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  1. #1
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    Effect of inverter on Pressure Ratio



    Hello Everyone!

    I just joined this forum as it seams there is a lot of knowleadgable people over here...

    I am learning how heat pumps work and I had a few questions where I could not find answers anywhere...

    I understand that an inverter varies the speed of the compressor to match the needed capacity.

    Now i also understand that the speed of the compressor will affect the pressure and the flow.

    So my question is: if the compressor is spinning at 50% of its speed. I assume the pressure is going to be also 50% of the maximum. The flow will probably be 50% as well.

    Now I don't understand what happens with the expansion valve....It is my understanding that the pressure drop increases with the flow increasing...Now reducing the flow and pressure on one side of the valve being reduced by 50%, what will hapen on the other side of the valve....

    For example at 100% compressor speed with 400PSI on the high side and 200 Psi on the low side. The pressure Ratio is 2.0.

    If the high side becomes now 200PSI, what will be the low side pressure?? Will the pressure ratio remain the same and then we get 100PSI????

    If the Pressure ratio changes, how do we control it???I also understand that this will greatly affect the COP

    I don't know if my question is clear or not....if you don't understand I can probably reformulate a little bit

    Thanks everyone for your time!



  2. #2
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    Re: Effect of inverter on Pressure Ratio

    You will find that most inverter driven AC units will probably have electronic expansion valves which are controlling the temperatures within the system.

    The compressor speed is controlled according to that temperature demand not the other way around.
    Brian - Newton Abbot, Devon, UK
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  3. #3
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    Re: Effect of inverter on Pressure Ratio

    So its like an advanced TVX with electronic control?

    I am guessing you control the pressure drop and the flow??? Where can I find more info about those expansion valves??

    Does it provide pretty accurate control? Say at a given outside temp I want a Pressure ratio of 3.1 and at different outside temp I want 2.6, this would be possible while still maintaining adequate superheat

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    Re: Effect of inverter on Pressure Ratio

    All split systems & VRV / VRF systems with inverter drive compressors have electronic control & pre programmed control stratagy using micro processor electronics.
    These systems range from the cheapest domestic split system with very simple control stratagy through to very complicated multi split VRF system with very complicated control stratagy & many temperature & pressure sensors to provide exact data for contol.
    The electronic expansion valves can have very exact contol.
    For instance the EEV ( electronic expansion valve)valves on many VRV / VRF system indoor units have 2000 possitions from fully open to fully shut but the actual needle valve only moves less then 2mm. So when the control stratagy makes a change in EEV valve opening possition it can move the needle with resolution down to less than one thousanths of one mm.

    So for instance
    Most VRF indoor units will have a pre determined target super heat for cooling mode which is programed into the control staratagy on the control circuit board.
    The indoor unit will constantly monitor two indoor unit heat exchanger temperature sensors & calculate the SH & then ramp open or close down a little to make the target SH.
    To help prevent overshoot 'fusy logic' is often deployed so the unit do not just consider the current data but look at a trend over a period of time to decide what LEV valve opening is required to achive target SH.
    At the same time the outdoor unit will trim the compressor speed & outdoor fan speed to meet a number of targets. The main one will be suction pressure / suction SH for cooling mode.
    Most VRV / VRF systems will also have a liquid line sub cooler circuit with EEV & the outdoor unit will also constantly trim the subcooler EEV to maintain the target liquid line subcooling.

    Clearly the part load COP of inverter compressor systems is dependant on the inovation deployed in the electronic control stratagy.
    Therefore manufacturers are reluctant to provide detailed information as it is regarded as trade secret.

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    Re: Effect of inverter on Pressure Ratio

    As long as you have superheat and subcooling you have saturated refrigerant in heat exchangers. Pressures are dependent on air in temperatures and TD of heat exchangers. Only flow of refrigerant varies.
    Last edited by nike123; 15-09-2009 at 04:27 AM.

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    Re: Effect of inverter on Pressure Ratio

    some of these units also have special service modes you can put them into that will keep the compressor and both fans at 100%.. the EEV still operates.. but in this mode the units tend to "look" to the technician more like a standard TxV type of system when the gauges are on...

    even some of the higher end units will allow you to read out the coil inlet and outlet temperature on each coil as well...

    -Christopher

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    Re: Effect of inverter on Pressure Ratio

    Quote Originally Posted by cadillackid View Post
    some of these units also have special service modes you can put them into that will keep the compressor and both fans at 100%.. the EEV still operates.. but in this mode the units tend to "look" to the technician more like a standard TxV type of system when the gauges are on...

    even some of the higher end units will allow you to read out the coil inlet and outlet temperature on each coil as well...

    -Christopher
    Yes alot of the Real inverter Brands have a Service mode for checking SuperHeat
    If its Does'nt Work Use a bigger Hammer

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