Alternative expansion valve in heat pumps

  1. Low-Q

    Low-Q 258
    Gold Member

    I have been looking at the basic operation of a heat pump. On these pumps there is an expansion valve. This valve is a narrow hole where the condensed working fluid is passing through, then evaporation happens in the low pressure side - after this hole.

    I assume this valve act as a resistance, and therfor I assume it also result in loss of energy.

    Would it be better to use an inverted pump to do the same job instead?
    I have been thinking of something similar to the motor in pneumatic tools - but instead this will be a hydraulic driven motor as the condensed working fluid is liquid. So using this kind of motor as a replacement of the expansion valve, the liquid pass through this motor, and the motor will start to rotate. This motor is some how mechanically connected to the pump, so the pump will require little less input to compress the working fluid (?)

    I understand that this new expansion valve (motor) must restrict the flow in order to maintain the different compression rate in the two sides, and therfor have less "pump capacity" than the pump. But it would not cause as much loss as a traditional expansion valve will do. Or what?

    The compression difference between the two sides I have read is about 1 to 5 at normal operation. However, this relationship will depend on the outside temperature and the wanted inside temperature, and the type of heat pump. So if I look at an ideal heat pump, the efficiency will increase with about 20% or so with the alternative expansion valve.

    Do this reasoning looks rational?

    Vidar
     
  2. jcsd
  3. russ_watters

    Staff: Mentor

    Not "inverted pump": turbine.

    No, a turbine would not be better for typical refrigeration because it just adds expense and complexity for little energy recovery.

    http://www.learnthermo.com/T1-tutorial/ch10/lesson-B/pg02.php

    But for certain, specialized applications it may make sense:
    http://en.m.wikipedia.org/wiki/Turboexpander

    Not a bad thought though.
     
    Last edited: Jan 29, 2013
  4. Low-Q

    Low-Q 258
    Gold Member

    TURBINE ofcourse :smile:

    I was also thinking of a turbine'ish thing but more like the ge-rotor below, except the oil...:

    [​IMG]

    Such "turbine" do not need much velocity of the fluid in order to start moving. It starts moving with acceptable torque almost at the first sign of pressure difference, and probably quite efficent for this application.
    A "regular" turbine (As I understand them) have free passage of fluid even if they are not rotating, and very inefficient for low velocity flow (Apologize for the terms if they turns out to be a little "loco").

    Thanks for the links - I did suspect that others than me had thought out the same idea, or at least a similar one, but I could not find the right word or phrase to google.

    Vidar
     
  5. jim hardy

    jim hardy 4,805
    Science Advisor
    Gold Member
    2014 Award

    The residential heat pumps i know of use phase change to effect movement of substantial heat.

    In the 1880's marine shipboard refrigeration used air as a working fluid, which made possible the meat trade between Europe and S America. They used a steam driven compressor that compressed air to around 100 psi, cooled the air back to ambient, then let it expand under the piston doing the initial compression. The work done in expansion cooled it to around zerto F. And it reduced the amount of steam required.

    Nowadays passenger airplane air conditioners use the same principle but with turbines instead of reciprocating equipment.
    http://www.boeing-727.com/Data/systems/System Descriptions.pdf
    If you ever used air tools you know their exhaust is cool.

    So your premise is fine. But i can't put a number on how well it works compared to a phase change cycle. Your thermo skills are doubtless more fresh than mine.

    old jim
     
  6. Low-Q

    Low-Q 258
    Gold Member

    Thanks for the link! I will get time to read it ALL :-) Interesting. I'm a noobe, but have some imagination :smile:

    Well, I would prefere a gerotor design for the pump as well (and not turbine or reciprocating equipment) because the compression rate can be kept stable even at low input work due to very little leakage, but also be able to shorten the time from increasing input work to increasing output heat if both expansion valve and compressor has the same gerotor design. A turbine will "leak" at low rews. Maybe not a problem, but I guess the gerotor is the most efficient, and least expensive at both purchase price and maintenance than a "turbo charger" or reciprocating equipment. I see, however, that turbine equipment is commenly used in the industry. Big scale compressors might be easier to build as "turbo chargers" rather than gerotors or reciprocating equipment...

    PS! Phase change also applies to the gerotor design - just for the record....

    Vidar
     
  7. jim hardy

    jim hardy 4,805
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    2014 Award

    I always admired a small georotor driven cooling fan that was on an air compressor aftercooler at work. This was an outdoor installation in humid south Florida. Motor was about 5" diamter.
    It ran for years unattended. The little air motor and its exhaust pipe were always coated with ice. On those 100 degree 95% humidity days it made you feel cooler just to look at it.

    Thanks - i hope this thread continues. I suppose you could experiment with an inexpensive air tool like die grinder or drill; i think they're a positive displacement motor. Imports start around $10...

    old jim
     
  8. Low-Q

    Low-Q 258
    Gold Member

    I already have a gerotor oil pump which I purchased on ebay. Fascinating how these pumps are designed. Works as "pistons" that has alternating "cylinders" just by rotating it. It is a positive displacement pump.
    I haven't yet started to experiment much with it, but it seems definitely to have a great potential as an expansion valve which can save some energy consumption of the pump - if its output work could be fed back to the pump.

    The pressure inside the high compression chamber will ofcourse power that expansion valve, but since the working fluid is liquid at the rotary expansion valve, it cannot move the same volume as the pump does. If it wasn't for the liquid fluid, the rotary expansion valve could feed back most of the energy that is required to run the pump - but then it would result in a very inefficient heat pump. Maybe you know this better than I do.

    Vidar
     
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