PH Diagram.......practically using it.

[nui]

Hello,

I'm up to the part where I'm sussing out PH Diagrams with my tech work and although kind of understand it would like to practically apply it.....at least once to say I know how to do it. I'll probably do it when I go to tech but would like to have a heads up. Can anyone give me basic instruction on where and what readings to take and from there I think I could work it out, i.e plot it. For arguments sake I will use a small coolroom. Does anyone use these in their day to day work for problem solving?

Thanks heaps,

[US Iceman]

I use PH diagrams all of the time. Remember, these diagrams have two axis; pressure and enthalpy.

Any data you record from a refrigeration system (pressure and temperature) can be plotted on the diagrams.

Have a look at this link for a good review.
http://www.sporlan.com/5-200.pdf

[carlfoster]

Hi

Nice link US Iceman. I'm also plotting a pressure enthalpy chart for an a/c system.

I hope you don't mind if I include a question in this thread nui?

When measuring the temperature of the evaporator, I've measured it after the cap tube. Now there is a distributor on the evap as well and I was wondering if perhaps the distributor is carrying on the restricting process and I should read the temp after the distributor instead.

Is this possible or should the distributor have no effect on the pressure at all?

[US Iceman]

When measuring the temperature of the evaporator, I've measured it after the cap tube. Now there is a distributor on the evap as well and I was wondering if perhaps the distributor is carrying on the restricting process and I should read the temp after the distributor instead.

Is this possible or should the distributor have no effect on the pressure at all?

When you look on a PH diagram for the expansion process this occurs as a straight vertical line (down). This is where the liquid (at condensing pressure) is being reduced in pressure down to the evaporating pressure.

What this vertical lines indicates is only pressure loss without any change in enthalpy. This is known as a constant enthalpy process.

Part of the pressure loss is caused by the pressure loss across the TXV, plus the refrigerant distributor (and orifice inside), plus the distributor tubes.

Once the refrigerant leaves the TXV, it can be assumed to be constant temperature, until the refrigerant is superheated in the evaporator.

Does that help?

[carlfoster]

That is what I thought, the normal graph shows an instant pressure drop without any increase in enthalpy, which would mean a reading after the distributor.

By the way, this system uses a cap tube, no TXV.

[nui]

Cool, thanks for that......sure there will be more.

[nh3simman]

The refrigerant p-h diagram (sometimes called the Mollier Chart after Richard Mollier) is the equivalent of the psychrometric chart in air conditioning.

It gives a great picture where you can see a lot of information at the same time.

Things like:

1. Superheat
2. Sub-cooling
3. Refrigerating effect
4. Evaporator entering quality
5. Compressor discharge temperature
6. Coefficient of performance
7. Compressor isentropic efficiency
8. Excessive component pressure drops
9. Coil DT's

and probably a few things that I have missed.

A company I do some work for was testing a cab cooler and not getting the expected duty. We plotted the p-h diagram and found the problem immediately (blocked distributor). A good fridge man would have found it anyway but this made it easy and gives people a way to visualize the problem.