tested some units, so we actually not quite new.

    Heatpump will be added besides rotary heat exchanger, so all in all two stage heat recovery will be performed. On the first step by rotary heat exchanger and on the second step by heatpump.

    The main aim, when unit is operating, is to provide as stable as possible supply air temperature. We've been testing units when reversing cycle, but in such scenario (depending on outdoor condition) supply temperature decreases for quite long period (try to image 24 C temperature drops till 10-15 C for minimum 5 minutes). You could also said that you should add additional heater that would turn on when getting defrost signal (for keeping supply temperature constant as much as possible). If hot gas bypass strategy won't work properly then there will not be any way like putting additional heater for temperature fall.

    I hope that information painted more detailed view.

  10. #10
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    Re: Hot gas bypass function

    Quote Originally Posted by ascorone View Post
    We are small air handling units manufacturer company. We are trying to integrate refrigeration systems into ventilation units. Refrigeration is quite new sector for us, but however we had tested some units, so we actually not quite new.

    Heatpump will be added besides rotary heat exchanger, so all in all two stage heat recovery will be performed. On the first step by rotary heat exchanger and on the second step by heatpump.

    The main aim, when unit is operating, is to provide as stable as possible supply air temperature. We've been testing units when reversing cycle, but in such scenario (depending on outdoor condition) supply temperature decreases for quite long period (try to image 24 C temperature drops till 10-15 C for minimum 5 minutes). You could also said that you should add additional heater that would turn on when getting defrost signal (for keeping supply temperature constant as much as possible). If hot gas bypass strategy won't work properly then there will not be any way like putting additional heater for temperature fall.

    I hope that information painted more detailed view.
    Hot gas bypass probably no good for your application.
    See link
    https://en.m.wikipedia.org/wiki/Air_source_heat_pumps

    http://www.mitsubishielectric.com/believe/heatpump/
    Last edited by RANGER1; 26-01-2018 at 05:10 AM.

  11. #11
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    Re: Hot gas bypass function

    Quote Originally Posted by RANGER1 View Post
    In some cases hot gas bypass will be inefficient or will not work at all. During unit software configuration there are set number of attempts of hot gas bypass. If after set number of attempts function end by timeout, then cycle will be reversed.

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