Pressures at Evaporating & Condensing Sides

[stephen.t]

Hi there!

Please could someone help me with the following question regarding the system pressures... :confused:

Is the pressure at the output of the compressor and the input of the TXV valve the same (i.e. at either end of the condenser) ?

Similarly, is the pressure at the output of the TXV valve and the input of the compressor the same (i.e. at either end of the evaporator) ?

If not, what factors generate a difference in pressures and can they be calculated,
allowing, therefore, the pressure readings to be adjusted ?


Many thanks in advance ;)

[Brian_UK]

Short answer - possibly.

Real answer - probably not because of frictional pipe losses.

You state that you are an "engineer", what trade are you in?

[The Viking]

In theory, yes they should be similar....

BUT

In practice, it depends on a lot of factors and on the design.

(And the true answer is hardly ever)

Take a small split A/C system for example.
The expansion device is in the outdoor unit but by design the pipe going to the indoor unit is too small for the refrigerant to start evaporate, it is an added restriction if you want. Therefore there is a great pressure drop between the expansion device and the compressor intake.

[Billy Ray]

To be precise, the presures will not be the same, although similar as per previous replies to your question.

When designing systems, the designer has to account for pressure drops in the system, as this will effect performance & system reliability. Oil return has to be considered, refrigerant flashing in the liquid line due to pressure drop & many other considerations.

Note also, no pressure drop would mean no flow, however negible this may be.

You can assume that less than 4psi on the discharge side may be seen, although 1-2psi is more desirable.

1 psi per hundred foot of suction is in the order. Typical commercial systems are designed at 2 psi (total suction mains).

Pressure drop in the suction at high applicatiion temperature (A/C etc) is not as crtical as pressure drop at low tempoerature application (-25 cold storage).

Note also, pressure drop due to vertical rise of liquid mains is quite significant & measures needs to be introduces to offset effect (i.e liquiud sub cooling).

Billy Ray

[wambat]

The simple answer is that they will not be the same pressure going in and going out but for most good designs the pressure drop will be negligible. For example, if the suction line is too small, oil return will not be a problem. However, there may be an excessive pressure drop through the suction line. This will reduce the system’s capacity. Deciding which pipe size to use for the suction line is generally a compromise between ensuring good oil return and maintaining a minimum pressure drop through the line. It is also important to correctly size the discharge and liquid lines. However, in the discharge line the refrigerant is traveling at a higher velocity, the oil is pushed along, and the pipe size is not as critical. In the liquid line, since the refrigerant is in its liquid form, it tends to mix well with the refrigerant and is carried along easily