Practical advise vs. textbook needed for this application.

[lobsterbob]

National Account OEM & consulting mech engineer would like to install 6 split systems with condensers on the roof (not a heat pump system); riser is approximately 220 feet, 70' of which is vertical. We have suggested smaller suction lines, though the engineer claims a larger line would be beneficial.

We have also asked OEM if they want a trap on the suction riser or an inverted trap on top and they say to just install solenoid at AHU and de-energize solenoid and compressor contactor at the same time (no pump down necessary). OEM does want a minimum 7 min on cycle though. We have suggested oil separators with heaters, but have not yet heard back.

We are in the process of decommissioning the 12 year old existing split systems and we are we are getting 2 1/2 gallons of oil from the line sets. Tenant has lost about 2 compressors a year with the former systems. Our recommendation is to use a CitiMulti unit which has no problem with oil returning to the compressor at 3285'.

Any suggestions I can make to further educate these textbook engineers to help the client get a problem free HVAC system? Client wants to hear from the field.

[R123]

Put in a small chiller.

[timjimbob]

Wow. I cannot agree with OEM about NO TRAPS NEEDED on this set up, but they are warrantying the compressors. I would get this in writing and also leave an access panel in the wall so you can add multiple traps later. Are they saying traps are only optional? I'd install them.

[syndicated]

70' rise and a 220' total run and they want to make the suction line bigger? Crazy

How about a double-suction riser setup? Maybe with some horizontal sections breaking up the elevation? There's no way you'll be able to maintain oil/gas velocity up a continuous 70' rise.
What kind of pipe size are they proposing? Unit capacity? There's established formulas to figure this stuff out properly, both copeland and trane have manuals out to help with this kind of system design.

I'm no old pro here, but maybe a different type of system would work better than a split?

[timjimbob]

Double suction is more suited for varying loads, like multuple evaps.

[lobsterbob]

2-20 ton with 2 circuits a 15 with 2 circuits and a 10 ton with 2 circuits. 1 3/8 x 1/2 on the 20 ton units 1 1/8 x 1/2 on the vertical then switch to 1 3/8 x 1/2 on the 15 and 1 1/8 x 1/2 0n the 1o ton unit.

[lobsterbob]

what about adding Temprite coalescent oil separators?

[timebuilder]

I'd do some research and find the charts that some OEM's publish that show line sizing to maintain the refrigerant velocity needed to get oil back up on the roof. Someone is being goofy here.

Don't let that someone be you.

[beenthere]

2-20 ton with 2 circuits a 15 with 2 circuits and a 10 ton with 2 circuits. 1 3/8 x 1/2 on the 20 ton units 1 1/8 x 1/2 on the vertical then switch to 1 3/8 x 1/2 on the 15 and 1 1/8 x 1/2 0n the 1o ton unit.

The 10 is 2-5 ton circuits then, and 1 1/8" is too large for good oil return, the velocity will be down to 1023FPM. 7/8" is better suited at a velocity of 1744 FPM, and a capacity loss of 1758BTU and a 3.9PSIG drop per 100 foot.

Better to use 1 1/8" for the horizontal runs, and then use 7/8" for the riser. Riser capacity loss would only be 1230 BTUs.

The 15 ton at 2-7.5 ton circuits will have oil return problems using 1 3/8" which only give s velocity of 1008FPM. 1 1/8" is better suited for oil return at a velocity of 1535 FPM, a capacity loss of 1522 BTUs, and a PSIG loss of 2.2 pounds per 100 foot.

Better to use 1 3/8" for the horizontal runs, and then use 1 1/8" for the riser. Riser capacity loss would only be 1065 BTUs.


The 20 ton at 2-10 ton circuits would have border line okay oil return at 1 3/8" with a velocity of 1344 FPM, and a BTU loss of 1241, with a PSIG loss of 1.4 per 100 foot. Oil return would be better with a 1 1/8" line at a velocity of 2046. But the capacity loss would be 3440 per 100 foot.Giving a total capacity loss of 7568 on 220 foot of suction line.

Better to use 1 3/8" for the horizontal runs, and then use 1 1/8" for the riser. Riser capacity loss would only be 2408 BTUs.


This is on the assumption that they are R410A systems.

[lobsterbob]

Wow, just what I needed for my meeting on Wednesday. I've been telling them from day one, that if I was doing a design-build, I would undersize the vertical suction riser one pipe size to keep the velocity up. Maybe if I let them read this, their eyes will be opened. The sacrifice in minimal btu loss is well worth a compressor that doesn't fail. What do you think about a TempRite coalescent oil separator as an addition to the system? Also, an ICM 450 phase loss monitor on each compressor? That would be a smart move I think. I've also suggested installing Plasma Air units on each AHU which would allow me to reduce the outside air from 5000 cfm to 2500 cfm. This would reduce the overall load 20%, which will reduce the run time on each compressor as well. Any thoughts? Thanks for your earlier response.

[beenthere]

Oil separator may help. With proper piping, you shouldn't have an oil return problem. I would have an inverted trap at the top of the riser to prevent oil from dropping back down to the evap. However, a separator won't hurt anything.

In refrigeration, a rack basically always has at least one compressor running. So velocity is lost when only one compressor is running, and oil separators are important to a long compressor life span. In comfort cooling you don't have as much of a variance in refrigerant velocity.

Phase loss protection is always a good idea.

[Poodle Head Mikey]

If it were me I would like to have big low-pressure-drop suction lines, oil separators at the compressors, and oil return pots at the coils. Of course getting the bid & spec boys to pay for all that will be the problem. <g>

[lobsterbob]

Thanks, looks like it more important to sell these guys on the fact that their risers are to big then sell them on a oil separator! Any thoughts on the Plasma Air units? I would think any reduction in run time is a reduction in risk? Plus they are heating with electric heat so this would calculate into about 8800.00 in utilities savings each year.

[beenthere]

I don't believe in them myself.

[Poodle Head Mikey]

Well; the better truth here is that forcing a DX system to do this envelope-pushing job is a little foolish to start with. For that kind of line run they'd be much better off with a chiller and water coils instead of evaporators.

PHM
------

Thanks, looks like it more important to sell these guys on the fact that their risers are to big then sell them on a oil separator! Any thoughts on the Plasma Air units? I would think any reduction in run time is a reduction in risk? Plus they are heating with electric heat so this would calculate into about 8800.00 in utilities savings each year.