Balanced port TEVs

[Prof Sporlan]

WHERE IS PROFESSOR SPORLAN?????!?!?

Some thoughts on balanced port TEVs for herefishy

With all TEVs, the sensing bulb provides the opening force on the valve, while the superheat spring and equalizer pressures combine to provide the closing force, i.e.:

F_bulb = F_spring + F_equalizer

These are considered the fundamental forces acting on the TEV. More often, TEV operation is described in terms of pressure:

P_bulb = P_spring + P_equalizer

Since bulb and equalizer pressures are easily understood.

When we refer to "force" due to bulb or equalizer pressures, we are taking pressure x the effective area of the TEV diaphragm (pressure x area = force). When we refer to spring "pressure", we are taking spring force and dividing it by the effective area of the TEV diapragm (force / area = pressure).

Using the pressure version of the TEV balance equation, and noting that the spring pressure is effectively constant once the TEV is properly set, we see that the TEV is simply controlling the difference between the bulb and equalizer pressures. This difference in pressure, converted to temperature, is of course the superheat the TEV controls.

So what is this "balanced port" stuff all about? Another force acting on the TEV arises from the actual pressure drop across the valve:

F_port = <font face="Symbol">D</font>P_TEV * Port Area

This force is normally an opening force with the TEV, and one can think of it as the refrigerant flow "pushing" the pin away from the port. Incorporting this force into our equation, we get:

F_bulb + F_port = F_spring + F_equalizer

Interestingly, if pressure drop across the TEV were to remain constant, this force has no real consequence on TEV operation. Spring force will offset it when the TEV is properly set. A problem occurs, however, when this force varies significantly, i.e., the more <font face="Symbol">D</font>P_TEV varies (as head pressure varies from summer to winter operation) and the larger the TEV port area.

As a practical matter, many if not most large capacity TEVs employ some type of balanced port design, and this is due to the fact they have large port areas.

Small capacity balanced port TEV can benefit systems, particularly if head pressure varies substantially from summer to winter operation. In addition, the single rod balanced port design, e.g., Sporlan Type "BF" allows for improved pin port alignment than the conventional two pushrod TEV design. So one can expect better control at low load conditions with the single rod design.

So do this mean we can unnecessarily oversize a balanced port TEV and expect good control? This is not a good conclusion. One should properly size the TEV for the application, whether the valve has a balanced port design or not.

Note that for some low temp applications, the TEV is oversized a bit to provided quicker pulldowns following defrosts. This is a good application of a balanced port TEV as long as one isn't too overzealous with the oversizing.