Categories Based On Methods Of Operation

Pressure Operated

hand expansion

fixed orifice

automatic expansion

Temperature Operated

thermostatic expansion

Volume Operated

high-side float

low-side float

Hand Expansion

needle valve (a hand operated valve, a change in load requires a manual adjustment of the valve – now commonly used where another device will control flow to the valve automatically, solenoid)

Fixed Orifice Tube (capillary tube)

a small accurate I. D. tube that offers high frictional resistance to liquid refrigerant flow there is no temperature change along the tube as flashing should not occur until the refrigerant leaves the capillary tube and enters the evaporator

Capacity effected by any change in


- length                        longer = less refrigerant

- inside                         larger = more refrigerant

- pressure difference   increase = more refrigerant



Characteristics

space temperature controlled by temp. stat

off-cycle eq. (as there is no actual separation between H. S and L. S.

critically charged system (a system that has just enough ref. for the load it was designed for, overcharging will cause flooding of the compressor, undercharging will cause evaporator to starve)

Used on System with fairly Constant Load

critically charged no reserve of ref. for load changes

The Capillary tube is often coiled to conserve space



Advantages of Capillary tubes

inexpensive requires low starting torque compressor

no moving parts

simple design

Disadvantages

does not respond well to load changes

the small inside diameter will plug easily with any debris in the system