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  1. #1
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    Evaporating Pressure Control - Will this Work? (and if not, why not?)



    Hi to all on my first post,

    Brief background - UK time served engineer, been working offshore (oil rigs) for 16 years.
    I have an idea for a soultion to an ongoing design issue, but can't believe I am the first person to think of this. So why don't we do it? What am I missing?

    The issue...
    A common system set up for offshore room temperature control is duct mounted DX coils with ATEX certified condensing units. For ATEX compliance we cannot use invertors or speed controllers, and most units are prety basic; open drive reciprocating compressors (although the last 5-6 years has seen more use of ATEX certified 'Bock' Semi-hermetic units).

    Because we have limited capacity control for the compressor there is a requirement to ensure evaporator pressure is not allowed to fall too low to avoid the risk of icing up the coil. This is of particular concern offshore, where supply airflow keeps modules and rooms positively pressurised as part of the safe design.

    If evaporator pressure regulation is controlled, then it will be with a hot gas bypass system, but the problem is, that almost every system I have seen or worked on has the bypass valve screwed in closed because they end up constantly bypassing and refrigeration duty falls off.


    So, I want to know if anyone has seen a set up as follows, or if there is any reason that it wouldn't work?

    Use cool gas from the top of the receiver tubed into the compressor suction line via a 'Close on Rise of Outlet' valve. The CRO valve would be adjusted to the lowest allowable evaporating pressure (e.g. 0-5degC for the refrigerant being used).
    On a drop in system duty, with the evaporator pressure falling, the valve would open and maintain the minimum desired pressure.

    This has the advantage of being completely removed from the evaporator and expansion valve operation and does not require a 3rd pipe run to the evaporator.
    By using cool saturated vapour from the top of the reciever there would not be a significant increase in superheat at the compressor inlet.

    The systems we install also utilise head pressure control valves to maintain condenser and receiver pressures so the removal of vapour from the top of the reciever should not have any detrimental effect.

    I would include a solenoid valve in the 'bypass' line to allow pumpdown and avoid migration when offline (the same as hot gas bypass systems), and would likely include a delay timer on the solenoid to let the system settle following start-up, before opening the bypass valve line.

    System sizes are anything from 5 - 30 kW cooling.

    So, that's the plan. Will it work? What have I overlooked?
    Please be critical...

    Thanks,
    Douglas



  2. #2
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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Quote Originally Posted by blue juice View Post
    Hi to all on my first post,

    Brief background - UK time served engineer, been working offshore (oil rigs) for 16 years.
    I have an idea for a soultion to an ongoing design issue, but can't believe I am the first person to think of this. So why don't we do it? What am I missing?

    The issue...
    A common system set up for offshore room temperature control is duct mounted DX coils with ATEX certified condensing units. For ATEX compliance we cannot use invertors or speed controllers, and most units are prety basic; open drive reciprocating compressors (although the last 5-6 years has seen more use of ATEX certified 'Bock' Semi-hermetic units).

    Because we have limited capacity control for the compressor there is a requirement to ensure evaporator pressure is not allowed to fall too low to avoid the risk of icing up the coil. This is of particular concern offshore, where supply airflow keeps modules and rooms positively pressurised as part of the safe design.

    If evaporator pressure regulation is controlled, then it will be with a hot gas bypass system, but the problem is, that almost every system I have seen or worked on has the bypass valve screwed in closed because they end up constantly bypassing and refrigeration duty falls off.


    So, I want to know if anyone has seen a set up as follows, or if there is any reason that it wouldn't work?

    Use cool gas from the top of the receiver tubed into the compressor suction line via a 'Close on Rise of Outlet' valve. The CRO valve would be adjusted to the lowest allowable evaporating pressure (e.g. 0-5degC for the refrigerant being used).
    On a drop in system duty, with the evaporator pressure falling, the valve would open and maintain the minimum desired pressure.

    This has the advantage of being completely removed from the evaporator and expansion valve operation and does not require a 3rd pipe run to the evaporator.
    By using cool saturated vapour from the top of the reciever there would not be a significant increase in superheat at the compressor inlet.

    The systems we install also utilise head pressure control valves to maintain condenser and receiver pressures so the removal of vapour from the top of the reciever should not have any detrimental effect.

    I would include a solenoid valve in the 'bypass' line to allow pumpdown and avoid migration when offline (the same as hot gas bypass systems), and would likely include a delay timer on the solenoid to let the system settle following start-up, before opening the bypass valve line.

    System sizes are anything from 5 - 30 kW cooling.

    So, that's the plan. Will it work? What have I overlooked?
    Please be critical...

    Thanks,
    Douglas
    First disadvantage of this I can think of is it is not controlling
    the evap pressure it is controlling the Suction temp and due to
    the nature of sytems that might not give the result you want?

    Why not just use normal evap pressure regulating valves?

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

  3. #3
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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Hi Rob,

    I suspect that using an evaporator pressure regulator on a single coil system will create pressures low enough downstream of the valve (comp suction) that L.P. switch will activate. I want to keep the compressor operating but at the reduced capacity at the evaporator.
    I use these on multi-temperature, muli-coil systems (e.g. cold room & freezer room) to maintain suitable evaporating temperatures in the 'warmer' coil.

