Liquid Injectors and how to make really hot water with a heat pump.
Graham Hendra
Heat pump product development engineer. Author of heat pump text books. Heat pump builder.
Heat pumps are getting hotter. Its a fact of life and I think its really good news. Higher temperature heat pumps which can literally be installed as a direct replacement for a boiler running at the temperatures people want and are used to makes perfect sense to me.
When I launched Samsung units into the UK in 2010, the units were amazing but they used R410A as a refrigerant so they were only really able to heat water up to 50 degrees C. It was standard technology back then and was completely normal. So we all embarked on our lower temperature crusade.
The only real technological change we have seen in heat pumps over the last 12 years is hotter heat pumps being available because new refrigerants are available. Its pretty common for the current batch of beige boxes from all the suppliers to be able to now give 60 degrees C water or 65 if its above 10 degrees C outsides.(trust me check the data). They use Refrigerant R32.
There have always been Super high temp units from Daikin and Hitachi but they were special cases using two heat pumps helping each other. I blogged about them here. https://www.dhirubhai.net/pulse/is2-heat-pumps-cascade-graham-hendra/
With the new super refrigerant R290, (barbeque gas to the layman) we are seeing water temperatures in 70 -75 degree C range and on hot days up to 80C.
But there is a problem. Every heat pump has a compressor in it. I talked about them here https://www.dhirubhai.net/pulse/who-makes-compressor-unit-mate-graham-hendra/
Compressors are lubricated with oil, the oil only works if its not too hot, ideally below 100 degrees C. The problem is the refrigerant and oil leaving the compressors can be very hot, typically 35 degrees C hotter than the leaving water temperature. It doesn't take a genius to work out that if you want water temperatures of 80 C the hot refrigerant and oil leaves the compressor it could be 115 degrees C or higher. And that's not good for compressor life.
Compressors can only compress gases, they cant compress liquid, so we fridge engineers do everything we can to stop liquid refrigerant entering the compressor. If liquid gets in the compressors get smashed to pieces. Its like pouring gravel in your car engine. Not advisable.
But if its controlled very carefully you can squirt a small amount of liquid refrigerant onto either the compressor part itself or into the gas coming back to the compressor to make it much colder. We call this liquid injection. Its the secret to getting higher temperatures and long compressor life. There are basically 2 ways to do it:
The cheap way, using an external valve and squirting cold liquid into the gas coming back to the compressor to pre cool it. like this.
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Or on high end units (RED etc) they have a special extra port on the side of the compressor allowing the liquid refrigerant to spray directly onto the hottest parts of the compressor.
Both have exactly the same effect, the compressor outlet or discharge temperature is controlled and kept below 100 degrees C, the oil continues to work and the compressors dont fail.
Now just before the marketeers jump on this and tell you how new and groovy it is, liquid injection has been used in aircon for years. It fell out of favour about 25 years ago but it looks like its back. Nothing like reinventing the wheel.
Divisional Director at Wates Group
2 年Absolutely love your posts Graham
Green Chemical Engineer
2 年Seems the wrong way to go, high temperatures and little squirts, simple is usually beautiful. More complicated and worse COP than a lower temp heat pump. The more I read about such issues the more I feel a simple high COP heat pump up to 40 or 50 C and an immersion to do PV legionella cycles slightly higher might work better for me.
Metering infrastructure and meter data management services for developers, building managers, and utility companies
2 年Benefit - compressor cooling Drawback - ? (how big a hit does COP take?)
Chief Executive at Forest Holme Hospice Charity
2 年Oliver O'Hare
Ultra [email protected] - Residential ultra-high efficiency Cool Heat + hot water production . Ultra-starts where others loose site of Efficiency .
2 年Interesting , I must wonder if its much like producing cold energy , effort needed to reach lower subgrade temperatures ultimately uses more energy and Mindsets go to believing its what we need , Human skin feel the effect of water above 38 C , I imagine this article is about high temp for sterilization duties ? Does it draw energy to reach such high temperatures or have they found a way to reuse heat not talked about to reapply it ? Old saying goes you get something for nothing !