In some of the larger commercial units, the suction line is connected to a multiple evaporator system requiring different temperatures, such as one evaporator in a frozen-food display with an average temperature of -20°F, and another in a walk-in cooler set at 40°F. Since, in this system, the two evaporators cannot be the same temperature, a reducer valve (two-temperature valve) is installed at the outlet of the warmer evaporator to increase the pressure inside the warmer evaporator by reducing the constant suction force that the compressor applies to it. Also, a check valve is placed at the outlet of the colder evaporator to stop the back flow of refrigerant, thus preventing the buildup of more pressure (and consequent warm-up) of the colder evaporator during the off cycle. (See figs. 96 and 95.) They are manufactured with different physical appearances.
There are two general categories of two-temperature valves: those using a spring inside their mechanism (including spring valves with thermostats or solenoids) and valves employing a sensing bulb, very much like TEVs.
Operation of the spring-type valve is quite simple: When the pressure inside the evaporator goes up (due to evaporating refrigerant), the spring opens the valve to allow refrigerant to flow out of the evaporator (drawn by the compressor suction power).
The thermostatic-type double-temperature valve is very similar in operation to the thermostatic expansion valve with the difference that it only opens to let vaporized refrigerant be drawn by the compressor suction power when its sensing bulb perceives the need for cooling. When the valve opens, the pressure built up in the evaporator drops, causing the temperature to go down.
In the solenoid type, as in figure 97, an independent thermostat is placed in line before the expansion valve to operate the solenoid. When the thermostat is satisfied, the solenoid valve closes, allowing no more refrigerant from the liquid line into the TEV valve.
A double-temperature valve can be used as a high- to low-side bypass to assure continuous compressor operation at low loads. A low-load condition means that the evaporator has little cooling to do as the selected temperature has
already been attained. This type of connection is normally used in commercial applications to limit evaporator temperature. (See fig. 97a.)
A constant-pressure valve can also be used to limit the compressor inlet pressure when the system starts after defrosting (at which time the suction pressure has greatly increased). By installing a constant-pressure valve in the suction line, the compressor is protected from overload or burnout. (See fig. 97b.)
Figure 97c shows a pressure-reducing valve installed in the evaporator suction line to prevent evaporator pressures from dropping below a desired minimum. Installation of these valves, on the suction line for example, can prevent liquid in chillers from freezing.