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  1. #1
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    Floating head pressure? How it works



    We hear the term floating head or suction pressure.
    Interested in peoples description on how to do it effectively.


    Thanks



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    Re: Floating head pressure? How it works

    To maximize energy efficiency of the refrigeration plant, it should be operated at optimum head and suction pressures. First we should know what is the optimum?
    For the plants with evaporative condensers optimum head pressure based on wet bulb temperature of the ambient air. T opt = T w.b.+ approach. Assume that optimum approach is 10 degF. If web bulb temp. is 75 degF, T opt. will be 85 degF or 150 psig(ammonia).

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    Re: Floating head pressure? How it works

    Thanks Sergei,
    So would assume that all condensor fans are on a VSD.
    They ramp up & down together to reach theoretical floating setpoint with wb db calculation.

    I've read some brief descriptions & they seemed to say they may still need optimizing for different seasons!
    Why would this be so?

  4. #4
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    Re: Floating head pressure? How it works

    I'm not sure what they suggest for different seasons. However, head pressure should float down minimum allowable head pressure. Every plant has own minimum head pressure. The lower minimum, the better, because we can run plant longer at optimum head pressure.

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    Re: Floating head pressure? How it works

    Hi Ranger,

    Page 8 through 12 gives a good overview.*

    Probably stuff that you already know, and obviously not the only way of "doing it", but a good document all the same.

    http://www.savepower.nsw.gov.au/Port...RprtLowRes.pdf

  6. #6
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    Re: Floating head pressure? How it works

    Thanks for the info. HVACRsaurus
    Thanks for an intersting question posted Ranger!
    Grizzly

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    Re: Floating head pressure? How it works

    I mentioned in one post that people over focused on floating head pressure. Floating saves condenser(pumps, fans) energy. However, condenser energy is 10% of compressor energy. So floating head pressure can give us 1-2% of energy savings for the limited period of time. This is the same energy savings that can be achieved by lowering minimum head pressure by 5-10 psig or 0.3-0.6 bar. We should focused on lowering minimum allowable head pressure.

  8. #8
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    Re: Floating head pressure? How it works

    .

    Something that is being used at the moment in supermarket systems is liquid amplification.

    Basically the head pressure is driven as low as possible, the compressors are controlled on
    inverters to control back pressure and then the liquid is hydraulically re-pressurised by a
    hydraulic pump to maintain the pressure difference over the expansion valve.

    This method is proving to be quite efficient and cost effective, fans and liquid pumps
    are cheaper to run than compressors.

    Rob

    .

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    Re: Floating head pressure? How it works

    Quote Originally Posted by Segei View Post
    I mentioned in one post that people over focused on floating head pressure. Floating saves condenser(pumps, fans) energy. However, condenser energy is 10% of compressor energy. So floating head pressure can give us 1-2% of energy savings for the limited period of time. This is the same energy savings that can be achieved by lowering minimum head pressure by 5-10 psig or 0.3-0.6 bar. We should focused on lowering minimum allowable head pressure.
    Segei,
    I would have thought that with a floating head pressure, with correct control of condenser fans, that this would be killing 2 birds with 1 stone.
    Its whole idea is run fans to suit the ambient conditionds of WB, DB.

    If you run the fans any faster than required, it will be a waste of energy, as it cannot be any lower!

    So to say if you drop head pressure by "X" amount it will defeat the purpose, won't it?
    I guess its how good your PLC or controller is set/programmed to achieve max results.

    In most systems, condenser fans are set to achieve a certain set pressure ie cut in, cut out or set to run flat out to get lowest head pressure.
    Floating head pressure if set up right, is a fine tune of above sentence to my understanding.

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    Re: Floating head pressure? How it works

    Your floating head pressure, should be related to ambient conditions and load of the system, and relation to required pressure drops over valve selections.
    For example on a screw, you could use your slide position, to determine load.(heat of rejection), web bulb for a wetted cond system and correction factor for the design load of the cond. (the point where you are not going to get any measurable benefit in drop in pressure)

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    Re: Floating head pressure? How it works

    Quote Originally Posted by Rob White View Post
    .

    Something that is being used at the moment in supermarket systems is liquid amplification.

    Basically the head pressure is driven as low as possible, the compressors are controlled on
    inverters to control back pressure and then the liquid is hydraulically re-pressurised by a
    hydraulic pump to maintain the pressure difference over the expansion valve.

    This method is proving to be quite efficient and cost effective, fans and liquid pumps
    are cheaper to run than compressors.

