View Full Version : pressure and temperature drops in R22 evaporator

10-05-2006, 08:24 AM
Hi, i want to know about formulas to calculate pressure and temperature drops in evaporator. :D

US Iceman
10-05-2006, 02:55 PM
The temperature drop is based on the differences in pressures due to the saturation temperatures of the specific refrigerant.

The pressure drop is very complex as it is a two phase (liquid and vapor) flow problem. The pressure losses are normally limited to reduce the temperature penalty to a minimum so the heat exchanger performance is not drastically affected.

The design value changes for the operating evaporating temperature and liquid feed type (DX, flooded, or liquid overfeed).

11-05-2006, 02:02 AM
It's too general question:).
The pressure drop affect directly the temp drop across the evaporator. It's needed to optimize the heat exchanger size and the pressure drop. I think you can get the formula based on the specified type of HX. There is no universal formula for it.:p


US Iceman
11-05-2006, 04:42 AM
If you want an estimated value without too much trouble do this....

Figure the mass flow through the evaporator as all vapor.

Find the refrigerant velocity through a single tube circuit.

Find the Reynolds number of the flow through the circuit tube.

Find the friction factor for the tube and inside surface of the circuit tube. You can find the friction factor by using the Moody diagram or the Colebrook equation.

Find the pressure loss using the Darcy-Weisbach equation.


To find the temperature loss find the saturation temperature for the entering pressure to the circuit.

Also find the saturation temperature for the outlet pressure of the circuit.

The difference in saturation temperatures is the temperature loss due to flow.

16-05-2006, 02:20 PM
Hi guys,

There is not only friction factor, but also a acceleration caused pressure drop, which is produced by gas velocity increasing. see this file

Then two phase outlet temperature got by its corresponding saturated pressure.

08-07-2006, 08:59 PM
Hello. I am new to this site and I want to know how and where to write your messages. Plz. provide complete information.


08-07-2006, 09:14 PM
Any body who can clear my doubts in NPSH calculation for the pumps in reverse return system. Plz. see the attachment.


09-07-2006, 03:44 PM
Hello. I am new to this site and I want to know how and where to write your messages. Plz. provide complete information.


From this page,
Select a forum, when new page appears press "New thread" on top left of list table to post you messages.

I have seen your file, may be you can calculate every duct loss by its design flow and diameter, include fittings as bends, tee branches, valves and equipments etc, select the maximal loss for each parallel flows, so that to get the total loss, finally plus spare capacity to determine the pump head.