In the last issue, we discussed the theory behind why operating refrigeration systems in low ambient conditions can be challenging. In this issue, we are going to discuss several of the methods used to control low ambient condenser operation.
Recall that we need to maintain an adequate pressure difference between the high side of the system and the low side of the system for proper operation. The challenge with low ambient conditions is that the condensers reject too much heat and therefore the refrigerant gets cold. Since the refrigerant is saturated, this means that the pressure decreases. In cold weather, refrigeration systems won’t work without some sort of intervention to make sure we can control the pressure on the high side of the system.
This is a bit simplified, but basically, the amount of heat a condenser can reject follows this formula:
The convective heat transfer coefficient is determined by the physical parameters of the condenser (ex. the fin material, the number of fins per inch, the tube diameter, the number of tubers, the tube layout, the shape of the fins and tubes, etc.) and the physical parameters of the air going through the condenser (ex. the velocity of the air, the mass or volume or air, the flow characteristics of the air, etc.). Once a particular condenser is built, the amount of heat depends only on a few parameters—the temperature of the refrigerant and the air, the mass of air moving through the condenser, the velocity of the air moving through the condenser, and lastly, the surface area of the condenser.
The temperature of the refrigerant and of the air are items we cannot really change at will. The temperature of the refrigerant depends on the condensing temperature, the physical characteristics of the compressor and the compression ratio. The temperature of the air is, well, the temperature of the air. As a short aside, there actually are systems that control the temperature of the air through the condenser by mixing outdoor air with indoor air, but in these cases, the systems aren’t really operating in a low ambient, so we are going to ignore them.
Controlling the air
The simplest way of decreasing the capacity of a condenser is to simply turn the fan off. With no air moving through the condenser, the heat transfer decreases rapidly. Since there is a lot of change in condenser capacity at once with this method, the condensing pressure can be quite volatile. The colder the air the less effective this method becomes because even a single fan cycle on can drive the pressure too low. On multi-fan condensers, this process is a little bit smoother because the fans can be shut off in sequence and each individual fan has less effect on the overall airflow than units that only have one fan.
Obviously if shutting a fan off can control the condensing pressure, so can slowing a fan down. This is most commonly done with a control system that is measuring the condensing pressure and modulating a control signal to control VFD motors on the condenser fans. There are a couple of non-VFD methods of doing this but they are hard on motors.
Using fan cycling and fan speed to control condensing temperature works in moderately cold climates fairly well providing the refrigeration load on the system (i.e. the amount of heat we are absorbing in the evaporators) remains fairly high and the system is running at high capacity even in the winter. If we have to run at low capacity in the winter or at really low outdoor temperatures, modifying the airflow is usually not enough.
Controlling the Surface Area
The only other tool we have available is to modify the surface area or the physical size of the condenser. It might seem like this isn’t really possible but there are a couple of different methods we can employ here. One is condenser splitting, which just means that we design the system with multiple condensers and isolate one of the condensers when it becomes cold outside. This is done manually by closing valves in many industrial plants and automatically with solenoids in grocery store-type systems. These types of setups also have limitations, but when combined with fan cycling or speed control can significantly increase the outdoor temperature range in which condensers can operate effectively.
Figure 1 shows a basic schematic of condenser splitting.
The second method we have available to decrease the surface area of the condenser is less intuitive. Instead of physically taking away surface area, we can fill the condenser with refrigerant. During normal operation, the refrigerant is de-superheating over the first part of the condenser and condensing throughout the rest of the condenser, with only the last little bit of the condenser completely full of liquid (see Figure 2 for clarification).
Imagine if we somehow could hold the refrigerant in the condenser and fill it up like shown in Figure 3. In this case, the refrigerant has much less room available to do the de-superheating and condensing. This decrease in volume has the exact same effect as making our condenser smaller.
There is however a consequence of doing this which is that heat does continue to be removed from the liquid refrigerant and the liquid can become very cold. This does not mean it’s at a low pressure, instead what is coming out of the condenser is subcooled liquid (and in really cold weather, it can be really subcooled). There are a few minor issues this causes with refrigerant flow in the receiver, but the more common problems come from the fact that in the winter the condenser drain piping can be very cold and cause condensation and dripping. In order to prevent this, the condenser drain line may need to be insulated in installations with remote condensers and condenser flooding controls.
Depending on the exact method employed to hold back the refrigerant in the condenser, there are other consequences to the operation of the refrigeration system that need to be addressed when we decrease the condenser surface area in this fashion. In the next issue, we will take a look at the exact valving and operational set-ups for condenser flooding controls that maintain head pressure in cold operating conditions. Additionally, we will discuss the operation of condenser flooding controls in combination with fan cycling and condenser splitting.