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Thread: constant flow control rather than constant pressure

  1. #1

    Default constant flow control rather than constant pressure

    For a ground source heat pump you need constant flow (not pressure).
    For multiple stages of heat pump (in parallel) you need variable flow (or staged flow).

    With a constant pressure system like Franklin Monodrive (variable speed drive) it is possible to regulate pressure, and then use multiple valves to change the flow for the fixed pressure. This either means wasting pressure at lower flow stages or using multiple pressure sensors (switched via a relay).

    Alternatively I wonder about just having a constant or staged flow system.
    I wonder if this could be done by replacing the pressure switch on the MonoDrive with a flow meter like this (http://www.proteusind.com/800_flow_m...S_800_001.pdf).

    Since the cost to me of adding a relay and extra pressure switch is less I will probably do that now. But if I was starting from scratch I wonder if using a flow meter in place of the pressure switch would work?

  2. #2
    DIY Senior Member Phil H2's Avatar
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    Either I am not understanding your question or your reasoning is flawed. My background is not with heat pumps nor wells. Rather, I have worked with other more complex pumping systems.

    If I understand your situation, you require a variable flowrate depending upon the number of heatpumps operating. A control system with a single flowmeter would be difficult at best. A single flowswitch would not work because the system has varying flow demands. A far simpler system is with a single pressure control. If the individual heat pumps have different demands or the piping isn't balanced, balancing valves (or a valve that can throttle flow easily) can be installed before each heatpump.

    If you already have the monodrive with a pressure control, I don't know what you hope to gain. Just set the pressure at the point that satisfies the most demanding heatpump running by it self. If this causes the other heatpumps to have too much flow when running simultaneously, then use manual valves to throttle their flow. If you want more control than that, you probably need something other than the monodrive. Some motor speed controllers can handle the logic and multiple inputs (signals telling which heatpumps are on) without an additional PLC or computer. I guess you could use the monodrive with a number of pressure switches (one switch for each combination of heat pumps) and relays or a plc to switch to the pressure switch required.

    I am not very familiar with Franklin's Monodrive nor its capacities. If it were me and I didn't have the Franklin unit or a single phase pump, I would investigate a more traditional motor speed control (VFD, VSD, pick the alaphabet du jour) with a pressure transducer and a 3-phase pump. Many companies make variable speed drives with an analog input for a transducer and may be programmed for a PID loop. Some of the smaller drives can use a single phase electrical input to supply a three-phase pump. The first caveat of this type of system is the pump would need to be sized to work with the varying speeds/flowrates; and the wider the spectrum of flow demands, the more difficult it will be to find a the right pump. The second caveat is you need to learn alot about this type of system or pay someone alot of money to design, program, and maintain it.

  3. #3

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    In the case of the flow meter that I linked to it appears able to support multiple trigger levels.

    I understand the alternatives, and my end system will be:
    Variable speed drive with two pressure switches controlled via by relay. Giving two constant pressures. This system will work nicely, and I'll adjust the two pressure points so that each pressure level is able to only just deliver the correct flow needed by the heat pumps (so no pressure is thottled or wasted, even though I have manual thottle valves).

    What I am wondering, rather hypothetically, is whether it is practical to control a variable speed pump inverter designed for constant pressure, with a flow meter, thus giving constant flow, instead of constant pressure. The issue of multiple stages was an unnecessary complication to my question.

  4. #4
    DIY Senior Member Phil H2's Avatar
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    Unless I missed something with the flowswitch in the link, it only has one set of output switch contacts. To switch between the 16 preset flowrates, a manual selector switch must be turned with a scewdriver.

    The company I worked for did not use many flowmeters. I don't know if the choice to use pressure transducers was driven primarily by cost, reliablity or another engineering concern. I don't know why a flowmeter would not work. But I would be careful if you are commited to the Frankin Monodrive. It would be worth a call to Franklin. I am not certain how they work. There may be something special about the switch they use.

    Everything I have seen for pump control used a 4-20mA analog signal rather than a switch contact. I can only guess how Franklin does it. Here is an analogy. Suppose you are driving a car on the freeway and you want to drive 65 mph. But instead of a speedometer, you have a light on the dash that illuminates when you exceed 65. Now, pretend that you can't see the hills, the only indication is a light. To make sure you are always driving 65 mph, you have to continually slow down and speed up so you can see the light go on and off. On the other hand, a transducer or flowmeter acts like a speedometer. The control unit will know what the current speed, and how rapidly the change in demand is occuring. With proper programming, it will react to changes as needed without hunting for the setpoint.

  5. #5

    Talking

    Phil, smart analogy! Made me laugh

    I read that Franklin swapped from using a pressure transducer because they have too much lag, but that's a whole new topic. Having a little lag wouldn't hurt for a heat pump, but for a house supply it could be an annoyance when taking a shower. So maybe for heat pump applications an alternative make variable frequency drive to Franklin would be better, one that uses a transducer.

    Anyway your answer is clear.

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