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07-21-2008, 07:26 PM #1Professional Member
- Join Date
- Jul 2008
Question(s) from a hvac/r beginner
Hello. I am relativley new to the trade, and have a somewhat of an understanding of the basics. There is just some things about refrigeration/ A/C that I'm having trouble understanding. First off, take a service call where the system is low on charge (any refrigerant). If im low on charge you'd think id have low suction pressure, but it is in fact it is a little high (?). Im I correct in thinking thats because the box temp is high due to the abscense of liquid in the evap and thus not allowing the box temp to drop keeping the boiling temp/suction prssure high? While i know the superheat is high because the suction line temp at the compressor (even though i know it should really be checked at the evap outlet and it feels warm there) is warm. The evap is clean as is the condenser and both have good airflow. Head pressure is "okay" cant remember exactly what but i remember it was about 120 condensing temp.
I was also wondering about pressure controls used for a tstat. Ive heard and accepted the fact that this application only works with a TXV and not a cap tube/ fixed orifice. Is this because the cap tube equilizes right away during the off cycle thus keeping the contacts closed and not allowing off cycle?
While i have your attention i was wondering about sight glasses. Ive heard it said that there only applied to TXV's yet ive seen them on fixed orifice A/C systems?! Thanks for reading!!!!
07-21-2008, 09:57 PM #2
Second I'm not sure what you are talking about pressure controlls for a thermostat. Are you talking about a compressor controll on a pump down system?
Third TXV's equalize they are either internally or externally equalized it may take a little longer than a captube type system but they have to equlize so the compressor does not start under high head pressure.
And as far as sight glasses go that is how you can easially tell if they unit is low on charge and if there is moisture in the system. You will usually see them installed on walkin's but I have seen them on residential split systems, they just let you know if you have a solid colloum of liquid leaving the condenser witch is what you want on any type of refrigeration or A/C system.
07-22-2008, 01:50 AM #3
Very confusing questions
Most of the time when the unit is low on refregerant
the pressures are low also.
Refer units very common that the cap tube start to get restricted
due to poly oil. ( caused by ditry coil ) dont confuse this by low on
Hvac - I had a heat pump once where the unit was low on refregerant
and my pressures was on the higher side.
I thought I had a bad valves at first
because my suction pressure was approx 90 psi
superheat was high.
I charge the unit then my suction pressure started to drop.
This unit had a bigger txv and it was wide open causing
my pressure to be high. as soon a my suction line temp
started droping the txv started closing down causing
the pressures to drop
Pressure switch used for temp control for cap tube. I have no problem
with those controls even on cap tube system.
Use superheat only if the system is already at or near design box
07-22-2008, 01:53 AM #4
The only way to know for sure if the unit has enough refregerant is
to weigh it in.
07-22-2008, 02:00 AM #5Regular Guest
- Join Date
- Jun 2008
07-22-2008, 02:50 AM #6
Most refer unit
2-3-4 doors cooler, freezers have a the proper
use a scale and charge it according to manufactures charge.
and I add approx 3 onces for my gauges
HVAC unit also have a charge stamped on it.
Bigger system is not so critical.
Dont always trust the sight glass.
most refregerant (new ones) has 2-3 blends of other refregerant
they all dont condense at the same rate.
Must check each refregerant on actual tempreture glide.
07-22-2008, 03:16 AM #7Regular Guest
- Join Date
- Jul 2006
These arent perfect answers but thoughts to think of. Good questions.
Remember higher temps = higher pressures. whatever the metering device. A slightly low system wont show the same pressures as an allmost empty system. For proper superheats the box or whatever needs to be near desired temps. A txv may open wider or lack of liquid can cause a higher cap system pressure
Keep a much closer watch on condensing pressures and temps where the real work gets done and subcooling. Unless your ambients are over 85 degrees try to maintain 95-105 degree target condensed refrigerant temps at proper space temps. Over 90 ambients add to that accordingly at 120 ambient then 150 refrigerant temps wouldnt be too far out of line, higher if conditioned area is hot.
I prefer pressure controls for the cut in temps over temperature controls on walk-ins etc. . Unless its below 10 degrees ambient Avoids a lot of evaporator freeze ups. I let the customer keep a few degrees on the temp control to play with. Be carefull and allways set by quages.
Sight glasses are your best tool on many systems to determine liquid levels but dont place total faith that this means full charge. Might be something plugged up or closed downstream.
07-22-2008, 09:26 AM #8Professional Member
- Join Date
- Jan 2005
- Veterans Home Yaphank, NY
First of all welcome dave1234. Second of all the rest of you had better start brushing up.
Take superheat at the evaporator outlet only. I guess you never heard of compressor superheat or Andy Schoens article "Evaporator Superheat vs Compressor Superheat."
Using a pressure control to regulate box temperature is quite common, except on "Capillary Tube" applications as the pressures must balance before restarting. Expansion valves adjust to the load and maintain constant evaporator superheat.
Sight glasses are used to observe the flow of refrigerant in the system and with blends you will see occasional bubbles because of fractionation.RAM Teaching Tomorrows Technicians Today.
07-22-2008, 07:03 PM #9
07-22-2008, 08:43 PM #10Professional Member
- Join Date
- Jul 2008
Hello and thank you (and everybody) for the reply. Thats also something I dont get. I guess it makes sense that you should have to check superheat while the box is near desired temp, but if the box is at or near temp why did the customer call for service? I understand that recovring and weighing in the charge is the best why to be sure, but that may not be an option at times. I guess what im trying to say is ithat if the most effective way to check charge (assuming there is no sight glass) is superheat, how can you efectivley do so if the box temp is high and cant/wont pull down. If it were at or near temp, I wouldnt be there.
07-22-2008, 08:44 PM #11
07-22-2008, 08:57 PM #12
07-22-2008, 10:13 PM #13