Rotary vs scroll compressor: what it tells you about the heat pump in front of you
The compressor is the heart of the heat pump: it is what raises the heat taken from the outdoor air to the temperature needed inside the house. Not all compressors are the same, though, and the type a manufacturer chooses says a lot about the unit it has in mind.
01
Four families
Compressors for cooling and heating come in four types. Piston and screw compressors are those of large industrial chillers, hundreds of kilowatts: we do not consider them here and we do not use them in our units. From the home up to industrial systems of 50-100 kW the other two are used: the rotary, with one rotor or two (the twin rotary), and the scroll.
02
The rotary: the one in air conditioners
The rotary is the compressor found in almost every air conditioner. Inside a cylinder an eccentric rotor turns: in the smallest air conditioners there is a single rotor, in larger ones there are two, offset to balance each other, and that is the twin rotary. It is light, small, inexpensive, and for a home it is fine: at small sizes a good rotary performs as well as a scroll. It has two limits, though. It is sensitive to liquid: a compressor should only draw in gas, and the rotary needs a tank on the suction side, the accumulator, which evaporates the droplets before they reach the cylinder. And as displacement grows, as industrial-building units require, it starts to vibrate: the bigger the eccentric rotor, the more it unbalances the machine.
03
The scroll: two spirals chasing each other
The scroll is made of two spirals, one fixed and one that orbits inside it without rotating on itself. The gas enters from the outside, is trapped in ever smaller pockets and leaves compressed from the centre. The movement is continuous, without knocks, and there are few moving parts. Three advantages follow. It is robust: the spirals tolerate a little liquid better, which the rotary suffers from. It does not vibrate even at the large displacements of industrial units. And it performs well across the whole operating range, which is what counts in a heat pump that goes from winter to summer.
04
Vapour injection
There is a fourth advantage, which on R32 units weighs more than the others: the scroll can have vapour injection. R32 is a refrigerant that leaves the compressor very hot. When it is very cold outside, or when very hot water is needed inside, the compressor has to raise the pressure a lot: the compression ratio grows and with R32 the discharge temperature climbs too high, which damages the oil and shortens the life of the unit. Injection sends part of the refrigerant, already cool, into the spirals halfway through compression: the discharge temperature drops, the output available in the cold rises, and the compressor always works within its limits. With propane, R290, the problem does not exist: the discharge temperature stays low and injection is not needed.
05
What we use
In our units we use almost exclusively scroll compressors, for their robustness and their efficiency. All our Kita air-to-air units on R32 have a scroll with vapour injection: it is one of the reasons why they heat well even on the coldest nights, and why units installed years ago still work as on day one. Our air-to-water units run on R290 and do not have injection, because with propane it is not needed.
06
How to check, before you buy
- ask which compressor the unit has: rotary or scroll, and what brand;
- if the unit runs on R32, ask whether the compressor has vapour injection, especially if it will work with low outdoor temperatures or high-temperature hot water; with R290 it is not needed;
- for an industrial or large building, ask the compressor’s displacement and how it is mounted: at those sizes a rotary vibrates.
07
Frequently asked questions
Scroll or rotary compressor: which is better?
For a small house a good rotary performs as well as a scroll. At large displacements the scroll is sturdier, does not vibrate, tolerates some liquid better and can have vapour injection. In our machines we use scroll compressors almost exclusively.
What is vapour injection?
It sends part of the already cold refrigerant into the scroll spirals halfway through compression: the discharge temperature drops and the output available in cold weather rises. It is needed with R32; with propane, R290, it is not.
What compressor do Templari heat pumps use?
Scroll. The Kita air-to-air units with R32 have a scroll with vapour injection; the air-to-water units use R290 and have no injection, because propane does not need it.
Why does a large rotary compressor vibrate?
The rotor is eccentric: the larger the displacement, the more it unbalances the machine. For an industrial building, ask for the compressor displacement and how it is mounted.
See also: R290 heat pump: what is inside a propane air-to-water unit · Hybrid boiler and hybrid heat pump: what they really are, and why we do not make them · How long do heat pumps last: years of service, what wears out and what makes them last longer
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