Demystifying the Thermostatic Expansion Valve w/ Jim Jansen

The refrigeration circuit has four key components: compressor, condenser, metering device, and evaporator. We can use a few different types of metering devices, including thermostatic expansion valves (TXVs or TEVs).

TXVs have a sensing bulb and capillary tube, which make up the powerhead. The valve itself has an inlet and an outlet, a diagram, two push rods, a pin working in conjunction with a port, and a closing spring that adjusts the inlet size. Some of them come with a removable strainer assembly to help control contamination. A few valves have a stem that allows you to adjust them.

The TXV responds to the temperature at the evaporator outlet to modulate the orifice size. The sensing bulb will pick up the suction line temperature and apply pressure to the diaphragm accordingly, which provides an opening force. A closing force that acts against the opening pressure is provided by the spring and an external equalizer. The orifice size is determined by the way these three forces act on the valve. TXV manufacturers typically want to prevent inlet pressure from acting on the valve.

TXVs control superheat by controlling the size of the orifice—and the way it meters refrigerant. Adjustable valves allow you to control preload from the spring, which doesn’t change the valve capacity but does allow you to control the superheat that the TXV aims to maintain. We want to make sure the superheat allows us to maximize capacity without flooding the evaporator.

Some TXVs have bleed ports, which allow inlet high-side pressure to bypass the part of the valve that regulates the incoming refrigerant pressure. This feature comes in handy when the compressor has a low starting torque; it allows the high and low sides of the system to equalize on the off cycle. You will want to make sure that you don’t replace non-bleed TXVs with one that has a bleed port or vice versa.

Valves may be internally or externally equalized. Internal equalization requires the valve to sample evaporator pressure, typically at the outlet fitting of the expansion device. External equalization requires a third fitting on the valve for an equalization line that samples pressure on the suction line. Mounting the valve properly will allow you to tap into the suction line properly, and it will allow you to get solid contact with the suction line to control the superheat most effectively.

Residential HVAC superheat values tend to be within the 8 to 12-degree (Fahrenheit) range, and evaporator temperatures may be as high as 52 degrees or as low as 40 degrees, depending on the manufacturer. Commercial refrigeration evaporator temperatures can reach subzero temperatures (Fahrenheit).

Common issues with TXVs include starving, overheating, or hunting. Starving results in high superheat, and overheating results in low superheat (or none). Hunting refers to excessive modulation and may indicate problems with your TXV setup. TXVs may also leak and prevent the bulb from responding to the suction line temperature; leaking valves need to be replaced.

Distributors help mix the vapor and liquid coming out of the TXV, and there is an additional pressure drop associated with them. Systems with distributors require externally equalized valves.