Temperature lift of a heat pump

[TSN79]

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?

 

[russ_watters]

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.

 

[TSN79]

"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.

 

[russ_watters]

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.