    I see the set-up I described as primarily controlling the low side pressure, but without a significant increase in superheat.

    Hot gas bypass systems that inject into the suction line, rather than the evaporator inlet, require a 2nd TEV to counteract the additional superheat. I don't want to over complicate matters and add cost.


    Cheers,
    Douglas

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Blue juice,
    Your description sounds pretty common in Industrial systems.
    You could use hot gas from discharge of compressor on inlet to evaporator to maintain a false load, TX valve still maintains superheat, or saturated gas off liquid reciever into suction line.
    THe saturated gas off reciever can be used but think compressor would have to unload, liquid reciever larger than normal with a decent level in it to be able to generate enough high pressure gas.
    COndenser bypass valve can divert discharge pressure to reciever so as to keep pressure up as well, as if connected to other units TX valve still needs high pressure liquid from liquid reciever.
    IN some cases the discharge line can run through liquid reciever to keep pressure up even if condenser fans are cycled off.
    A downstream pressure regulator like a DAnfoss CVC pilot on a main valve or it's smaller version (Rob will know) for the saturated gas off reciever method. This valve feed saturated gas from liquid reciever to suction line.
    Also a evaporator pressure regulator would work, but if compressor cannot reduce capacity suction pressure at compressor may be way to low, as you mention.

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Quote Originally Posted by RANGER1 View Post
    You could use hot gas from discharge of compressor on inlet to evaporator to maintain a false load, TX valve still maintains superheat, or saturated gas off liquid reciever into suction line.
    This is the method most commonly employed, but as I mentioned; almost all these systems end up being virtually closed off because over time they seem to get "stuck" in bypass with minimal refrigerating effect but a system demand for more cooling.
    I have tried to determine why they do this, but haven't worked out a definate answer. I originally thought that long pipe runs to the evaporator allowed the hot gas to cool and start condensing, but I have seen similar operating conditions on small integrated units with minimal pipe lengths.

    Quote Originally Posted by RANGER1 View Post
    liquid reciever larger than normal with a decent level in it to be able to generate enough high pressure gas.
    COndenser bypass valve can divert discharge pressure to reciever so as to keep pressure up as well, as if connected to other units TX valve still needs high pressure liquid from liquid reciever.
    Ok, so maybe the volume of vapour required from the top of the receiver would put too much demand on the condenser bypass valve to maintain receiver pressure?
    That ORD valve is usually only trying to flow enough vapour to maintain receiver pressure when the ORI valve is closing and backing up liquid.

    So would a larger ORD capacity valve would solve the problem?
    Hmmm...

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Quote Originally Posted by blue juice View Post
    This is the method most commonly employed, but as I mentioned; almost all these systems end up being virtually closed off because over time they seem to get "stuck" in bypass with minimal refrigerating effect but a system demand for more cooling.
    I have tried to determine why they do this, but haven't worked out a definate answer. I originally thought that long pipe runs to the evaporator allowed the hot gas to cool and start condensing, but I have seen similar operating conditions on small integrated units with minimal pipe lengths.



    Ok, so maybe the volume of vapour required from the top of the receiver would put too much demand on the condenser bypass valve to maintain receiver pressure?
    That ORD valve is usually only trying to flow enough vapour to maintain receiver pressure when the ORI valve is closing and backing up liquid.

    So would a larger ORD capacity valve would solve the problem?
    Hmmm...
    I guess it would as it usually would be sized for pressurizing liquid receiver, not taking a large amount of gas off it.
    But if you did this it may defeat purpose & you would end up with hot gas again unless it was injected into the liquid level to cool it or hot gas in, cool gas out pipes as far away from each other in the liquid receiver to avoid short circuit of hot gas.
    Does it have to have an ORI or ORD valve at all or is it to cold to not have it?

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    This isn't a particular system, but a general design issue, but yes North Sea ambient temperatures require head pressure control and the preferred method is ORI, ORD valves.
    If the ORD was connected into the liquid line closer to the ORI outlet than the reciever then the hot gas would be cooled by the time it entered the receiver.

    I think I'll have to build one and experiment. Starting with a standard, single ORD valve and try to determine what volume of vapour would be required by the bypass to maintain an elevated evaporating pressure.

    Thanks for the input!

    Douglas

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Can you put unloader heads on compressor?.

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    Re: Evaporating Pressure Control - Will this Work? (and if not, why not?)

    Quote Originally Posted by blue juice View Post
    This isn't a particular system, but a general design issue, but yes North Sea ambient temperatures require head pressure control and the preferred method is ORI, ORD valves.
    If the ORD was connected into the liquid line closer to the ORI outlet than the reciever then the hot gas would be cooled by the time it entered the receiver.

    I think I'll have to build one and experiment. Starting with a standard, single ORD valve and try to determine what volume of vapour would be required by the bypass to maintain an elevated evaporating pressure.

    Thanks for the input!

    Douglas
    You could determine to a certain extent if you plot a running system under the conditions you want to avoid compared to pressures you do want.
    This may give a capacity shortfall allowing you to size valve.

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