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  1. #1
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    Variable load charge?



    I have a mini split condensing unit (1HP, fixed orifice) which I've rigged up to a suitably sized heat exchanger to use for cooling water. I haven't used it much but I pulled it out again the other day and gave it a run, and was disappointed with the performance. I realized at that point I had connected the heat exchanger in reverse to what it should be ideally, so I've switched that around.

    I also noticed that the discharge port would frost up almost immediately, as would only the bottom part of the heat exchanger, so figured it was low on refrigerant (R22, static charge showed ~120PSI). I ran a constant flow of tap water through the heat exchanger and added refrigerant until the frost disappeared, and the suction line was sweating back to the suction accumulator, and it's running much better now.

    That said, the only service port on the unit is on the suction side, so I don't really know what's going on inside it beyond that. With a decent flow of probably ~25C tap water going in, the suction pressure is around 80PSI. If I reduce the flow I can get it to drop to around 60PSI, and the suction line will start frosting up at that point. Static pressure is now around 180PSI. I checked compressor current draw and it is within spec.

    But, seeing as the load is so variable, and I only have access to the suction side to measure pressure, I'm really not sure how I should go about charging the system optimally. From much Googling I've done, I've basically figured charge it until the suction line back to the compressor is sweating, the heat exchanger is evenly cool, and make sure that the compressor isn't being overloaded. Is there much more I can do? Should I perhaps err on the side of over or undercharging in this situation?

    This system is just for my own personal use (and a fun hobby project), so basically my only "worry" is damaging the unit from over/undercharging it.

    Cheers



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    Re: Variable load charge?

    Things,
    Usually instal a crankcase pressure regulator.
    It can be set for maximum amps &/or pressure.
    Basically only lets enough gas into compressor by restricting it, then when pressure drops below set pressure it opens up more.
    Always nice to know discharge pressure & if system had to be emptied, suggest instal everything at once.

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    Re: Variable load charge?

    Hmm, I'll keep that in mind if I have to empty it. I don't really have a way of recovering refrigerant so I've just been pumping it down into itself to do work on it

    It's quite an odd setup so I understand if it's not really optimal to start with. I figure the water temp should never be too high, so charging it so there's condensation on the suction line at "ambient" water temp should help keep the compressor a bit happier maybe. I'm really just concerned about the compressor overheating I guess.



    I've had it to pieces searching for a hidden discharge port and nothing - cheap units for you I guess.

  4. #4
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    Re: Variable load charge?

    It also sounds like you have inadequate water flow through your heat exchanger. If you are going to put it on a closed loop, I would install a pump to give you heat exchanger design flow rates for pressure drop through plates. It is far easier to regulate refrigerant design over a TD of say 6 degC.

    How are you metering refrigerant through heat exchanger?
    Last edited by hookster; 25-08-2015 at 08:12 AM. Reason: missed question
    I love the smell of Ammonia in the morning!

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    Re: Variable load charge?

    It will be a closed loop system eventually, just waiting on a pump. I can get good flow rates through it, it's just the ambient/return water temp that might vary a bit. I know the compressors do run hot though so I'll have to give it a solid run on a warm day and see how it does.

    The metering device is just a capillary inside the condenser, I guess they put it in there instead of the indoor unit to reduce noise. It is actually a reversible unit but I think the heat exchanger is too small to hold all the liquid, and pressures get too high for my liking, so it'll be for cooling only I think. I'd have to install a "reservoir" to store the excess liquid if I wanted to do that, I figure.

    In addition to a water cooler, I have a small cold room evaporator that I've modified to accept water, so essentially it can be a portable split system. If I use that in a closed loop with it, I can probably install a valve and adjust the water flow rate to keep the suction line nice and cool if the ambient temp is too high. But I'll have to wait for my high pressure pump to test that properly.
    Last edited by Things; 25-08-2015 at 11:44 AM.

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    Re: Variable load charge?

    Hmm, just thinking about this from a practical standpoint.

    Once I get my high pressure pump and can get decent flow through the heat exchanger, would my best bet for tuning the refrigerant charge be basically the same as a regular split? As in, measure the temperature of the air coming out of the evaporator and just ignoring the fact it's coupled via water? If the water flow is sufficient than the temperature of the evaporator/water should be basically the same as if it were coupled by the refrigerant itself, I would think?

    I know charging by weight is preferred for these mini splits, and the spec sticker says 560g, but presumably it'll be a fair bit lower seeing as I don't have long linesets.
    Last edited by Things; 27-08-2015 at 10:06 AM.

  7. #7
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    Re: Variable load charge?

    I spent a bit of time messing around with it today, and basically decided my best option was to just run a constant stream of water through it, and "tune" the refrigerant charge until the water leaving was coolest.

    After I did this, I adjusted the water flow so the water leaving was 5C cooler than it was going in, and timed how long it took to fill a 2L coke bottle. Then I could calculate I was getting roughly 2.8kW cooling power, which gives a COP of ~3.5, which I'm pretty happy with

    Then just for fun I ran it without any water flow at all. The evaporator suction line has a temperature sensor on it which is meant to prevent it freezing the water inside if the flow stops for some reason. Turns out I messed up some logic in my controller code, and well ..





    Temp was still dropping rapidly at that point, but I could hear the compressor wasn't happy so I stopped then. Fun experiment

  8. #8
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    Re: Variable load charge?

    Running without water flow is the quickest way to destroy your plate heat exchanger.

    If the unit charge is stated on the condensing unit then that is the charge to use.
    Brian - Newton Abbot, Devon, UK
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  9. #9
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    Re: Variable load charge?

    Having a bit of a difficult time using this thing as a portable split system. The air coming out of the "evaporator" doesn't seem to get below 20C or so, yet it's only rated for about 1kW, so I should be having issues with it freezing up, not barely getting cool.

    It's odd as the water flow is reasonable, the air coming out of the condenser is nice and warm, and the suction line is nice and cool too, so it doesn't seem like it's low on refrigerant. Not really sure what's going on with it.

  10. #10
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    Re: Variable load charge?

    What are the actual water flow/return temperatures serving the remote evaporator?
    Brian - Newton Abbot, Devon, UK
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  11. #11
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    Re: Variable load charge?

    After running for a few minutes, water entering the plate exchanger (after it's been through the "evaporator" coil) is around 17C. Water leaving the plate exchanger is around 11C, which is before it enters the "evaporator" coil. I have a temp sensor attached to the suction line where it leaves the plate exchanger, and I'm reading around 0-1C on average, and pressure maybe 30cm or so down the suction line is ~65PSI. The water has a bit of glycol in it to prevent freezing. The water flow rate is perhaps a bit lower than I'd like (new pump should be here any day now), but I'd kind of expect in that case the plate exchanger would just freeze up. Ambient air temp entering the coil is probably 23C or so.
    Last edited by Things; 08-09-2015 at 10:55 AM.

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