Refrigerants will always migrate to the coldest point within the system; this is important to understand with condensers operating in colder climates as all the refrigerants could be sitting outside.As many know, cold weather can have a significant impact on refrigeration systems. It is time to brush up on our cold weather refrigeration system operation. There are several reasons why cold weather presents a challenge and the types of challenges we face depend somewhat on the type of system and expansion device being used.
To understand one of the more fundamental reasons that cold weather causes problems, it is useful to look at how refrigerants move between the components in a closed system. Figure 1 shows a simple case of two refrigerant vessels that contain R507A, connected with a closed valve. The left vessel is at 70F and the right vessel is at -20F. Since both vessels are saturated, the left vessel is at a pressure of 153 psig and the right vessel is at a pressure of 18 psig.
Figure 1: Two refrigerant vessels at different temperatures connected with a closed valve. The left vessel is at 153 psig and the right vessel is at 18 psig(R507 refrigerant).
Understanding pressure
If the valve is opened, the vapour from Vessel 1 will flow to Vessel 2 because there is a pressure difference. As the refrigerant leaves Vessel 1, the pressure will decrease. Since the refrigerant is still saturated, this decrease in pressure causes a decrease in temperature and a decrease in temperature means energy is removed. Energy could be removed by something external like blowing cold air over the vessel or energy could be removed by an internal change in the system like the boiling of some of the refrigerant.
At 70F, it takes about 60 BTUs per pound to boil R507A; so, as the pressure decreases, the refrigerant is boiling to make the rest of the liquid colder. Anyone who has ever added refrigerant to a system can see this first hand because the refrigerant tank gets cold while you are adding the refrigerant.
Similarly, as the refrigerant flows into Vessel 2, the pressure increases. Since the refrigerant is saturated, this pressure increase must cause an increase in temperature. The only way to add a significant amount of energy into Vessel 2 internally (not by doing something external like blowing warm air over the vessel) is for the refrigerant to condense. At -20F, 80 BTUs of energy would be added to the vessel for every pound of refrigerant that condenses. This interaction is probably the hardest to understand but both the boiling and condensing are necessary in order for the refrigerant to achieve steady-state while being saturated.
Figure 2: Refrigerant moving from a warm vessel to a cold vessel.If these vessels were not subject to anything external, they would find an equilibrium where the exact amount of refrigerant moved to balance the temperatures/pressures. This isn’t something you would ever have to figure out in the field, but Figure 2 shows what would happen in this particular case if the two vessels had absolutely no external influence at all. Vessels starting at 50 per cent full would end up equalizing with about 1/3 of the refrigerant moving and at a temperature of 30F. If you are trying to speed up a recovery job you might want to start with a cold tank!
Indoor/outdoor components
In the real world, there are always external influences. What if we had the scenario where the indoor “vessel” was a walk-in cooler evaporator and the outdoor “vessel” was a condenser, and imagine the system was off?
A remote condensing unit using a single-valve flooded head pressure control (To be discussed next article)In this case, the evaporator fan would constantly be blowing 35F air over the coil and the condenser would be sitting outside in the cold ambient temperature, say -20F. In this case, the refrigerant will move, just the same as it did in our example, but it will have to keep moving because the cooler is always adding heat (since 35F is much warmer than -20F) and the condenser is continuing to remove heat. Eventually, the bulk of the refrigerant will end up wherever it is coldest. Unless something is done to stop it, refrigerants will always migrate to wherever it is coldest in the system.
This is important for us to understand because it means that in the winter, it is very easy to have all of your refrigerant sitting outside in a condenser. When this happens, the system will often not start properly.
Another consequence of this is that when compressors are sitting off outside, the refrigerant likes to find its way to the crankcase. When the compressor starts, the pressure in the crankcase drops and all the refrigerant flashes and washes the oil out of all the places it’s supposed to be lubricating the compressor.
Thermosyphons
What we have described so far is exactly why thermosyphon cooling works. While most common in ammonia-oil cooling applications, there are several direct thermosyphon cooling applications in use and CO2 thermosyphons are showing up in some neat places like data centers. Figure 3 shows how a thermosyphon works by using the density difference between liquid and vapour refrigerate and gravity, to have the migration happening in a cycle with no compressor or pump.
Figure 3: Basic Thermosyphon arrangementHead pressure control
Another component of cold weather operation to understand is head pressure control. This is related to what we’ve discussed so far but occurs when the system is running.
All metering devices require a pressure drop across them in order to be able to control the refrigerant flow into the evaporator. Newer balanced port TX valves and electronic valves have decreased this pressure drop requirement a lot in the last 10 to 15 years but nonetheless, there is still a need to have a high enough condensing pressure.
I like to look at the head pressure control problem by approaching it from the extreme. Imagine an R507 cooler operating at 25F SST and a suction pressure of 65 psig. Typically, condensers are sized for a 10 to 20F TD (i.e., the temperature difference between the condensing refrigerant and the outside air is usually about 10 to 20F). In many climates in Canada, this means that the condenser was designed to reject heat to a 95F ambient by condensing at 105F or higher.
The math doesn’t hold exactly, but for our purposes, we can track this relationship linearly. This means that if the condenser was in a -20F ambient, it would be able to reject enough heat at -10F condensing. At -10F condensing, R507 would have a pressure of 25 psig, which is lower than the 65 psig we needed for a suction pressure. This would obviously not work and as we learned earlier, all of the refrigerants would simply end up in the condenser while the compressor was stuck running doing nothing and ultimately tripping off on low pressure.
This scenario is exactly what happens if we do nothing to manage low head pressure in cold weather operations and it doesn’t need to be -20F outside to get in trouble. In fact, you have to start paying attention to head pressure in ambient conditions between 60F and 70F in many cases.
In the next issues, we are going to explore techniques for managing the condensing pressure in refrigeration systems, including condenser flooding controls, discharge pressure regulators, and reducing condenser surface area. We are also going to look at some of the cold weather accessories that are used on commercial equipment such as receiver heaters and crankcase heaters so that we can understand when we need them.