Figures 107, 107a, and 111 show a solid-state type of TEV. This type of valve can be used on any system. Unlike the bulb-type TEV, the electric valve eliminates the problem of placement of the sensing bulb.
The electric valve is operated by and responds to low-voltage electricity. Its operation is simple and easy to understand.
A liquid-sensing thermistor (see fig. 112) is installed in the suction line at the outlet of the evaporator where a complete change of refrigerant state from liquid to gas occurs. It is wired in series with the electric valve. Here, the thermistor reacts, increasing or decreasing voltage in the valve circuit, depending on the state of the refrigerant passing through that part of the line. When it is exposed to hot refrigerant gas, the thermistor is heated to a high temperature by the voltage applied to it. The thermistor resistance drops as it self-heats. This negative-coefficient thermistor causes an increase in voltage to the bimetal heater inside the valve head.
The amount of low voltage applied to the heater bimetal within the valve controls the degree of valve opening (see figs. 208 and 209). At zero voltage, the valve is closed. As voltage is applied, the bimetal heater is deflected. The needle follows the bimetal deflection and opens the valve. The more voltage applied, the greater the valve opening.
The thermistor may be installed nearly anywhere in the system to perform a variety of functions. Depending on where the thermistor is installed, the electric valve can control
head pressure,
maximum evaporator pressure,
minimum evaporator pressure,
flooded evaporator (in low-side float system),
flooded condenser (in high-side float system) (see figs. 114 and 115).
The electric valve can make system analysis and system troubleshooting fast and easy. Service personnel need only attach a voltmeter to the electric valve. The readings obtained from the voltmeter will tell how the valve is operating at a glance.
A single check of system conditions will
indicate valve reaction,
identify problems elsewhere in the system.
Complete servicing details are given under the thermal electric valve troubleshooting guide in the Refrigeration Fluid Flow Controls and System Troubleshooting.
