Types of TX valves

• Standard Internally Equalized
• Standard Externally Equalized
• Block Type Valve
• Fixed Orifice Tube

The types of TX valve listed above are all similar in function and they may be described as the carburetor of the air conditioning system. Briefly, TX valves have a small, variable orifice which opens and closes when temperature changes at the coil outlet are sensed by the capillary, which is always clamped to the coil outlet. Internally equalized valves have a small refrigerant charge which expands and contracts. This movement exerts pressure on a diaphragm to open or close the valve against the pressure in the evaporator coil itself. The pressure on the evaporator coil is exerted on the opposite side of the diaphragm by means of a bleed in the valve. Externally equalized valves are similar, except that a direct line to the evaporator coil allows evaporator outlet refrigerant pressure to the underside of the diaphragm, rather than by direct bleed-back as in the internally equalized valve. The block valve is inherently equalized, ensuring accurate flow without sensing bulbs or equalizer tubes. The fixed orifice tube is simply a set, or fixed, orifice which allows a metered amount of refrigerant through to the evaporator coil whenever the compressor is pumping. See valve operation schematics opposite.

TX Valve Application

• Internally equalized valves are used for small to medium coils
• For large coils, externally equalized or block valves should be used
• Fixed orifice tubes are used in conjunction with accumulators

TX Valve orifice stuck closed or blocked

This problem is indicated by low pressure readings and lack of cooling, as refrigerant flow is restricted, the low side can actually drop into vacuum. Blockages may be due to foreign material or moisture freezing at the orifice.

Incorrect TX Valve

TX valves are rated in tons. The tonnage of a valve refers simply to the capacity of the valve. (1 ton equals 12,000 btu/hr). A valve which has a rating too low will ‘starve’ the coil reducing performance. A valve which is too big will flood the coil, and in extreme cases, liquid slugging may occur. Liquid slugging can seriously damage the compressor.

TV Valve with capillary incorrectly positioned or damaged

The bulb must be securely clamped to the evaporator outlet tube.

TX Valve with incorrect superheat setting

TX valves are set to a particular superheat setting, usually around 5-10°C. If set too low for the coil, the valve will open too early and flooding may occur. If set too high, the TX valve will not open as early as it should and starving will result. Please note that altering the superheat setting is not recommended, the correct valve selection is preferable. 
Definition: Superheat is the additional heat absorbed by a gas after vaporization from a liquid state.In the case of an evaporator it is therefore simply the temperature rise across the coil.

TX Valve Diagnosis Tips

• Valve stuck closed or blocked - almost no cooling and low pressure readings (may even be in a vacuum on the low side).
• Valve stuck open - poor cooling performance and high pressure readings on both sides of the system.
• Valve too small - poor cooling and low reading on low side.
• Valve too big - poor cooling and high reading on low side.
• Capillary incorrectly clamped or positioned in the cold part of coil will result in the valve tending to stay closed.
• Capillary damaged or incorrectly positioned. A damaged capillary or a capillary bulb not clamped to the evaporator outlet will result in the valve tending to stay open.