Low Pressure tripping

[Beginner]

Hi

Any help would be appreciated, please be patient I'm still learning

I have a problem with a chilling circuit that trips on low pressure.This is a basic 15 kw R407c system just comp, tev(ext equalised) Condenser and evap. There is a main fan which circulates air over the finned evap coil and another fan which circulates air over the finned condensing coil.( both fans are running ok) there are 2 tranducers for low and high side readings. The unit is tripping on low pressure switch ( set approx 2 bar)and judging by the readings this appears to be correct. The pipework from the tev and distributors to coils is frosted over.I thought maybe we had a leak ( short of gas or tev partially blocked ?)
According to the on site info the system charge is 6kg r407c.I recovered the gas into a recovery cylinder which weighs in at 5kg....would that cause the prob?
Appologies for the grammer and spelling , any further info ( what bit I know ) will gladly be made available.System running readings were :-

low side = around 2 bar but trips
hight side = around 9 bar.
Room temp = 28 c
outside temp = 16

Hope I've made sense

Thanks

[nike123]

According to the on site info the system charge is 6kg r407c.I recovered the gas into a recovery cylinder which weighs in at 5kg....would that cause the prob?

Thanks

Of course that 17% less refrigerant will cause the problem. More in to that is, that this refrigerant is blend of 3 components with high temperature glide (7K).
Now, not only that you have less refrigerant then required, but you also have unknown composition of components.

[Brian_UK]

Hi and welcome to the forum.

Your spelling looks OK so don't hide in the corner yet...

Well if you took out 5kg and there should be 6kg it appears that 1kg has escaped.

This means that there is a leak somewhere, all you have to do now is find it and fix it. Sounds easy but probably won't be.

[Beginner]

Hi

Thanks for the replies. May I ask another quick question? Yes?...ok..lol

What would be the differing symptoms in terms of high and low side pressures of :-
1.....low charge
2.....restricted tev

Thanks any help is appreciated.

[Peter_1]

What would be the differing symptoms in terms of high and low side pressures of :-
1.....low charge
2.....restricted TEV
Thanks any help is appreciated.

Low charge = low LP and low HP
Restricted TEV = Low LP and and low HP (low load to the condenser) but higher HP then when low on charge (liquid will back up in the condenser)
Less or more normal HP.

More important low charge = high SH and low SC
Restricted TEV = high SH but very high SC (the high SC is the biggest difference)

Decomposition of the R407c due to the leak is possible.
SI there something to control Hp (fan controlled by HP?) Your HP seems very low for an ambient of 16°C outside.

[nike123]

Hi

Thanks for the replies. May I ask another quick question? Yes?...ok..lol

What would be the differing symptoms in terms of high and low side pressures of :-
1.....low charge
2.....restricted tev

Thanks any help is appreciated.

Differing is that, when we have restriction at TEV, we experience lot of subcooling in the condenser, which is not the case with low charge.

[Peter_1]

Nike 123, we're exactly on the same line.

[nike123]

Where is delete button???

[nike123]

Nike 123, we're exactly on the same line.

Yep, i saw that you already posted same answer when I pressed submit reply button!

Newer mind, this way, message is twice stronger!

[Beginner]

Thanks all for replies. Can I also ask how for each case ( low charge and restricted tev ) the discharge side is affected.
Thanks

[Beginner]

oops ..missed a word out there ,,,,should have said discharge side temperature in each case...thanks

[hendry]

Hi beginner,
for a start, i'll connect external pressure gage on high, low & oil path for better readings.

next, i'll run the system with few guys standing at different points to observe each equipment esponse.

and, if the lo press trip too fast, i'll check from liquid supply line downwards until comp suction port.
otherwise, i'll increase refrigerant charge to allow me longer hours to identify the possible cause.


hope that helps.

hendry, on wesak day holiday.

[Peter_1]

oops ..missed a word out there ,,,,should have said discharge side temperature in each case...thanks

Well, in both cases because the evaporator is underfeed, you will have a high SH, so high to very high discharge temperatures. This is the same for both.
You can see this in a log/P chart

[Beginner]

Hi

Now this is were I get confused ....I thought because little gas getting through tev = high superheat but I also thought maybe little gas = low duty and therefore low load so no gas to compress so lower discharge temps...put me out of my misery someone...lol...thanks

[nike123]

Hi

Now this is were I get confused ....I thought because little gas getting through tev = high superheat but I also thought maybe little gas = low duty and therefore low load so no gas to compress so lower discharge temps...put me out of my misery someone...lol...thanks

http://www.hvacwebtech.com/1marks.htm

[Peter_1]

Or the PPP at http://www.hvac-pro.org/articles/superheat.pps

Your question goes back to the real basics of a log/p chart.
Without the intention to being rude or to humiliate you, I'm sure nobody ever learned you how a log/P chart works.
As long as you don't really understand this, there will always be gaps and faults in your way of thinking.

But your first post and your nickname says you're a beginner and these are normal questions but you surely should take some time to read a good book which explains the beginning of cooling process and more specific the log/p chart.

I suggest that you read once Roy Dossat's book which is a very good start.

[Beginner]

Ok thanks for your patience