TEVs were selected by most residential a/c equipment manufacturers because they:
• Isolate the refrigerant charge in the condenser during off-cycles;
• Provide better evaporator performance over entire operating range;
• Protect compressor from liquid flood back; and
• Provide better performance with high or low refrigerant charge.
How TEVs isolate refrigerant charge in the condenser during off-cycles:
When the thermostat is satisfied and the compressor turns off, a slight increase in evaporator pressure closes the TEV preventing the migration of refrigerant charge from the condenser to the evaporator. When the thermostat calls for cooling again, the evaporator reaches design temperature faster and less energy is used to pull-down the evaporator. This is how the TEV helps the equipment manufacturer achieve a higher SEER rating, but, as we mentioned before, the TEV also performs other important functions that help the unit performance and reliability.
How TEVs provide better evaporator performance over entire operating range:
Unlike fixed restrictors, TEVs adjust the size of the orifice in the valve to increase or decrease refrigerant flow to the evaporator based on the temperature of the suction line leaving the evaporator, the suction pressure, and the pressure differential between the condenser and the evaporator. The TEVs operation based on suction temperature and pressure to maintain evaporator leaving superheat is well explained in any TEV manufacturer’s application literature. What is less understood is how the valve adjusts to pressure differential changes related to outdoor ambient in conjunction to the evaporator load changes. As outdoor temperatures increase, condensing temperatures and pressures increase leading to higher pressure differential between the evaporator (low side) and condenser (high side). This higher pressure differential decreases the amount of refrigerant the compressor is able to pump. In systems with fixed restrictors, the mass flow through the restrictor will actually increase as the compressor mass flow is decreasing. (See Figure 1.) Fixed restriction expansion devices must be properly sized such that high ambient conditions will not cause liquid refrigerant to return to the compressor under low evaporator load conditions. This sizing practice limits refrigerant flow through the restrictor during lower or normal outdoor ambient temperatures, and operates the evaporator at a higher superheat, thus decreasing the heat transfer performance of the evaporator and making the unit operate longer to pull down to the thermostat setting.
How TEVs protect the compressor from floodback:
The article already described how floodback can occur under extreme conditions or with improperly sized fixed restrictions, but how do TEVs prevent floodback when the pressure differential between the evaporator and condenser increases from high ambient temperatures or a dirty condenser? To understand this, you must understand the basic operating forces that control the position of the TEV pin or the size of the orifice. Figure 2 represents the three basic controlling pressures. Force 1 is working to push the valve open, while forces 2 and 3 are working to close the valve.
The thermostatic charge (force 1) pushing on the top of the diaphragm is created by the pressure in the sensing bulb measuring the suction line temperature leaving the evaporator. This force is designed to balance against the spring (force 2) pushing the pin closed, and the suction pressure or equalizer pressure (force 3) pushing on the bottom of the diaphragm. This relationship between the bulb temperature and the equalizer pressure represents the superheat that creates balance or a stable valve position. Most TEVs are designed so this balance allows the valve to open at a minimum superheat referred to as static superheat or opening superheat (see Figure 3). Typically the spring in the valve is preset at the factory to open the valve between 2 to 10ºF of superheat for most TEVs on residential OEM equipment using nonadjustable preset valves. Replacement valves may have an adjustment to allow the contractor to set the superheat on the system. These are typically set at the factory for 10° opening superheat. As superheat increases, referred to as superheat change, from the opening point the valve opens further until it reaches the maximum opening point or maximum orifice size. Most TEVs take between 4 and 12ºF of superheat change beyond the opening point to reach the maximum opening. The stroke position where the valve balances the forces and matches the compressor flow will be the system superheat or operating superheat at the bulb location. If high condensing pressures or low evaporator loads (dirty air filter, indoor fan failure, dirty evaporator coil, etc.) lower the amount of heat the refrigerant in the evaporator can absorb, the valve will close before it allows liquid refrigerant to leave the evaporator. This may cause the coil to frost or freeze and suction pressures to run lower than design, but it will protect the compressor from seeing liquid refrigerant.
How TEVs provide better performance with high or low refrigerant charge:
In Figure 1, we saw how the pressure differential in the system for different ambient temperatures can affect the superheat at the evaporator for systems using fixed restrictors. However, improper refrigerant charge in a system can also lead to poor system performance. If a system is overcharged, the high to low side pressure difference can increase just like running at a higher ambient. If your system is overcharged and running at a high ambient, the results could be excessive flooding and damage to the compressor. If the system is low on charge, the system can operate at a lower pressure differential and/or flashing may occur in the liquid line and starve the evaporator coil. This can cause the coil to freeze condensate, or just not provide enough cooling to control the room temp, in which case the unit will just keep running, driving up energy cost and not fully cooling the space. In both cases a TEV will adjust the orifice size to either prevent liquid floodback in the case of overcharged systems, or open the orifice to try and maintain the highest possible suction pressure in the case of low-charged systems. In fact it was this TEV functionality that prompted the California Energy Commission to include reference to the use of a TEV on residential a/c systems in the state’s Title 24 building code.
How do EEVs improve performance over TEVs?:
TEVs have been applied to refrigeration and air conditioning systems for over 80 years. Although the designs have become more compact and cost effective over the years, very little has changed relative to the basic operating principles of the product. All these years of experience have helped TEVs remain a cost effective and reliable solution to evaporator feed control. However, over the last 30 years, the use of electrically actuated valves and electronic controllers or EEVs (Electric Expansion Valves) have evolved and increased in popularity since, in addition to being capable of performing the functions of a TEV, the EEV control system can also provide multiple performance benefits and include features that TEVs are technically incapable of providing, such as:
• Providing electrical means of liquid line shut off for “pump-down” and further improvement on cyclic efficiency.
• Maintaining stable minimal superheat control over a wider range of evaporator loads and outdoor ambient temperatures;
• Providing multiple control functions like leaving evaporator air temperature control while allowing superheat control to float from a minimum safe operating point;
• Allowing for remote adjustment of superheat settings without having to remove panels to adjust TEVs;
• Temperature and pressure data along with valve percentage opening can help with system problem troubleshooting/diagnostics; and
• Allowing remote monitoring or investigation of refrigeration system performance.
The EEV is a system in itself. At a minimum to provide the same function of a TEV, the valve must be coupled with a controller capable of positioning the electrically actuated valve and the necessary temperature and/or pressure sensor data inputs to measure superheat. However, to realize some of the features and benefits listed, it is necessary to have additional hardware and software features in the controller to provide this functionality. System manufacturers can address this by, at a minimum, adding electronic superheat control as a standalone controller from the valve manufacturer, or integrating superheat control into a higher level system control with additional data input and capabilities to control compressors, fans, thermostat functions, etc. Whether developed by the system manufacturer or by a third-party controls manufacturer these electronic control solutions offer benefits a TEV cannot provide.
Publication date: 08/29/2011