Thermostatic Expansion Valve (Constant Superheat Valve)

Operates on 3 pressures

Spring (closes)

Evaporator (closes)

Bulb (open)

As load on the evaporator increases superheat will increase

TXV senses the change in superheat through the bulb

As superheat increases bulb pressure increases bulb pressure increases putting more pressure on the diaphragm of the valve overcoming spring pressure and evaporator pressure to force the valve to open

TXV Valves Theory of Operation

Basically, TXV Operation is determined by three fundamental pressures:

P1 : Bulb pressure acts on one side of the diaphragm, tends to open the valve

P2 : Evaporator pressure acts on the opposite side, tends to close the valve

P3 : Spring pressure: which also assists in the closing action – is applied to the pin carrier and is transmitted through push rods to a buffer plate on the evaporator side of the diaphragm

When the valve is modulating, bulb pressure is balanced by the evaporator pressure plus the spring pressure
P1 = P2 + P3

 

TXV Types:   

TXV internally equalized

TXVE externally equalized

Used when evaporator coil pressure drop is excessive, and when a coil is equipped with a distributor this allows evaporator pressure the bulb location to be sensed by the valve

Metering Device

1. Seal cap
   
   
2. Adjusting stem
   
   
3. Bottom cap assembly
   
   
4. Spring guide
   
   
5. Spring
   
   
6. Strainer
   
   
7. Pin carrier
   
   
8. Pin
   
   
9. Inlet
   
   
10. Body
    
    
11. Outlet
    
    
12. Push rods
    
    
13. Thermostatic element
    
    
14. Seat
    
    
    
    
    

TXV Types

Internally equalized externally equalized used when evaporator coil pressure drop is excessive, and when a coil is equipped with a distributor this allows evaporator pressure at the bulb location to be sensed by the valve

TXV Sensing Bulbs

The bulb charge provides temp. operating characteristics for the valve

Type of Thermostatic Bulb Changes

Liquid Charge

Same refrigerant as in the system contains enough refrigerant liquid/ vapor for a wide range of temperatures

Liquid Cross Charge

Refrigerant is not the same as that which is in the system. (different characteristics)contains enough refrigerant liquid/ vapor for a wide range of temperatures

Metering device Gas Charge

Same refrigerant as in the system contains just enough liquid for valve operation within its temperature range above this bulb temperature, a temperature increase does not increase bulb pressure. Also called ( MOP or Pressure Limiting)used to limit compressor loading during hot pull down or after a defrost this pressure limiting charge causes TEV to remain closed until the system evaporator is reduced below Max Operating Pressure (MOP) of the charge valve body must be located in a warmer location than the bulb to prevent liquid migration

Gas Cross Charge

Refrigerant is not the same as that which is in the system (diff. characteristics)operating characteristics similar as Gas Charged Valve

FOR MORE INFO GO TO OEM MANUAL OR WEBSITE

TXV Sensing Bulb Mounting

Clean area and bulb before mounting must have good thermal contact mounting must be secure and well insulated location between 8 to 4 o’clock on horizontal suction 7/8” and larger diam pipe located at least 18 “ from where suction line exits the conditioned space cannot be located on coil suction header excess capillary tube on top of thermostatic element where possible bulb should be mounted up-stream of the equalizer connection on the suction line

 

TXV Superheat

Superheat Adjustment: 8 to 15 degree F

- to increase superheat            turn adjusting stem CW

- to decrease S. H.                    turn adjusting stem CCW

Adjustment of Superheat Setting

 Methods:  Compare temp to temp

Temperature reading at saturation compared to temperature at sensing bulb

Compare Temperature to PT check Saturation Temp (most practical)

(A) To suction pressure ( at compressor ) 66 PSIAG (B) Add estimated suction line loss       + 2 PSI to obtain (C) Pressure (at bulb)                   68 PSIG