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View Full Version : Low suction and discharge pressures, amperage is fine







romanowski72
20-01-2017, 04:12 AM
Hey guy, I had a 5' True sandwich prep table yesterday customer says not cooling, but had been in storage for 2 years, they purchased it used and when they picked it up, it was running and at correct temperature, they took it to storage right after they purchased, when they plugged it in couple days ago no cooling.
I got there and compressor and all fans run fine so I hooked up low side gauge, soon as I took the cap off the access port it was leaking I could hear it, so I tightened up the schrader valve and it stopped, put low side gauge on purged line and plugged unit in pulled to vacuum this unit is r134a, unit did not have a access port for discharge line I put in 8oz r134a thinking it leaked out through schrader while sitting for the two years. tag call for 9 oz.
Pulled straight to vacuum as soon as i turned off the gas, I put a temporary valve on high side off of filter dryer and my discharge pressure is only about 80psi. MY clamp-on temperature sensor went bad so i could not get superheat or subcooling, going back out there on monday to pull out the gas and weigh it, oh and my amperage was 7.1-7.3amps run amps for table rated 7.8amps.
small little section of frost at end of cap and inlet of evap, couple 3-4 inches maybe.
If i overcharged my amps would be high.
If it was undercharged amps would be low.
Plugged cap tube high side pressure would be high.
Bad valves and amp would be low
I did put a cover over condenser to see if pressure would go up and needle did not move.
Any ideas guys I am going back on monday thanks much for your time and input!

Tayters
20-01-2017, 08:13 PM
Just a thought, blocked cap tube and unit not run long enough to get any head pressure?
On similar systems I've measured subcooling with partial blockage is 8K, normal system about 2K and never seen more than 4K.

nike123
21-01-2017, 10:59 AM
Plugged cap tube high side pressure would be high.

Not necessarily! Pressure on high side is dependent on air inlet temperature, heat gain in evaporator and compressor and condenser TD! Condenser TD is determined with construction and air flow and cleanliness of fins etc....



MILD RESTRICTION (factory charge) = High superheat. Low suction pressure, evaporator TD & current draw. Normal to high subcooling. Normal to low head pressure.


http://www.refrigeration-engineer.com/forums/entry.php?355-9-superheat-amp-subcooling-scenarios



If heat gain is low on evaporator (because of restriction), than high side pressure will be lower than normal with mild restriction and all other factors normal!

High side high pressure is sign of severe restriction!

qcfish
21-01-2017, 11:54 PM
I do not agree with Roman's assumptions re amps. pressures, over charge, etc. I do agree with Brainiac's methodologies.
If you add the correct charge and it stays in a vacuum then a restriction is about the only normal cause. Assuming the compressor has not been replaced with a larger model etc. Head pressure should be low if suction is in a vacuum regardless of the other conditions. Amperage is actually high considering the suction vacuum. Normally if you at 85% FLA then you have a huge load on it unless the voltage is way off. Run cap ok? Wild guess......compressor worn which caused debris in the orifice and is also causing high amps. It can be verified easily with the SC. Isn't easier to replace the temp probe than keep adding *****, evacuation, etc.?

romanowski72
22-01-2017, 01:49 AM
Thanks for the help everybody! Brainiacs idea does make sense, maybe i have only seen hard cases of blockage in cap tube, where i have seen frost on the tube. I will trim the cap tube on monday and run some nitrogen through it and see what i can find.
Thanks again for help I will reply back what I find.
I will pick up a new sensor on monday morning as well it literally just died that day i always roll them up and they get caught on stuff in my bag lol.

nike123
22-01-2017, 07:28 PM
Thanks for the help everybody! Brainiacs idea does make sense, maybe i have only seen hard cases of blockage in cap tube, where i have seen frost on the tube. I will trim the cap tube on monday and run some nitrogen through it and see what i can find.
Thanks again for help I will reply back what I find.
I will pick up a new sensor on monday morning as well it literally just died that day i always roll them up and they get caught on stuff in my bag lol.

If it is transported with compressor in position where oil can get in condenser, and later in filter and capillary (discharge tube vertically pointing down), partial obstruction is possible because of that oil. Than nitrogen blowing will be temporally solution until majority of oil is still left in condenser.

If it is not worked a lot, and recently transported, that is my bet!

Tayters
22-01-2017, 08:05 PM
Thanks for the help everybody! Brainiacs idea does make sense, maybe i have only seen hard cases of blockage in cap tube, where i have seen frost on the tube. I will trim the cap tube on monday and run some nitrogen through it and see what i can find.
Thanks again for help I will reply back what I find.
I will pick up a new sensor on monday morning as well it literally just died that day i always roll them up and they get caught on stuff in my bag lol.

Hard to tell if a cap tube has a partial blockage by blowing nitrogen. It comes out the end but you have no way of telling if it comes out enough.
Need to confirm with subcooling or weigh out charge.
For what it's worth wouldn't keep the cap tube if suspected, save grief and replace it.