SUBCOOLING Problem

[hvac4178]

Hello Everyone,
I am a colllege student and I am currently enrolled in a class dealing with Refrigeration engineering.I am attempting a homework problem but I am having a rather difficult time with it. It deals with Subcooling of refrigerant R134A and states for R134a analyze the effect of subcooling from 0 to 10 Deg F and plot the refrigeration effect and COP as a function of sub degrees of cooling. I have no clue on this one. I am trying to plot my points on my PH Diagram but i cant get my points to calculate COP or refrigeration effect. How do I solve this?

I know COP in this case would be=(h2-h1')/(h3-h2)

I am also going to try to use EES for this problem. Anyone ever use it? I am in need of help, i dont understand.

[icecube51]

play whit coolpack for a wile.

you wil figgered out.

Ice

[hvac4178]

is coolpack available in English Units by chance?

[Chef]

.....analyze the effect of subcooling from 0 to 10 Deg F and plot the refrigeration effect and COP as a function of sub degrees of cooling.

Is this the actual thing you are looking for? It seems you want to plot a graph of COP versus subcooling.
Your idea to use EES would be quite easy but maybe the course tutor wants to see how you solve it, so maybe rewrite your equation for COP to include a term for the effect of subcoling.
Chef

[icecube51]

yep, coolpack is standaard English.

and try to follow the tips from Chef. now you gone learn something.

Ice

[Chef]

I know COP in this case would be=(h2-h1')/(h3-h2)

.

hvac - I am not sure we are using the same nomenclature here but your COP seems to negative or even an imaginary number (like SQRT(-1) or similar). But the problem seemed interesting so investigating this in more detail brings more complications.

The problem seems to be with what type of system has the subcooling.

Case 1
If the system was designed to have subcooling, with say an after cooler, then the equation might look like COP=(h1-h4')/(h2-h1).

Case 2
BUT if the system actually needs subcoolng to regulate and balance then it would be very different and be more like COP=(h1-h4)/(h2"-h1)

We can see here that h3"=h3 but h2">h2 so again it shows a drop in the COP by requiring more power input for the same cooling effect.

In case 1 the subcooling gives an increase in COP and is therefore 'useful' but case 2 gives a reduction in COP and is therefore 'not useful' so the type of system being modelled is key to defining the slope of the final graph of subcool versus COP.

Bottom line is subcooling is not always good but maybe necessary.

The PH diags I have show it better.
(h' is deltaT and h" is deltaP)

Chef