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  1. #1
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    Re: Liquid receiver on a reverse cycle VRV system

    Quote Originally Posted by BradC View Post
    I asume the heat absorbed by the receiver from the ambient air is going to cause the heating of the gas/liquid it contains…
    This assumption is valid only if the energy absorbed by the receiver (from the air) is greater than the energy you need to bring the subcooled liquid from your PHE back to saturation and then super heat it (only then you would produce the pressure differential).

    This effect will depend on the refrigerant velocity and the air velocity around the receiver.

    Common practice is to insulate not only the receiver but all of the liquid line when ambient temperature is most of the time well above condensing temperature.

    Insulating the liquid line is a mistake when the opposite occurs.

    Quote Originally Posted by BradC View Post
    a) ..., and actively subcool the liquid both entering and leaving (a couple of tube-in-tube subcoolers)
    You are using EEV with an orifice that should be dimensioned taking into account the average amount of subcooling your system has.

    Having too much subcooling affects the stability of the control of the EEV if the minimum it can open already meets the demand of the evaporator due to excessive subcooling.

    So if you cannot choose your orifice you should test how the system operates without/with insulation. Maybe preventing air currents around the receiver is enough or maybe a small amount of insulation around it is enough.

    Quote Originally Posted by BradC View Post
    I asume the heat absorbed by the receiver from the ambient air is going to cause the heating of the gas/liquid it contains…
    b) Bypass the receiver in normal operation.
    [/QUOTE]

    The only reason to bypass the receiver is if it is indeed reducing the system subcooling and you want the subcooled refrigerant from the condenser go straight to the liquid line with neither the pressure losses nor temperature gains of a receiver.

    In order to do this effectively you have to use a number of designs like the “Through Type” receiver, widely used with shell and tube condensers, you can read about it in Ashrae’s Fundamentals Chapter 2 under Receivers. In all of these designs you have to respect a certain height between the receiver, condenser and liquid line depending on pressure drops. A correct design is problematic and an empirical design, impossible.

    If you simply bypass the receiver then your “normal” operation of the evaporator must be a constant load or you will not benefit from the ability of the receiver to store and deliver excess refrigerant to the evaporator. If this were the case I would save my EEVs for a more demanding application.

    I´d stick to the first part of plan a, if more subcooling is then needed ok proceed to subcool more, if everything fails then go to plan b but get data for pressure drops first (these are small values and you need differential pressure gages).

    Have fun!

  2. #2
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    Re: Liquid receiver on a reverse cycle VRV system

    Quote Originally Posted by aramis View Post
    Common practice is to insulate not only the receiver but all of the liquid line when ambient temperature is most of the time well above condensing temperature.

    Insulating the liquid line is a mistake when the opposite occurs.
    This will almost always be the case.

    Quote Originally Posted by aramis View Post
    You are using EEV with an orifice that should be dimensioned taking into account the average amount of subcooling your system has.

    Having too much subcooling affects the stability of the control of the EEV if the minimum it can open already meets the demand of the evaporator due to excessive subcooling.
    I discovered this accidentally last weekend. The EEV I have is probably slightly undersized for the application at standard air cooled subcooling levels, however as I increased the superheat by closing the eev the subcooling also increased and my overall supply air temperature dropped.

    I have to say I was absolutely astonished until I thought about what was actually happening.

    I've actually revised my control algorithm to maximise the Td across the evaporator coil rather than maintain a steady superheat. As the outdoor unit has quite a large accumulator, I can sustain temporary control glitches that might ordinarily cause a bit of floodback. Overall this control strategy has dropped supply air temps by between 1 and 3C on average, and additionally reduced overall power consumption as it appears to be running with a slightly lower mass flow (and resulting lower condensing temperatures), but higher superheat/subcooling.


    Quote Originally Posted by aramis View Post
    Have fun!
    I'm actually having a ball. I really appreciate the feedback. I'm way out of my depth, but you guys keep throwing me enough life rings to stay afloat.

  3. #3
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    Re: Liquid receiver on a reverse cycle VRV system

    I forgot to mention in my previous post that my Daikin supplier said he does not know your valve either.

    I discovered this accidentally last weekend. The EEV I have is probably slightly undersized for the application at standard air cooled subcooling levels, however as I increased the superheat by closing the eev the subcooling also increased and my overall supply air temperature dropped.
    For a slightly undersized valve you have two strategies to play with:

    You mentioned this already. Subcooling affects the SYSTEM performance. You can get more capacity from your system by increasing subcooling. This is a good strategy but don’t exaggerate.

    The EEV’s performance is affected by the applied pressure differential, so if you need more from your fixed EEV you need to increase the pressure differential. You can easily do so controlling your PHE flow. Of course you also have the opposite effect if your suction pressure lowers, that the compressor reduces capacity. So you must play with this balance until you get what you want or may need a bigger compressor.

    Also don’t forget to check for high discharge temperatures, the dark side of increasing superheat.

    I have to say I was absolutely astonished until I thought about what was actually happening.
    This is called learning! Good for the brain and soul!

    I've actually revised my control algorithm to maximise the Td across the evaporator coil rather than maintain a steady superheat. As the outdoor unit has quite a large accumulator, I can sustain temporary control glitches that might ordinarily cause a bit of floodback.
    What differential are you talking about here?

    It is most normal for superheat to vary, this only means that your EEV is working as it should: trying to keep superheat stable! Though there are a number of controllers designed for other purposes like keeping discharge temperature low but I haven’t seen those in ACs. Without manuals, only by trial and error you can discover where to place your sensors.

    Accumulators must be big enough to capture most of the liquid refrigerant going to the compressor but small enough not to trap oil. So be careful at low demand that oil gets back to the compressor.

    Cheers

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