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Temperature lift of a heat pump

  1. Dec 15, 2008 #1
    I've always been told that systems that involve a heat pump need to be designed for a delta T of about 10C, not the more usual 20C, apparently because the pump can't lift the temperature more than this. Is this correct, and why is this so?
     
  2. jcsd
  3. Dec 15, 2008 #2

    russ_watters

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    Staff: Mentor

    What is "usual"? Water-based heat exchangers (ie, in chillers, water source heat pumps, cooling towers, etc) are typically designed for 10-14F delta-t's. Even for airside heat exchangers of the cooling variety (dx or chilled water air conditioning coils) 20C is a huge delta-T for air for an HVAC application: 20 F is more typical.

    The approach temperature would be the reason for all of that and for a heat pump in particular, the efficiency is highly dependent on the difference between the hot and cold sides.
     
  4. Jan 20, 2009 #3
    "Usual" is my opinion of a system that uses boilers, gas, or whatever for heating. Here I use a delta T of 20C. But, as soon as a heat pump is to take care of it all, I'm always told it must be designed with a lower delta T, i.e. 10C. I was just wondering why this is.
     
  5. Jan 20, 2009 #4

    russ_watters

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    Some devices are more sensitive to temperature variations than others: whether you send 90 C water or 80 C water to a radiator, the difference in performance will be small - maybe 10%. But if you send 0 C water instead of 20 C water to a heat pump (in heating mode), you will utterly destroy your efficiency. Refrigeration cycles require very specific operating temperatures to run efficiently - fuel-fired heating systems are a completely different animal. Many heat pumps list performance at different temperatures - have a look at how fast the output changes with it.

    Still, it surprises me that you would design a boiler plant for a 20 C temperature rise. They are typically rated for much less for safety reasons: i've seen boilers melt their burners due to insufficient water flow and you also don't want to accidentally boil the water.

    A gas furnace, on the other hand (if that's what you are talking about) is designed to operate at an extremely high temperature (stainless steel burner), so it can handle a very wide range of temperatures: you might see a 40 C delta-T for the air going through a gas furnace. As long as you keep the output temp below about 60 C (so you don't burn anyone), you can do just about whatever you want with it.
     
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