Controlling discharge and liquid pressure at low heat sink temperatures – Part I
By Phil Boudreau
This two-part series discusses common approaches to high-side pressure control during periods where the temperature of the cooling medium is relatively low.
Why high-side pressure control is necessary
While there are a few reasons why high-side pressure control is necessary, we will focus on the most common. The first involves a consideration for the minimum allowable saturated discharge temperature (SDT) that was established for the compressor. SDT tends to vary depending on the compressor model and refrigerant type used.
When the SDT of a compressor is allowed to drop too low, insufficient sealing of reed valves may take place. Also, the discharge superheat should be kept at a level high enough to support the best possible conditions for the compressor to operate at. For example, a low discharge superheat tends to increase the amount of refrigerant content of the oil travelling to the various lubrication-dependent components within the compressor. Operating at an excessively low SDT level can also result in condensation within the compressor cylinder heads.
Condenser fan cycling has experienced widespread use over the years. By reducing the number of condenser fans in operation, the capacity of the condenser will also be limited. Fan cycling can be accomplished by using a temperature control strategy, which cycles the fans on and off according to the ambient temperature. This is perhaps the simplest approach. However, these types of fan-cycling switches only sense the ambient temperature. This method is generally limited to applications where there is more than one circuit connected to the condenser.
Another approach is to use pressure fan-cycling. With this method, a pressure control strategy is used to cycle fans on and off as needed. Although this method involves accessing the refrigerant pressure in the high-side of the system, it is more responsive to load changes and cycling of heat reclaim. Also, when a central control system such as a rack controller is used, another fan can be cycled on to compensate for a fan motor and/or blade failure.
Fan cycling is not able to provide enough reduction of the condenser capacity. In this case, other approaches may need to be applied in addition to conventional fan cycling.
Let’s look at another common approach to high-side pressure control. The condenser flooding technique has also been applied extensively for many years on condensing units, supermarket racks and process cooling systems.
There are two flooding methods that are used. One approach is referred to as the condensate or drop-leg regulation method. The other is the discharge regulation method.
With the condensate or drop-leg method, a regulator is installed between the condenser outlet and the receiver inlet. This regulator can be of the direct-acting or pilot-operated type and modulates according to its inlet or condenser pressure. The regulator contains an adjustment spring. This spring opposes the pressure at the condenser’s outlet.
As the outdoor ambient temperature and/or condenser load decreases, the condenser pressure will also decrease. As the condenser pressure drops below the spring pressure, the spring will overcome the condenser pressure and force the regulator’s port to move towards a more closed position. As this happens, a limited quantity of the liquid can leave the condenser. Essentially what is happening in this case is that the condenser is being partially flooded with liquid refrigerant. This liquid occupies some of the condensing surface and, as a result, reduces the condenser capacity.
At this point, the use of this single regulator has enabled the discharge and condenser pressure to be kept above some minimum pressure level but this approach alone is not sufficient. As the condenser is flooding and liquid flow from the condenser is restricted, the receiver pressure will decrease. As the expansion valve regulates flow from the receiver to the evaporator, the receiver pressure will drop. This is because as liquid is being drawn from the receiver, the vapour volume above the liquid tends to increase as the liquid volume decreases.
If the receiver pressure drops off too much, then another issue will quickly become apparent. Expansion valves, whether they are of the thermostatic or electronic type, provide a certain flow rate or capacity that is largely dependant on the pressure drop. So, as the pressure difference across the expansion valve decreases, the flow rate will be less.
However, the role of the expansion valve is to fill the evaporator up as much as possible, while leaving room for essential superheating to protect the compressor and allow for stable control of refrigerant flow through the refrigerant circuit. The load on the evaporator along with the superheat setting of the expansion valve determines the per cent opening of the valve.
As the receiver pressure decreases, there will be less “push” from the receiver to the expansion valve and therefore less flow across the valve. Having a pressure regulator in the condensate line is not enough. With the discharge and condenser pressure held at the required level, an additional and smaller receiver pressure regulator is needed. This regulator is installed between the discharge line and the receiver inlet. It essentially bypasses the condenser. This is a downstream or outlet regulator, which is constantly sensing the receiver pressure.
The regulator contains an adjustment spring, which opposes the outlet or receiver pressure. As the receiver pressure drops off, at some point the spring pressure will overcome this pressure and force the port towards a more open position. As this happens, hot gas from the discharge line is permitted to bypass the condenser and enter the receiver. Since the hot gas is at a higher pressure than the receiver, it pushes down on the liquid, forcing the liquid pressure up and increasing the capacity of the expansion valve. The result is that the differential pressure across the expansion valve will be kept at a level that is high enough to fill the evaporator to a sufficient level.
Next Time: In Part II in Mechanical Business January/February 2022, Phil will discuss the discharge regulation method and cover the topic of condenser splitting for low ambient operation