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    simple hot gas defrost circuitry



    In the process of fitting some second hand kit for a good customer the kit is mine so I know it works,sadly the evaporator has no heater elenents it's only going to run at minus two c so decided to install basic hot gas defrost as the ice build up should not be excessive,a solenoid valve in the delivery line putting hot gas into evaporator all controlled by elliwell controller suction regulator,what's the thoughts on fitting an accumulator? it's a two horse hermetic condensing unit.



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    Re: simple hot gas defrost circuitry

    I would fit a suction accumulator, as when you defrost with hotgas you are going to blow all the liquid back into the suction line.

    Wouldn't it be easier to fit heating elements on the evap?
    Last edited by Tycho; 25-04-2016 at 09:59 PM.
    -Cheers-

    Tycho

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    Re: simple hot gas defrost circuitry

    There is no space in the coil for heaters,and I have all the valves in my old stock now I will have to dig out an accumulator.

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    Re: simple hot gas defrost circuitry

    You will need to install at the accumulator inlet a pressure regulator valve like danfoss KVL to protect the compressor from liquid return from the evaporator.

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    Re: simple hot gas defrost circuitry

    Yes a reg is being fitted.

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    Re: simple hot gas defrost circuitry

    One other thing to consider is high pressure fan control to keep discharge pressure up in cold weather.

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    Re: simple hot gas defrost circuitry

    If you fit a crankcase limit valve , fit it after the accumulator as liquid entering the valve will damage the bellows to destruction, as what often happens in transport systems. Let the accumulator do its job

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    Re: simple hot gas defrost circuitry

    Re-read, and converted HP to KW, 2 hp is approx 1.5 kw, meaning a small walk in cold room, this is for a cold room, not for a freezing room? am I correct?

    The desired temperature in the room is +3-5C, meaning you have to keep a evaporator temperature of -6-2C

    I would skip the hotgas defrost and just do a natural defrost.
    use the defrost timer in the elliwell to close the solenoid and pump down the system every 8-12 hours, but keep the evaporator fans running while the compressor is stopped, and set the defrost time to 45 minutes.
    it should be more than enough time to defrost the evaporator.






    correct me if I am wrong
    -Cheers-

    Tycho

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    Re: simple hot gas defrost circuitry

    Also, at -2C...there is no need to run a pump down. Just have the liquid solenoid de-energize when defrost cycle initiates.
    Last edited by mikeref; 28-04-2016 at 09:31 AM. Reason: 28/4/16. Read my explanation below.
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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by mikeref View Post
    Also, at -2C...there is no need to run a pump down. Just have the liquid solenoid de-energize when defrost cycle initiates.
    I don't want to sound pedantic, "me pedantic" I know, I know..................

    But not wanting to be pedantic is that not a pump down anyway?

    Controlling the liquid so it is pumped into the receiver whilst on
    defrost or at temp is called a pump-down cycle in the UK?????

    Edit I have just re-read your post and I get it, "by Jove he's got it", I think?
    You mean turn of the comp on the controller and de-energise the mag valve
    at the same time????

    If that is what you mean, I understand what you mean, if not sod off I'm confussed



    Rob

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    Last edited by Rob White; 27-04-2016 at 03:31 PM.
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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by cadwaladr View Post
    In the process of fitting some second hand kit for a good customer the kit is mine so I know it works,sadly the evaporator has no heater elenents it's only going to run at minus two c so decided to install basic hot gas defrost as the ice build up should not be excessive,a solenoid valve in the delivery line putting hot gas into evaporator all controlled by elliwell controller suction regulator,what's the thoughts on fitting an accumulator? it's a two horse hermetic condensing unit.

    What is the product?
    Because fresh meat cases / rooms run with an ambient of about 0 degC to 2 degC
    temp and they run on off cycle defrost. What is the product and will it not allow that?

    Regards

    Rob

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by Rob White View Post
    I don't want to sound pedantic, "me pedantic" I know, I know..................

    But not wanting to be pedantic is that not a pump down anyway?

    Controlling the liquid so it is pumped into the receiver whilst on
    defrost or at temp is called a pump-down cycle in the UK?????

    Edit I have just re-read your post and I get it, "by Jove he's got it", I think?
    You mean turn of the comp on the controller and de-energise the mag valve
    at the same time????

    If that is what you mean, I understand what you mean, if not sod off I'm confussed



    Rob

    .
    Haha Yes Rob. May i re-submit. No need to pump the system down for a defrost cycle.

    When power to the Compressor is cut via thermostat or Defrost cycle, power to the Liquid line solenoid is shut down. ( Comp And LLSV on the same control circuit.)
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by mikeref View Post
    Haha Yes Rob. May i re-submit. No need to pump the system down for a defrost cycle.

    When power to the Compressor is cut via thermostat or Defrost cycle, power to the Liquid line solenoid is shut down. ( Comp And LLSV on the same control circuit.)
    I'm with you on that one mike. Just one little thing, LLSV should as close as possible to the TXV.

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    Re: simple hot gas defrost circuitry

    The product is meat in as much it's a meat material used in the petfood market,the customer normally will want zero c,but on occasions will want minus ten set point to firm up the product,so fitting a defrost is must because fitting it later would be near impossible as it's a very busy factory.

