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  1. #1
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    Lightbulb Large tonnage De-coupling

    Can any body explain the basic points for a large tonnage district / Zone A/C cooling plant's ( Centrifugal - water cooling system )De-coupling with VFD secondary pumps. And how important is the Supply and return Diff Pressure.( IN every site . i:e in primary and every where in the secondary load site)
    Last edited by MANIS; 14-08-2004 at 02:59 AM.

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    Re: Large tonnage De-coupling

    Ya Marc ;
    you are very much clear to the point , Actually i have ask this to know " due to any / slightly changes in the main plant DP ( even for a 0.01 Bar ) more or less or for run-test any main secondary VFD pump in manual speed for a while it disturb many distrct buildings tertiary site flow and gives very hard time to settle the temperature.
    one of my query is what should be the ratio or setting in betwen the main plant out put Diff-Press and tertiary site Diff-Press. (any rough caculation )

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    Re: Large tonnage De-coupling

    Do all the braches have a metering valve (don't know the correct name for a 'strangenregelaar') wherbey you turn in via a handwheel a certain DP before or after the AHU so that you can balance the whole system?

    That's one I found via Google http://www.hga.honeywell.de/pdfs/v50...5ge25r0702.pdf

    I doubt that 0.01 bar give such a problems. There must be other reasons. Are your PID algorithm settings on the VFD not too narrow setted?
    Why not increase slightly main pressure on the system. It will increase of course slightly power consumption but if the actual settings give you such a headaches...?

    One of the benefits of a hydronic circuit is that it can be controlled/balanced very well and that it forgives more faults then with a VRV or VRF system.

    Try to monitor pressure on secundary circuits while you change on the primary side. You will notice - I think - much larger variations. You only can be sure if you measure, otherwise it's guessing.

    As long as your pressure drops over the AHU remains the same, your flow will remain the same.

    How is each AHU coil controlled and balanced for the moment?
    When an AHU has reached setpoint, what actions are taken now?
    Are there 2 or 3 ways valves fitted? Because you spoke of a VFD pump, I think they fitted 2-way valves.


    We installed twice a Carrier Chiller of +/- 500 kW (production machine of Rieter) with +/- 30 branches on 2 extrusion machines (for some sort of very thin polyprop wire). All the cooling coils where independent from each other regulated. We had also a VFD controlled pump and we never had any problems with it.
    Last edited by Peter_1; 10-08-2004 at 06:49 AM.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Large tonnage De-coupling

    The link you posted looks like what we would call a balancing valve Peter. You can see the gauge ports each side of the orifice plate.

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    Re: Large tonnage De-coupling

    If you don't install this balancing valves on a plant with different coils, you'll never be able to balance the system correct.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Large tonnage De-coupling

    Solutions that involve mixing return water with supply water undermine the thermodynamic efficiency of the system, destroy the capacity of the coils to meet their loads, and add further to low delta T problems. To solve the types of distribution problems that lead to low delta T, the design or retrofit needs to follow these rules:
    1. Eliminate all possibility of direct mixing between chilled water supply and return: This means eliminating all decoupling lines and three way valves. In this era of networked DDC and variable speed control, pumping systems no longer need to be decoupled. Furthermore, modern chillers accommodate varying flows over substantial ranges without any loss of efficiency or operational stability. By selecting equipment wisely, it is not difficult to design "all-variable speed" chilled water generating and distribution systems without any mixing so that every bit of supply water must pass through a load before returning to the plant and supply chilled water at design temperature is available to all loads at all times.
    2. Employ a direct coupled distribution system: This means that when multiple pumping circuits are employed they need to be connected directly in series rather then isolated with the use of decoupling lines. Primary/Secondary systems become "Primary/Booster" systems in which "all-variable speed" pumping stations are operated in series. Such systems are extremely effective and can save capital cost when compared to decoupled Primary/Secondary schemes because Primary/Booster configurations can incorporate built-in backup without the need for redundant equipment.
    3. Focus delta T attention at each and every load: Once decoupling lines and three way valves have been eliminated, the only source of low delta T problems is overflow through individual loads. Overflow can occur because of improperly sized valves and varying pressure differentials across valves. It can also occur when the air side of cooling coils becomes clogged or other maintenance failures take place. A simple means for preventing overflow is to install a temperature sensor on the return water line at each load and to use this temperature as a limit for the control valve serving the load. When the return water temperature falls to approach the design leaving water temperature for the coil, the valve is limited from opening further. This step eliminates the problem of low delta T at the load and gives the designer a little more flexibility in sizing valves for each load. The simple logic that limits the valve operation can also be employed to notify the operator that a maintenance problem may be affecting the operation of the load.
    For a full read go here :http://www.automatedbuildings.com/frame_resources.htm

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