# Thread: What determines the static pressure?

1. Originally Posted by ICanHas
or standing/still state/equalized refrigerant loop pressure of a stopped system.

If the entire system is at uniform temperature, it will be whatever the unit is at, so if you leave a window A/C in your trunk and its at 120F, the entire hermetic system will pressurize to 264 psi or so for HCFC-22

In the case of a split system where the condensing unit's sun baked to 120F and evaporator coil is at 90F, what can I expect the static pressure to be?

Lowest temperature point, highest point, or something in between?

For a package RTU in Phoenix, AZ on a 105F baked in the sun, is it unreasonable to assume it will reach 135F? (vapor pressure of R-22 is 315 psi).

This means you can leak test any component using nitrogen at whatever R-22 vapor pressure system is expected to see under normal design parameters, no? So for a RTU package, even if you don't know the maximum permissible pressure, its ok to pressurize the system to at least 315 psi, no?
The name plate has some pressure testing info too. One a split system, pump it down and run the CFM to bring temp to normal and then take your pressure reading. On a RTU let it rest, get an accurate temp reading (kinda tough with the radiant heat from the sun...)

2. Originally Posted by timebuilder
Wouldn't that be standing pressure?
Potential pressure exerted in all directions by a fluid at rest; for fluid in motion, measured in a direction normal to the direction of flow.

Standing is probably the laymen term.

I'm fairly certain, he used the proper term.

With the possible exception of using the term/phrase saturation pressure.

3. Originally Posted by beenthere
Potential pressure exerted in all directions by a fluid at rest; for fluid in motion, measured in a direction normal to the direction of flow.

Standing is probably the laymen term.

I'm fairly certain, he used the proper term.

With the possible exception of using the term/phrase saturation pressure.

When I use the words "standing pressure," I am indicating more than the fluid being at rest, but also having achieved pressure and temperature equilibrium, usually so that the saturation temperature for that pressure can be compared to the ambient temperature, for determination of the presence of non-condensibles in the system.

"Static pressure" has a familiar and clear meaning, as we saw from some of the responses above.

4. On a hard shut off TXV. "Standing pressure" won't be at equilibrium.
Or a pump down system.

However. While the unit is off, you will still have a static pressure in the lines, condenser, and evap, etc.

5. ## What determines the static pressure?

I do. That's my job.

6. Originally Posted by beenthere
On a hard shut off TXV. "Standing pressure" won't be at equilibrium.
Or a pump down system.

However. While the unit is off, you will still have a static pressure in the lines, condenser, and evap, etc.
I have not seen many non-bleed txv's lately. Maybe they are favored by a brand I don't usually run into.

Pump down systems are usually refrigeration setups, so that would be a unique sector in the industry. I don't think we are dealing with refrigeration in this situation.

Most of the AC systems I see have no trouble equalizing. In fact, many don't use a TXV at all.

I have to agree that if I want to measure "static pressure," it will involve my 510, and not my gauge set.....and I have to admit, this is the first time I have heard someone mention static pressure outside of a discussion of fans and ducts.

7. And what would be the point in knowing this, I wonder? Would the poster state why this is necessary information.

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As long as the system is physically able to equalize (no hard shutoff TXVs, no solinoids) The system pressure will correspond to the portion of the system that contains liquid and saturated vapor which is usually the coldest point unless the coldest point is already full of liquid.

Imagine the following system:

coil capable of holding 3 LBS in the basement is 70 degrees

Linset capable of holding 2 LBS in the walls is 80 degrees

condenser capable of holding 6 LBS outdoors is 90 degrees

If that system has less that 3 LBS of refrigerant, it should show a pressure that corresponds to 70 degrees.

If it is holding more than 3 but less than 5 LBS it will correspond to 80 degrees

If its holding more than 5 it will correspond to 90 degrees

9. Originally Posted by timebuilder
I have not seen many non-bleed txv's lately. Maybe they are favored by a brand I don't usually run into.

Pump down systems are usually refrigeration setups, so that would be a unique sector in the industry. I don't think we are dealing with refrigeration in this situation.

Most of the AC systems I see have no trouble equalizing. In fact, many don't use a TXV at all.

I have to agree that if I want to measure "static pressure," it will involve my 510, and not my gauge set.....and I have to admit, this is the first time I have heard someone mention static pressure outside of a discussion of fans and ducts.
Static pressure is mentioned in water systems/piping/plumbing, air planes, and many other things.

Static pressure, is simply the pressure exerted by something in all directions.

10. Originally Posted by craig1
As long as the system is physically able to equalize (no hard shutoff TXVs, no solinoids) The system pressure will correspond to the portion of the system that contains liquid and saturated vapor which is usually the coldest point unless the coldest point is already full of liquid.

Imagine the following system:

coil capable of holding 3 LBS in the basement is 70 degrees

Linset capable of holding 2 LBS in the walls is 80 degrees

condenser capable of holding 6 LBS outdoors is 90 degrees

If that system has less that 3 LBS of refrigerant, it should show a pressure that corresponds to 70 degrees.

If it is holding more than 3 but less than 5 LBS it will correspond to 80 degrees

If its holding more than 5 it will correspond to 90 degrees
I believe what you posted would violate the laws of psychics.

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Originally Posted by beenthere
I believe what you posted would violate the laws of psychics.
explain?

12. Your basing your saturation on all the refrigerant being in only X part of the system, if it only has X amount left in it.

Even if it only has 3 pounds in it. There will be some liquid in the condenser, and the line set.

13. Originally Posted by DeltaT
And what would be the point in knowing this, I wonder? Would the poster state why this is necessary information.
Is it a crime to ask questions without stating the purpose of necessity?

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