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by cadwaladr View Post
    The product is meat in as much it's a meat material used in the petfood market,the customer normally will want zero c,but on occasions will want minus ten set point to firm up the product,so fitting a defrost is must because fitting it later would be near impossible as it's a very busy factory.
    Well, that moved the Goal Posts on your project.

    Minus 10 C will be a problem as you have an Evaporator suitable for coolroom conditions.
    You'll have to find the maximum weight of product (KG), temperature of incoming product, exposure time @ -10 C, and air circulation for maximum heat rejection...among a few other variables such as humidity and whether your 2 Horse kit is big enough for.... say 5 Hours run time before the evap chokes on Ice.
    Last edited by mikeref; 29-04-2016 at 09:50 AM. Reason: Looking forward to Rob W's response. Blue highlight is fine with me :)
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by chemi-cool View Post
    I'm with you on that one mike. Just one little thing, LLSV should as close as possible to the TXV.
    Better to have the defrost controller close the solenoid and pump let the compressor run till it stops on low pressure. This way you remove the refrigerant from the evaporator and make an ambient defrost faster.

    That is basically what a pump down is, closing the liquid supply and let the compressor stop on the low pressure switch, but start again as soon as the LP switch rises when the solenoid opens after a defrost.


    on the other hand, you have Pump out, which means that once the compressor stops on the LP switch, you have to manually start the compressor again.
    -Cheers-

    Tycho

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    Re: simple hot gas defrost circuitry

    How much is a new electric defrost evaporator with correct fin spacing etc?

    To many calls, mate it's iced up again, can you come & have a look at it!

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    Re: simple hot gas defrost circuitry

    Do not go there, first myth is hot gas defrost, by the time it gets there it cool gas and evap acts as a condenser and floods coil with liquid. Bingo flood back.
    Short answer get a new electric defrost evap.

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    Re: simple hot gas defrost circuitry

    I have a few tricks up my sleeve that will make sure it's capable of clearing the coil and getting heat into the evaporator,I have converted many transport units and they have worked.

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by Magoo View Post
    Do not go there, first myth is hot gas defrost, by the time it gets there it cool gas and evap acts as a condenser and floods coil with liquid. Bingo flood back.
    Short answer get a new electric defrost evap.
    Magoo in a "normal" hot gas defrost we would agree it's correct to say that it does not have to be hot & the change of state or latent heat does most of the work.
    Of course liquid will condense in evaporator & suction trap would have to be used, unless cadwaladr has some unique way of doing it.
    I've never really looked at reverse cycle aircon, but as far as i know they have a suction trap.
    In our game you need to get a load to generate "hot" gas to defrost unit & for it to work you need other working evaporators, or heat load to keep compressor working.
    I guess we would prefer heaters so we go home & forget about it.

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    Re: simple hot gas defrost circuitry

    Reverse cycle is more efficient from every prospective,but a lot of extra components have to be used the reversing valve another expansion valve no p trap this system is used on many transport units over here as when your mobile no electric supply defrost times on a freezer unit are about 15 minutes maximum and that's running at minus 25c,condensor fan control is used to keep pressure at an optimum the only thing transport units suffer from is drain heaters but they are fitted normally using the 12 or 24 vdc.

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    Re: simple hot gas defrost circuitry

    Quote Originally Posted by Tycho View Post
    Better to have the defrost controller close the solenoid and pump let the compressor run till it stops on low pressure. This way you remove the refrigerant from the evaporator and make an ambient defrost faster.

    That is basically what a pump down is, closing the liquid supply and let the compressor stop on the low pressure switch, but start again as soon as the LP switch rises when the solenoid opens after a defrost.


    on the other hand, you have Pump out, which means that once the compressor stops on the LP switch, you have to manually start the compressor again.
    The "standard" Pump down cycle on many commercial cool room kits are using the method i mentioned above. Two reasons. 1. It's a waste of energy to constantly pump down on every cycle.

    ( Credit to Chemi for mentioning the position of LLSV.)

    2. Mechanical contacts on a LP switch tend to fail after X number of cycles.
    To the optimist, the glass is half full. To the pessimist, the glass is half empty.

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    Re: simple hot gas defrost circuitry

    .

    The most simple way. with little protection but it is used
    or has been used for years with most smaller commercial
    freezers.

    Run a hot gas line to the evap and tee it into the evap inlet
    directly after the TEV. Put a mag valve on this line and when
    it goes into defrost, it opens the hot gas line, hot gas passes
    through the evap and the evap coil defrosts.

    Trouble is, running in defrost will overheat the suction gas back
    to the compressor and overheat it, so timing the defrost long
    enough to defrost the coil and not too long so the compressor
    overheats and fails.

    After less than about 5 mins the heat from the discharge gas will
    be lost and the defrost must be completed in this initial period.

    Protecting the compressor with the use of a suction accumulator
    would be an added precaution but this process would work with
    little additional cost and works well.

    There are drawbacks to using hot gas defrost though, clamping the
    pipes to stop movement through expansion is a must and remember
    the coil will be subjected to extreme temperature changes and that
    can result in shock failures to evaporator pipe returns and fittings.

    It is very crude, a bit agricultural in design, but it is the cheapest
    option and it will work.

    Rob

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    Re: simple hot gas defrost circuitry

    I agree Rob,accumulator and crankcase regulator are being used to protect or help to protect the compressor on the pump down cycle I have not had issues with the lp switch contacts only when it's wired direct to single phase compressor if it's ran powering a contactor/relay the load over the contacts would be extremely low.

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