    Rob

    .
    LPA has been around for many years. (many have had problems with cavitaion with LPA),

  12. #12
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    Re: Floating head pressure? How it works

    Quote Originally Posted by Segei View Post
    I mentioned in one post that people over focused on floating head pressure. Floating saves condenser(pumps, fans) energy. However, condenser energy is 10% of compressor energy. So floating head pressure can give us 1-2% of energy savings for the limited period of time. This is the same energy savings that can be achieved by lowering minimum head pressure by 5-10 psig or 0.3-0.6 bar. We should focused on lowering minimum allowable head pressure.
    I agree that you should try to lower head pressure as much as possible to save power, also wear & tear.
    Its mainly new plants that would have this floating system installed, not to many older installs would have it yet.
    Some older or poor designed plants have max pressure limits due to motors on compressors sized on limits, so you have to run fans flat out anyway to keep them in limits.

    You still have to keep to basics as you suggest Segei.

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    Re: Floating head pressure? How it works

    Quote Originally Posted by RANGER1 View Post
    Segei,
    I would have thought that with a floating head pressure, with correct control of condenser fans, that this would be killing 2 birds with 1 stone.
    Its whole idea is run fans to suit the ambient conditionds of WB, DB.

    If you run the fans any faster than required, it will be a waste of energy, as it cannot be any lower!

    So to say if you drop head pressure by "X" amount it will defeat the purpose, won't it?
    I guess its how good your PLC or controller is set/programmed to achieve max results.

    In most systems, condenser fans are set to achieve a certain set pressure ie cut in, cut out or set to run flat out to get lowest head pressure.
    Floating head pressure if set up right, is a fine tune of above sentence to my understanding.
    Idea is to keep optimum(the best) head pressure as long as possible. At this pressure total(compressors +condensers) energy use is minimum. Typically, optimum head pressure float up(day) and down(night) or when weather change. It works fine during summer operation, but you need sophisticated PLC to control this floating.
    During periods of cool weather head pressure can not reach optimum because optimum is below minimum allowable head pressure. We should focused on lowering this minimum allowable head pressure and we will be able to run plant longer at optimum head pressure. There are several barriers to run plant at low head pressure but every barrier has a solution.

  14. #14
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    Re: Floating head pressure? How it works

    Quote Originally Posted by RANGER1 View Post
    I agree that you should try to lower head pressure as much as possible to save power, also wear & tear.
    Its mainly new plants that would have this floating system installed, not to many older installs would have it yet.
    Some older or poor designed plants have max pressure limits due to motors on compressors sized on limits, so you have to run fans flat out anyway to keep them in limits.

    You still have to keep to basics as you suggest Segei.
    During floating of head pressure PLC will switch off some fans or reduce fan speed, when head pressure is optimum. Typically, older plants have undersized condensers. Condenser fans can run at full capacity but optimum head pressure will not be achieved. To float head pressure, condenser capacity should not be undersized relatively to current refrigeration load.

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    Re: Floating head pressure? How it works

    Segei's description is correct and depending on just where he is in Canada his point on the value of Floating Head Pressure Control vs Minimum Tolerable discharge pressure is well taken: in come climates the circuit will spend more time at Minimum Tolerable than it will in the "Float" control interval so investing in high end "float" control will not be as cost effective as eliminating the causes for a Minimum Tolerable pressure thats higher than it needs to be.

    That approach may not be as effective in Miami or Houston. Or where your load is largely ambient dependent as it is for a fruit storage....So the economics does change with typical weather cycle and plant duty cycle. In the system design stage, the plant duty cycle can be hard to define; and the typical weather cycle may be hard to source. The meterologists, domestic heating and energy policy folks have got lots of data on degree-days but very little on grain-days (or wet bulb days). Establishing an optimum differential and cost-benefit for an air cooled circuit is pretty simple, but for an evaporative on remote sumps its considerably more involved.

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    Re: Floating head pressure? How it works

    Thanks...
    good information about floating head pressure

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    Re: Floating head pressure? How it works

    Actually, it is large topic. Sometimes floating head pressure can be very useful for the plants with significant fluctuation of the load and/or over sized condensers. I have several newsletters related to this issue and you can read them on my website http://www.skenergy.ca

  18. #18
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    Re: Floating head pressure? How it works

    Hi, HVACRsaurus

    Quote Originally Posted by HVACRsaurus View Post
    Hi Ranger,

    Page 8 through 12 gives a good overview.*

    Probably stuff that you already know, and obviously not the only way of "doing it", but a good document all the same.

    http://www.savepower.nsw.gov.au/Port...RprtLowRes.pdf

    Thanks for sharing ...

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

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