Originally Posted by
Gary
Think of the system charge as consisting of low side charge and high side charge.
Assuming there is proper airflow through both coils (and that everything else is functioning normally):
On the low side, at design refrigerated space temp, if the coil outlet superheat is right, then the low side charge is right.
Coil outlet superheat varies with type of system. Generally speaking, a freezer should have 6-8F/3.5-4.5K superheat, a cooler should have 8-10F/4.5-5.5K superheat, and an A/C should have 12-16F/6.5-9K superheat.
On the high side the subcooling should be no more than 15F/8.5K because this is the point at which liquid will start to back up into the condenser.
On a cap tube/fixed orifice system, if the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then the system has the right charge.
Note that minimum subcooling is not a factor on cap tube/fixed orifice systems. They are critically charged.
On a TXV system there is a minimum and maximum charge. When the superheat is right (at design space temp) and the subcooling is not more than 15F/8.5K then there is enough refrigerant in the system to do the job. This is the minimum charge.
Additional refrigerant can then be added to bring the subcooling up to 15F/8.5K. This is the maximum charge.
It is good practice to bring the charge up to maximum because vapor bubbles flowing through the TXV will wear out the needle and seat. This also provides additional refrigerant for heavy loads.
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