Refrigeration 101

[chillyblue]

If the room temp is high, the superheat is supposed to be high. That's how a TXV works. If the room temp doesn't come down, then something else is wrong.

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,
Some rooms i've worked on can take hrs to pull down, there must be some indication via evap td or air dt that indicates correct operation.
Mistakenly so i've always gone by evap superheat, and now i'm trying to correct myself and an understanding of why is always helpful.

Thanks CB

[Gary]

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,
Some rooms i've worked on can take hrs to pull down, there must be some indication via evap td or air dt that indicates correct operation.
Mistakenly so i've always gone by evap superheat, and now i'm trying to correct myself and an understanding of why is always helpful.

Thanks CB

Let's say you have a room that is very warm. You increase the orifice size and you turn the adjustment screw out until you have what you believe to be the right superheat at the bulb. You have matched the refrigerant flow to the heat load.

You are currently overloading the compressor and condenser by absorbing a very heavy heat load in the evaporator. But if the compressor can handle the overload, the room temp will drop quickly, leading you to believe that you have done the right thing.

As the room comes down to its design temperature the TXV starts hunting (oversized orifice) and flooding (incorrect TXV adjustment). Why? Because the heat load is now insufficient to boil off the excessive flow of liquid refrigerant through the TXV.

If you have already left the job site, you may not even be aware that the system is hunting, flooding, flushing the oil out of the compressor and potentially going into self-destruct.

[Gary]

How high is high, 8, 10 ,20, 30, 50???????????
and if it is high what indication can you get from the evap that it is performing correctly,

Your indication comes from the condenser. Given a normal load, and assuming good condenser airflow, the SCT should be 20-35F/11-19K above the ambient (condenser air in) temperature (20-35F/11-19K cond TD), depending upon the relative coil sizing. If the heat load being rejected by the condenser is normal, then the heat load being absorbed by the evaporator must also be normal.

[chillyblue]

Your indication comes from the condenser. Given a normal load, and assuming good condenser airflow, the SCT should be 20-35F/11-19K above the ambient (condenser air in) temperature, depending upon the relative coil sizing. If the heat load being rejected by the condenser is normal, then the heat load being absorbed by the evaporator must also be normal.

Thanks Gary, thats great help thanks.
could you confirm what happens if you have a high load and a low load??
I'm assuming with a high load the SCT will reduce below 11-19K above air on to condenser and with a low load the SCT will increase to more than 11-19k above air on to cond.
I'm also assuming that under high conditons your subcooling will be less and with low load conditions your subcooling will more

[Gary]

Thanks Gary, thats great help thanks.
could you confirm what happens if you have a high load and a low load??
I'm assuming with a high load the SCT will reduce below 11-19K above air on to condenser and with a low load the SCT will increase to more than 11-19k above air on to cond.

Just the opposite. High load increases SCT and low load decreases SCT... and SCT above ambient (cond TD) should remain within the 11-19K range.

[chillyblue]

Just the opposite. High load increases SCT and low load decreases SCT... and SCT above ambient (cond TD) should remain within the 11-19K range.

If the ambient air onto the condenser remains the same and the sct decreases with low load then the cond td must get bigger.
If the ambient air onto the condenser remains the same and the sct increases with high load then the cond td must get smaller.

do you mean that with a high load the cond td would be around 11k (smaller) and low load would be around 19k (larger)

CB

[Gary]

If the ambient air onto the condenser remains the same and the sct decreases with low load then the cond td must get bigger.
If the ambient air onto the condenser remains the same and the sct increases with high load then the cond td must get smaller.

do you mean that with a high load the cond td would be around 11k (smaller) and low load would be around 19k (larger)

CB

No... No... and No.

Let's say the ambient is 25C and the SCT is 36C. The TD is 36-25=11K.

The SCT then rises to 40C. The TD is 40-25=15K .

[daviddwilson]

Just the opposite. High load increases SCT and low load decreases SCT... and SCT above ambient (cond TD) should remain within the 11-19K range.

There will be relatively little change in subcooling as the liquid line temp will tend to rise and fall with the SCT and a TXV system stores sufficient surplus liquid refrigerant to compensate for variations in load without substantial changes in subcooling.

[Gary]

I'm also assuming that under high conditons your subcooling will be less and with low load conditions your subcooling will more

There will be relatively little change in subcooling as the liquid line temp will tend to rise and fall with the SCT and a TXV system stores sufficient surplus liquid refrigerant to compensate for variations in load without substantial changes in subcooling.