I'm working on a newer Hussmann parallel rack and under moderate load my drop leg is running consistently 1-5psi higher than the dish charge. This changes when the rack is under a higher load (i.e. higher than 85 or 90 ambient or case just coming out of defrost) I've searched quite a few threads and have found discussions about A8 and A9 adjustments but none of these apply since it all hinges on having higher discharge than drop leg. I agree that this is highly unusual and have exhausted my resources including some very intelligent experienced techs and some highly educated engineers. I'll describe the system starting from the discharge header at the rack. From the header it passes through the oil separator and then enters the heat reclaim valve. Between the separator and the HR valve is the discharge line feeding the A9 valve (solenoid controlled). After the HR valve the line continues 20 vertical feet and about 60 horizontal. To the inlet to the air cooled condenser on the roof. Out of the condenser it returns to the rack and first passes through the A8 then passes through a tee. On the top of the tee sits the vertical surge receiver. After passing under the receiver at floor level it continues to the LL filter drier then sight glass then liquid header. The receiver has a vent line that runs from the top side of the housing to the condenser inlet line at the point where you would place a purge valve. There is a check valve in the vent line allowing gas to escape the receiver but not enter. This is a low temp rack running 404a and electric defrost. Floating head based on condenser TD floating suction. The heat reclaim is reheat for the seasons 4. My theory has to do with gravity as it relates to different refrigerant densities. After the condenser the refrigerant is at a higher density than it is when entering this makes it heaver. If you have two pipes, each on their own scale, one is full of water and the other only contains air. Which one exerts more weight or psi on the scale? This is as far as I've gone with this theory. Does anyone have another explanation or something to add that might disprove the gravity theory? This is the root of my issues involving logging all of my refrigerant in the receiver and only leaving a small river of liquid in the liquid line. Note: when I close the vent line from the receiver to the condenser my receiver drains enough to give me a solid column of liquid. This is significant because it enters the condenser piping where the condenser line goes horizontal after the vertical riser. This could be an area of lesser pressure than what's at the bottom in the liquid filled receiver. This also spells a possible issue with low velocity in condenser piping?
AverageRefTech
Its good to have some confirmation. It seems logical but I've never had it cause a problem like this. Then the issue is solely with the design of the surge receiver and related components. The engineer for the customer has a flexible combiner that locks out the A9 valve unless the saturated condensing temp is greater than 90 deg f and the receiver is over 50%. As long as I'm in the lockout range I'll likely be logging liquid in the receiver. Without changing the program config the only way to stop this from happening is either by closing the vent line or raising the A8 to fill the gap. So adjusting it to hold back at around 200 psi would allow it to empty. Its allow like they intended the receiver to work passively as opposed to using the controls to drain, fill or bypass the receiver. So expecting the surge receiver to act more like an expansion tank for a plumbing system.
According to all of the generalized installation info I've read, the vent line is to allow the receiver to fill. Some systems use a "sewage drain where the drop leg is large enough to allow a stream of liquid from the condenser to drain to the receiver while still allowing enough free space in the line for vapor to move freely. This allows vapor to be displaced by the liquid as it fills the open cavity of the receiver. The other way is to size the drop leg to the calculated size required to handle only liquid and then vent the top of the receiver to allow the vapor to be displaced. Hussmans manual refers to this as an equalizer line and shows that the installation of that linen is correct to their specs. Unfortunately the Hussmann manual does not offer instructions for a system with a vertical receiver. The difference being that hussman. Designs their diagrams for non-trapped drop legs and by using a vertical receiver in the way they have on this rack it makes it a trapped drop leg system. The A9 taps into the upper side of the receiver about 3/4-7/8 up the side.
Other than the tank filling up what other issues are there? It's very common for the liquid line sight glass to be 'rivering' and not filled to the top. It sound like the A9 solenoid should be cycling ON when the main liquid line subcooling drops below 2 or 3*F, to flush the tank. Similar to how the old Attron setups use to be.
Given the set points you described above, are you only having this problem during periods when the ambient is below 62F?
The flooding valves and condenser fan cycling controls need to be set such that you always have a higher pressure in the condenser so that the A9 can actually pressurize the receiver.
You will have the additional pressure at the bottom of the drain leg due to the weight of the liquid. If you don't have an offsetting pressure drop in the holdback valve, allowing a differential between the condenser pressure and drain leg pressure, you'll not be able to pressurize the receiver. When a demand for liquid exceeds what is coming from the drain leg, and that pressure is higher than the receiver pressure, you've got a problem.
The flooding valves and condenser fan cycling controls need to be set such that you always have a higher pressure in the condenser so that the A9 can actually pressurize the receiver.
You will have the additional pressure at the bottom of the drain leg due to the weight of the liquid. If you don't have an offsetting pressure drop in the holdback valve, allowing a differential between the condenser pressure and drain leg pressure, you'll not be able to pressurize the receiver. When a demand for liquid exceeds what is coming from the drain leg, and that pressure is higher than the receiver pressure, you've got a problem.
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