Telltale Signs of An Overcharged Refrigeration System

It is imperative to ensure that a refrigeration system is optimally charged to prevent overconsumption of power & exposure to the uncertainties that arise from a high-pressure or an overload protection trip. This article presents an in-depth view of some of the telltale signs of an overcharged system & the physics behind each of them. We will primarily focus on a capillary configuration with brief comparisons against the thermostatic & in some cases the automatic expansion configurations.

Increased Suction Pressure

In a capillary configuration, an overcharged system will function at a higher suction pressure than designed. This is attributed to the greater mass flow rate of refrigerant into the evaporator. This means that more liquid molecules are available to change phase to gas than usual. Upon absorbing heat from the refrigerated space, a higher number of gas molecules fill up the evaporator & the suction line, thereby raising the overall suction pressure. In an automatic expansion valve, the suction pressure is held constant & in a thermostatic expansion valve, there may be a slight increase in the suction pressure if the operation continues beyond its designed range. The feedback loop offered by the remote bulb in a TXV ensures a regulated pressure but only within the operating range.

Lower Superheat

In capillary systems, the greater mass flow rate fed into the evaporator causes the superheat to drop drastically at the evaporator outlet. In simpler terms, the refrigerant changes phase to gas much further along the length of the evaporator than when compared to a system that is optimally charged. This raises the risk of liquid flooding back into the compressor, especially in low load conditions. In a TXV configuration, the superheat is maintained constant by the feedback loop thereby reducing the chances of liquid flooding back. However, if the system overshoots the working pressure range of the TXV, the drop in suction temperature & a possible flood back scenario is plausible.

Greater Current Draw

The greater rate of gas molecule generation in the evaporator of an overcharged refrigeration system would mean that the specific volume of the refrigerant in the suction line will be low. This means that the gas molecules would occupy less space per kilogram in the suction line than its designed conditions. In turn, more molecules of gas would flood the cylinder of the compressor in its suction stroke compared to normal operation. Also, the discharge line will have a buildup back pressure against which the compressor must operate. This means that the compressor has to do a greater amount of work to compress this new packet of refrigerant than usual load scenario. A higher current draw is evident due to this reason supplemented by a high compression ratio.

Higher Discharge Pressures

The buildup of pressure in the suction line has a domino effect on the condensing line as well. The back pressure from the evaporator leads to a build-up of liquid & gas molecules in the condenser just waiting to be expanded into the evaporator. This elevates the discharge pressures & temperatures of the system. In a TXV configuration, this is a major indicator as the back pressure generated by the closure of the expansion valve orifice greatly elevates the discharge pressure of the system. The compressor continues to pump refrigerant into the condenser at a rate faster than is released to the evaporator.

Greater Subcooling

As mentioned in the previous point, in an overcharged system, there is a build of refrigerant in the condenser just waiting to be released into the evaporator. During this waiting time, the refrigerant is exposed to the cooling medium for an additional amount of time. This greatly subcools the liquid near the drier & in many cases can be used as a measure of overcharge in a system.