Basic refrigeration software questions

[moondawn]

hi all am just having a go at basic refrigeation CDROM at home.
I have advanced troubleshooting boards.

i have uploaded a screenshot so you can see what program i am using.

i am having a go at some problems and i have to check subcooling superheat which i can work out but i also need to know if the head pressure is high or suction pressure is low depending on the different refrigerants the system is using and also different applications.

for example one trouble shooting board is on r401a gas and is a reach in cooler.

My question is How do i work out whether the Head pressure is HIGHER OR LOWER than it should be.

And also The Same for the Suction Pressure.

I know i would have to use a comparator to find out but is there a calculation for finding if the pressure is indeed to high or too low.

I know air cooled condensors in moderate weather should reject around 40 degrees is this my answer for head pressure.

Not sure for evaps.

please advice

lee

[Tradewinds]

Hi Lee,

These are just rules of thumb:

High Side
SCT = Ambient + 15'c


Low side
SST = Air off - 10'c

[moondawn]

hi tradewinds, okay thanks will give that a try and see how i get on.

cheers lee

[moondawn]

hi trade winds just done a trouble shooting board like the picture which is shown in this thread.

on the pic it shows the ref diagram P1 p2 etc which are system pressures
t1 t2 etc are temperatures
the answer on the troubleshooting board i am working on is a inefficient compressor and it says the diagnosis are as follows
Head Pressure LOW
Subcooling HIGH
Superheat HIGH
Suction Pressure HIGH
Discharge temp LOW

i will list the temps and pressures from the troubleshooting board , here goes
Pressures SUCTION P1 = 44psi SST = 8.8 deg c
DISCHARGE PRESSURE P2= 103psi converted to temp SCT= 31.6 deg c

Temps
T1 suction temp 22.8 deg c
T2 Discharge pressure temp = 65 deg c
T3 Liquid Line = 23.3 deg c
T4 Liquid line past drier = 23.3 deg c
T5 Suction pipe after evap =18.9 deg c
T8 Air in condensor = 27.8
T9 Air Out condensor = 29.4 Difference of 1.6 deg c
T6 Air in Evap = 20
T7 Air Out Evap = 18.9 Difference of 1.1 deg c

From This data i work out Sub cooling as SCT 31.6 - 23.3 = 8.3 deg c
and superheat as SST 8.8 - 18.9 T5 suction pipework after evap (Expansion bulb temp) = 10.1 deg c

So sub cooling is 8.3 HIGH?? What figures should i be looking for ?
Superheat is 10.1 HIGH?? Again what figures am i looking for?

liquid line temp is fine no change across the liquid line. Am i looking for anything over 1 deg difference as problematic ??

delta t air cond 1.1 LOW What should i be looking for figures wise 15-20????

Delta t air evap 1.6 LOW again what temps should i be looking for please.

HEADPRESSURE

27.8 ambient + 15 deg c = 42.8 deg c SCT is 31.6 so Headpressure is LOW

SUCTION PRESSURE
air off is 18.9 - 10 deg c = 8.9 deg c SST = 8.8 So suction Pressure okay? or high cos its 0.1 degc over??

Discharge temp is at 65 deg c. says its low but what figures should i be looking for please.


So i seem to be tripping up on Suction pressure High as i am only getting 0.1 deg c over
but me thinking the suction pressure is fine would not be good enough for diagnosing inefficient compressor

Really keen for help on this so i can try to have a crack at these boards to better myself

cheers lee

[mad fridgie]

There does seem to be some error with your program or your calculations, look at the liquid temperature leaving the reciever, it is somewhat cooler than the ambient air "air on to the cond" where is the sub cooling coming from, what is the pressure at this point P3.

[cool runings]

.

Hi Lee.

Trade winds rule of thumb is correct.

In the UK with most air cooled condensers you will only see about 4 to 6 degs of subcooling.

You claim it is R401 and the screen shot shows MP39 refrigerant which is 401 and if I remember correct? Refrigeration basics is an American product??

Maybe they work to different rules of thumb.

Put the numbers in to give you 5 deg subcooling and 6 to 8 degs of superheat and see what the numbers are.

All the best

coolrunnings

.

[moondawn]

mad fridgie - the liquid pressure is at 130 psi which on the refrigerant comparater is 94.It does not say so but this must be fahrenheit so 94 fahrenheit to degrees = 34.4 degrees

T3 is 23.3 degrees so maybe there is a mistake in the program or i am missing something here.

coolrunnings - my subcooling i got was 8.3 and superheat 10.1 so that would mean both high only the superheat only just?

i am finding it really difficult to grasp as whithout a figure in mind whats the point in me taking temps and pressures.
For instance the answer tells me the discharge temp is low but it is 65 degrees i dont know if this is low or not
frustrating really but thanks for the replies guys hopefully try to get there in the end.

This is the heading from the advanced troubleshooting board if anyone has it. MP39 (R-401A) Reach In Cooler - Problem 3

[mad fridgie]

If this is a training CD, I would suggest you go through the program, as the writer is likely to have used his experience for what he classes as a rule thumb.
I would suggest if you want to learn more (which is good), it that you go back to the very basics of the 4 main components comp, cond, evap and expansion, understand how each part is effected by different conditions and how then each part then effects the other parts and different conditions. A refrigeration system is a loop and in theory and mostly in practice will reach equalibrium. If you can really get a handle on this (and many have not) then your problem solving will become somewhat easier.
In todays times, when efficiency and/or competative advantage is required the old rules of thumb are out of the window.

[cool runings]

.

Unless the cond is too large you normally only get about 4 to 6 deg subcooling on an air cooled cond.

So the subcooling is greater than I would expect and if you say the discharge temp is low that also points to an over sized cond.

Your discharge needs to be about at least 6 or 7 bar greater than the suction for the Thermostatic Expansion
Valve to work correctly so if you have 2.5 b (40 psi) suction and 7 or 8 b (100psi) discharge combined with the
TEV you might need to look at raising the head (discharge) pressure?

All the best

coolrunnings

.

[moondawn]

hi guys

madfridgie, i have a basic understanding of the components and how they work in an ideal world i would go through the whole training cd but hard to get the time sometimes, i flicked through the titles on the cd to find this info before posting but not sure its there, it does however on the first troubleshooting board give a working system as the first problem maybe it just means go off this guide maybe i need to do the whole program to come across the info i am after.

Hi coolrunnings
thanks for your input will try to check for that when troubleshooting

thanks all
cheers lee