Thermal expansion valve

Thermal expansion valve (TXV) The component in the HVAC system that controls the amount of refrigerant that goes into the evaporator. 

At stage 3, moving out of the condenser, the refrigerant is now a saturated liquid and is routed through the thermal expansion valve (TXV). 

At the thermal expansion valve (stage 4), adiabatic flash evaporation of part of the liquid occurs. Autorefrigeration lowers the temperature of the liquid and vapor refrigerant mixture so that it is colder than the temperature of the enclosed space to be refrigerated. 

Usually limited to systems of 1 ton or less, it performs all of the functions of the thermal expansion valve when properly sized. 

Thermal Expansion Valve The metering device or flow control which regulates the amount of liquid refrigerant which is allowed to enter the evaporator. 

The geometry of the chamber-condenser valve (Figure 14.10) must be chosen in such a way that there is a good water vapour distribution so that the ice can grow evenly over the condenser pipes installed in the condenser. Temperature control of the condenser pipes is effected over thermal expansion valves the cooling agent is injected directly into the condenser pipes (evaporator). 

Storage and Handling. The acid should never be allowed to stand in a line completely sealed between two closed valves or check valves. Excessive pressure caused by thermal expansion of the Hquid can cause leaks or pipe mptures. AH lubricants and packing materials in contact with chlorosulfuric acid must be chemically resistant to the acid. Elanged connections are recommended over screwed fittings and flange guards should be used. 

Full liquid containers require protection from thermal expansion. Such relief valves are generally quite small. Two examples are... 

Consideration should be given to the effects of thermal expansion of liquids and pressure-relief valves installed unless ... 

In cases where vessels are provided with PR valves for protection against overpressure from fire exposure or an operating failure contingency, additional thermal expansion protection is not required. 

Control Valve - Double-seated control valves are considered to pass sufficient leakage flow that equipment blocked in by such valves need not be provided with thermal expansion protection. 

In offsite locations, thermal expansion PR valves may discharge to a flare header upstream of a knockout drum, if available, or to the equipment (e.g., a tank) on the opposite side of one of the blocking-in valves, or to the atmosphere. Atmospheric discharges must be at grade level in a safe location... 

Thermal expansion PR valves in onsite or offsite locations which release severely toxic fluids must discharge to an appropriate closed system. 

Each thermal expansion PR valve should be provided with an inlet CSO valve (and an outlet CSO valve in the case of closed discharge) to permit isolation for inspection and testing, if permitted by local codes. 

PR valves handling materials which are liquid or partially liquid at the valve inlet. An exception to this is made for certain thermal expansion relief valves as described below. 

Thermal Expansion Relief Valves - Routing of thermal expansion relief valve discharges was covered earlier. 

Design Temperature of Outlet Piping - The design temperature of outlet piping from PR valves discharging to the atmosphere is normally ambient. However, autorefrigeration and need for brittle-fracture-resistant materials or thermal expansion forces should be examined if the release pipe is unusually long. 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

Themtal. Thermal relief is needed in a vessel or piping run that is liquid-packed and can be isolated, for example pig launchers and meter provers. Liquid is subject to thermal expansion if it is heated. It is also incompressible. The thermal expansion due to heating by the sun from a nighttime temperature of 80°F to a sun-heated temperature of 120 F can be enough to rupture piping or a vessel. The required capacity of thermal relief valves is very small. 

Smaller relief valves are often used in isolated parts of the system where a check valve or directional control valve prevents pressure from being relieved through the main system relief valve or where pressures must be relieved at a specific set point lower than the main system pressure. These small relief valves are also used to relieve pressures caused by thermal expansion of fluids. 

The purpose of the evaporator is to receive low-pressure, low-temperature fluid from the expansion valve and to bring it in close thermal contact with the load. The refrigerant takes up its latent heat from the load and leaves the evaporator as a drygas. Evaporators are classified according to their refrigerant flow pattern and their function. 

The signal from a suitable thermistor placed at the evaporator outlet will vary, depending on whether it senses dry refrigerant gas or traces of liquid. This can be used directly to control the current through a thermal element to modulate the expansion valve. This de vice usually has no separate adjustable controller and so cannot be incorrectly set (see Figure 8.10). 

---