low pressures

[sean1]

hi all
what can cause low suction and disharge pressure/temp on one unit at the same time

[Brian_UK]

A simple answer would be a change of temperature...

Do you any more information? Type of unit, situation of the change?

[sean1]

cellar cooling unit on r407c and suction at 1 barg discharge only at 8barg - unit is a beermaster - indoor a bmi 407
ambient temp is 5 celcius. did think of low temp.
what would people class as a low ambient temp?

[Colin G]

maybe over condensing. Condenser fan jammed on or is it water cooled? Its definatly not lost any gas?

[sean1]

unit air cooled condenser fan running but no fan controller(on all the time the compresssor runs)did originally think low on charge but after adding some refrigerant no real change and sight glass clear.

[Colin G]

is this a new system, thats just been installed or new occouring fault?

[sean1]

site i just took over. as far as i know its a new fault. on original call found unit iced up defrosted but didnt have gas.returned and leak checked and tried to charge the pressures stay in the same sort of range. it did have a hp/lp switch replaced by another before xmas.this unit must be about 6 yrs old at least.

[Colin G]

evap coil clean? what super heat you getting over coil?

[sean1]

i have heard people talk about low ambient tempertures affecting refrigeration equipment. but no-one seems to be able to tell me a temp or how to check for it.

[sean1]

evap coil clean - done whilst i defrosted it. the tev is a mop and room temp up on +18 - so i didnt check superheat ( should i have? thought superheat should be checked when room nearly at set temp especially with mops)

[mad fridgie]

i have heard people talk about low ambient tempertures affecting refrigeration equipment. but no-one seems to be able to tell me a temp or how to check for it.
Think of refrigeration as a circle or wheel, you change one pert of the circle, the rest of the circle changes. With a circle there is no start or end.
So to explain i will pick a starting point. TXV (TEV)
This is designed to allow a certain amount of liquid through at a certain pressure (can vary abit because the valve does open and close) But lets just say it is fixed. If the pressure going in is low (liquid pressure) then flow is reduced, (if high flow is increased)
Evap if flow is low then refrigeration work can not be done, so for work is to be done the pressure (evap temp) must reduce. Compressor if flow to comp is less then the compressor sucks a lower pressure. Less mass (weight of refrigerant) is going through the compressor.
Condenser with less refrigerant going through the condensor needs to reject less heat, so the difference between the condensing temp and the ambient is reduced to liquifiy the refrigerant. The lower the ambient, the lower the condensing temperature. Thus lower the liquid refrigerant pressure. At all times equalibrum is reached.
In simple terms
Lower condensing pressure (than design)
Lower suction pressure (colder evap temp)
Evap freezing up!
As far as an actual (fixed) figure, if your suction temp (SST) is below -2C, then freezing may start to occur. many ways to fix simple off cycle defrost, head pressure control (fan or valve) LP switch setting., to name a few.

[taz24]

i have heard people talk about low ambient tempertures affecting refrigeration equipment. but no-one seems to be able to tell me a temp or how to check for it.


Just look on your gauges to see what differance the ambient temp will make.

Just imagine the discharge condensing pressure temp relationship being about 15 deg above the ambient temp.

at 5 deg C ambient it would condense at 20 degC which = about 9 bar

at 0 deg C ambient it would condense at 15 degC which = about 8 bar.

It's basic fridge science.

Temp - Pressure relationship.

taz

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[sean1]

tanks
so have i got it right - eg.+5 ambient +15 for the condensing factor =20 or 9 bar either ways you are going to be on a below " 0 " presssure on the suction side for r407c which in tern means evap frosting/icing.
if i fitted cond fan controller would this have a effect

[taz24]

tanks
so have i got it right - eg.+5 ambient +15 for the condensing factor =20 or 9 bar either ways you are going to be on a below " 0 " presssure on the suction side for r407c which in tern means evap frosting/icing.
if i fitted cond fan controller would this have a effect

In very general terms....

For a beer celar the refrigerant will boil inside the evap about 10 deg lower than the air off the evap.

So if the room is at about +5 deg C the air off could be as low as 0 C to -5 deg C. At those temps
the suction pressure would be at about 3 or 4 bar.

If the outside ambient is say 10 deg C then the discharge could be as low as 7 or 8 bar.

Most TEV's need a minimum of 5 bar differance accross them to work properley.

With the ambient temps we have been haveing it has been colder on the outside than
it needs to be on the inside. Opening the celar doors would be enough as long as the stock did not get nicked :)

Beer celars struggle in this weather, because it can be colder outside than it is inside.

taz

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