3 ways to adjust hot-gas bypass valves

Question: What is the proper method for adjusting a hot-gas bypass valve? How do you know when you have too much discharge has feed back to the suction line? Can the compressor be damaged?

-Quincey McIlwain, Durham, N.C.

Answer: Discharge bypass valves provide an economical method of compressor capacity control in place of cylinder unloaders or the handling of unloading requirements below the last step of cylinder unloading.

On air-conditioning systems, the minimum allowable evaporating temperature that will avoid coil icing depends on evaporator design and the amount of air passing over the coil. The refrigerant temperature may be below 32 ° F, but the coil icing usually will not occur with high air velocities since the external surface temperature of the tube will be above 32 ° F.

For most air-conditioning systems, the minimum evaporating temperature is 20 ° F to 25 ° F. However, when air velocities are reduced considerably, the minimum evaporating temperature should be 26 ° F to 28 ° F.

Discharge bypass valves can be set so they start to open at an evaporating pressure equivalent to 32 ° F saturation temperature. Therefore, they would be at their rated capacity at 26 ° F evaporating temperature.

Another typical application would be a low-temperature compressor designed for operation at a minimum evaporating temperature on R-22 of -40F. The required evaporating temperature at normal load conditions is -30F. A discharge bypass valve would be selected, which would start to open at the pressure equivalent to -34F and bypass enough hot gas at -40F to prevent a further decrease in suction pressure.

Three common methods of hot-gas bypass are:

•  Bypass to evaporator inlet with distributor. This method of application, illustrated in Figure 1, provides distinct advantages over the other methods, especially for unitary or field build-up units where the high and low sides are closed or coupled.

•  Bypass to suction before the evaporator TEV sensing bulb (at least 3 feet before bulb). This method offers added flexibility for multievaporator systems because the hot-gas bypass components can be located at the condensing unit (see Figure 2). However, neither method (Figure 1 or 2) ensures oil return unless special care is taken in the system piping to accomplish satisfactory oil return to the compressor from the low side.

•  Bypass to suction after the evaporator TEV sensing bulb (desuperheating thermostatic expansion valve). On those applications where the hot gas must be bypassed directly into the suction line downstream of the main expansion valve, commonly called a desuperheating TEV or a liquid injection valve, is required. The purpose of this valve is to supply enough liquid refrigerant to cool the hot discharge gas to the recommended suction temperature. Most compressor manufacturers specify a maximum suction gas temperature of 65 ° F (see Figure 3).

Always check with the equipment manufacturer for its hot-gas bypass recommendations.

- Steve Esslinger, senior application engineer, Sporlan Valve