Refrigerant piping question 1

[amazing azza]

Hello friends,
We are about to start commissioning our HVAC system. However, I just noticed the following refrigerant piping connection that has been made at the outdoor/compressor unit.

Will the U-pocket not accumulate an oil plug? (Refrigerant is R407C). Also, does the filter-drier need isolation valves around it to facilitate changing it?

 

[itsmoked]

Yes on the oil plugging.

No on the filter because a closed refrigerant system should not continually have moisture or particulates that need to be continually filtered or dried out. That filter is just to catch whatever got into the opened system during installation. Same with the moisture and hopefully a vacuum was pulled on the system over night before charging so there should've been very little moisture but what was there will be trapped permanently in the filter.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

The oil plug will depend on the gas velocity in the pipe. Relatively low (< 2-3m/sec) and it will develop. Higher and any gas will be swept along.

I have no idea what gas velocities are in a small refrigerant system.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

May be this article will help OIL IN REFRIGERATION SYSTEMS - RSES.org

http://www.rses.org/assets/serviceapplicationmanual/620-1071.pdf

 

[itsmoked]

azza; I just ran your questions by my buddy who is a refrigeration 'god' around here. Laurence Livermore Labs always asked for him and only him to help them and that's a two hour drive one way.

He confirms while having a removable section for the dryer is convenient it's not necessary and wouldn't typically be done.

As for the oil... That low loop is acting as suction accumulator. The intent is to prevent any slugs of oil from falling from above straight into the compressor and so they should instead deliberately pile up in that U.

He goes on to say that the refrigerant velocity in that pipe is insane and once the oil piles up deep enough the gas will pick it up and in the short vertical section atomize it so a slug never does make it to the compressor. He said it's a poor-mans suction accumulator. He'd rather a proper suction accumulator but that will do. It works because of the short vertical and then the long sloped horizontal. It's acting as a P-trap.

He went on to say the most important thing is that the system should have pump-down to be reliable so you should make sure it does have that feature.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[chicopee]

To alleviate the problem of oil slugs, crankcase heaters and oil separators are normally installed on commercial systems.

 

[itsmoked]

By the way, if that is a compliant scroll compressor then pump=down is not actually very import. Definitely important if it's a reciprocating compressor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

Thank you itsmoked, LittleInch and chicopee

It's a relief to hear about the poor man's suction accumulator. I was not looking forward to redoing that piping Also good to hear about the filter/dryer not needing isolation.

Inside is a Copeland Scroll compressor (w/ crankcase heater). Why the difference with regard to pump-down importance between the scroll and the reciprocating?

 

[itsmoked]

With pump-down the thermostat controls only a liquid-line solenoid. Need cooling? The liquid-line solenoid is opened allowing refrigerant access to the expansion valve(s). The compressor is controlled by the suction pressure only. Once the liquid line solenoid is shut by the thermostat the compressor cares not, it just keeps pumping until it has reduced the suction pressure down to the appropriate cut-off pressure then shuts down. All the refrigerant is bottled up in liquid line receiver between the compressor's output check valves and the liquid line solenoid. There is no free refrigerant available in the system to return to the compressor. Once cooling is called for the liquid line solenoid is re-opened and eventually the suction side pressure rises and the rising pressure re-starts the compressor. You'll never slug the compressor.

Scroll compressors, since they don't have pistons, can't easily be slugged because a bunch of refrigerant has come wandering back during the off cycle and filled the compressor cylinder(s). While they can be controlled exactly the same way and may need to be in some applications they aren't as likely to be slugged and so are often controlled by the thermostat directly banging on and off without benefit of a liquid line solenoid.

Here a spectacular little demo link

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

itsmoked, would it be fair to say that a stand-alone refrigeration controller should not drive a reciprocating compressor motor contactor directly, but rather control the solenoid valve only? The motor would then be started/stopped via internal pressure switch mechanisms? Or is this more for prolonged periods of being powered down?

 

[itsmoked]

itsmoked, would it be fair to say that a stand-alone refrigeration controller should not drive a reciprocating compressor motor contactor directly, but rather control the solenoid valve only?

Yes, except in special fully constrained systems engineered to avoid the need. Absolutely not for long term situations but for safe daily functioning.

Yes, the compressor should then be fully controlled by pressures.
Low side #1 => start/stop control.
Low side #2 => Low pressure Lockout. (Loss of refrigerant. Running will fry the compressor because of the loss of cooling.)

High side #1 => condenser fan(s).
High side #2 => Lockout for over pressure protection. (Clogged condenser fins, dead condenser fan. Running will either cause a burnout as more HP is demanded than is available or the crank or rods will break. It's a race to disaster.)

Lockouts require human intervention for obvious reasons.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

by #1 and #2 do you mean progressively lower thresholds? For example, Low #1 would be "low", while Low #2 would be "very low"? Vice versa for high (#1 = high, #2 = higher)?

 

[itsmoked]

That was not my intent but does happen to apply here.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

itsmoked, so the pressure sensor would have to be multi-setpoint (low low, low, high, high high) in order for this to work, right? Or is it somehow possible to implement this algorithm with a pressure sensor consisting only of an N.C. contact (for locking out the compressor contactor) and a N.O. LOW pressure signal contact? (high and low thresholds are settable mechanically)

 

[itsmoked]

Almost never sensors. Switches. Typically you'd get as many as you need and set them with a screwdriver to what you need for each function.

They come combined. Separate. Integrated. What Ever.

Pressures switch madness

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

I see... and so you would feed all the LOW switches from a small "header" pipe that is connected to the low side of the compressor and all the HIGH's likewise from the high side?

 

[itsmoked]

Yep. Often called "service valves". They're gas tight capped valves that allow you to isolate the pressure switches for replacement without having to reclaim the entire system charge. There is often ports with said service valves in the compressor or even threaded ports on the compressor you can install the service valves in.

Typical example:

Keith Cress
kcress -

http://www.flaminsystems.com

 

[amazing azza]

thank you, itsmoked!