Andy Schoen's Receiver Subcooling Article

[DTLarca]

This matter of subcooling and receivers is a good example of the nonsense that pervades our industry and probably because historically those who write the books and design the courses just do not properly know what they are talking about. I know because I have scrutinised nearly all the books and have presented some of the key courses myself.

Andy Shoen is correct - and I want to make more salient some of the key principles. But I will get a little more technical in the article version in a few months time. I just want to give at least Mad Fridgie something to think about since he has shown a little skepticism toward Andy's arguments.

Here's the article in question...

http://www.sporlanonline.com/Februar...Cold%20WAR.pdf

I'm just going to start off with the main and very basic points - in case no one is interested anyway.

Consider water in a pot on the stove. It has a vapour pressure always. When that vapour pressure equals atmospheric pressure the water boils. The water boils at 100°C at atmospheric pressure. At atmospheric pressure it cannot properly boil at any temperature above 100°C. This is to say that when adding heat the water rises and rises but eventually reaches a certain temperature above which it cannot go - this highest temperature is a function of the prevailing pressure.

What is important to notice here is that I am talking about things as they happen when we are ADDING heat. Not removing heat. ADDING heat. We generally expect that no matter how fast we add the heat the water will not rise above 100°C - this is a reasonable expectation to have when thinking of how things are within the confines of refrigeration systems.

Because the vapour at 100°C is leaving the water body the vapour just above is also at 100°C. This vapour rising above the water will have a pretty constant density.

Now here is a key point overlooked relevant to scenarios where we are going in the opposite direction - its oversight is the source of confusion - which is a principle not brought into salience by Andy.

Consider a rigid container with 1 bar of nitrogen gas at 20°C where the weight of gas is 1kg. I can double the pressure of the gas by either halving the volume of the cylinder slowly allowing the gas to cool thus also maintaining a gas temperature of 20°C. I can also double the pressure of the gas by raising its thermodynamic (absolute) temperature - say 290K to 580K. But I can also double the pressure of the gas by simply adding another 1kg Nitrogen or by adding 1bar of any other gas. If I added another 1kg of Nitrogen and thus another 1 bar of nitrogen then I would have doubled the density of the gas. So I can have double the pressure of the same gas while maintaining the same low temperature - just that the density of the gas has increased (doubled in this case). This is very important to picture mentally.

When we are removing heat from a liquid vapour mixture at a constant pressure, as happens to say R22 in the condenser, the liquid and vapour temperature is by no means prevented from dropping below the saturation temperature. Who says just because when you are adding heat the liquid cannot reach a temperature above saturation that when you are removing heat the temperature cannot drop below saturation? It just tends not to in the early stages of the condenser. Hell, we regularly see 4K subcool with vapour presence in liquid lines - even slow liquid lines where there is minimal vapour carry-over occurring on account of sweeping.

All that happens then - toward the end of the condenser - is that we have a cooler but higher density vapour above the subcooled liquid where it simply is the case that both the liquid and the vapour are subcooled for the existing pressure. The higher pressure in the condenser/receiver is being caused by the hotter, higher pressure, lower density gas coming in the from the compressor and also that at saturated condensing temperature where all the condensing is taking place. But the vapour with the liquid toward the end of the condenser and in the receiver is equal in temperature with the liquid - its density is just greater having been compressed by the higher temperature, less dense, hotter vapour entering the condenser. Any heat that tries to get to the liquid at the end of the condenser from the hot gas at the beginning of the condenser is quicker removed along its way by the cooler heat exchanger walls than it is conducted by the separating liquid vapour bridging beginning to end.