Thermostatic expansion valves Introduction

[meybotyhvac]

A thermostatic expansion valve is built up around
a thermostatic element (1) separated from the
valve body by a diaphragm.
A capillary tube connects the element to a bulb
(2) and a valve body with valve seat (3) and a
spring (4).
A thermostatic expansion valve works like this:
The function of a thermostatic expansion valve is
determined by three fundamental pressures:
P1: Bulb pressure which acts on the upper
surface of the diaphragm, in the valve
opening direction.
P2: Evaporating pressure which acts on the
underside of the diaphragm, in the valve
closing direction.
P3: Spring pressure which also acts on the
underside of the diaphragm, in the valve
closing direction.
When the expansion valve regulates, balance is
created between bulb pressure on one side of the
diaphragm and evaporating pressure plus spring
force on the other side.
The spring is used to set superheat.