Please Explain Subcool/Superheat Vs. Ambient Temp

[soiset]

I'm just a dumb civil/structural, so please forgive my ignorance. A residential split AC system is clearly intended to operate over a wide range of ambient outdoor temps. Here in Dallas, we can expect a system to run from 78 degrees to 112 degrees F. I know that excessive subcooling or inadequate superheating can result in liquid refrigerant at the compressor, which is a very bad thing. Would it be true that either or both of these conditions is more likely at lower ambient temps?

If that is the case, I would guess that AC techs would calibrate the charge and temperature expansion valve to ensure that no liquid reached the compressor at the lowest ambient temps at which the unit could be expected to run. Is that true? If it is, what happens if you run the ac when it is 60 degrees outside? Are you sending liquid to the compressor?

Also, if the above is true, does this mean that you have less and less subcooling, and more and more superheating at higher ambient temps, thus making the system less and less efficient as the ambient temps rise? Is the system most efficient at the presumed lowest temperature for which the system was calibrated? I'm flying blind, here.

Thank you,
Chris

 

[MintJulep]

Wow, how long have you got?

Yes, inadequate superheat at the evaporator exit may result in liquid slugging of the compressor - definitely a bad thing.

Sub-cooling, in and of itself, will not lead to liquid slugging. In fact, I would argue that there is no such thing as "excessive" sub-cooling. Greater sub-cooling gives greater performance and efficiency.

The only way for sub-cooling to lead to liquid slugging would be if the throttling device cannot throttle down enough, thus leading to no or insufficient superheat at the evaporator exit. If this condition were to exist, it would most likely happen at low outdoor temperature low indoor load conditions.

Preventing this is mostly a function of selecting the correct expansion valve or other throttling device. An excessive amount of refrigerant charge could mess things up, possibly leading to slugging - and other problems. Too little charge will lead to low suction pressure and evaporator freezing.

As a general rule, lower ambient temperature = increased sub-cooling.

There is a stronger relationship between superheat and evaporator load then there is between superheat and ambient temperature - although obviously evaporator load is a function of ambient.

The relationship between load and superheat is different for different types of throttling devices. A thermal expansion valve is an active control device, and its job is to maintain a constant superheat as the load changes. They do this will, provided you stay within the operating envelope. As noted above, if the valve can't restrict flow sufficiently, you may get slugging. On the other end, once the valve is wide-open, it can't do anything more. If full flow isn't sufficient to absorb the load, then superheat will start to increase.

 

[imok2]

MintJulep , I believe your statement: "In fact, I would argue that there is no such thing as "excessive sub-cooling " may be misleading. One of the problems of excessive sub cooling assuming that the system perameters are normal is that the condensing surface area is deminished thereby leading to a hi head pressures. A 10 to 15*F subcooling is usually taken as the norm for a TXV system with a 12 *f superheat at the sensing bulb for air conditioning.
Now when we are dealing with a fixed oriface or a cap tube situation then excessive sub cooling does/can lead to slugging if the system is overcharged that is why we charge by weighing or by the superheat method as the charge is more critical.

Low condenser entering air temperature will cause low head pressure from the excessive heat transfer between this cool ambient and the condenser coil. Low head pressures may reduce flow through some metering devices, particularly those that have capacity ratings dependant on the pressure differences across them.
A 30-psi pressure difference is usually the minimum across TXVs. This reduced refrigerant flow causes a starved evaporator that will in turn cause low suction pressures and high superheats. However, this may be offset by increased subcooling at low ambients.

The entire drop in capacity may decrease the air conditioner’s heat-removal abilities if it is not designed for it. If not designed properly, liquid will start to back up in the condenser. But, because of a low heat transfer rate caused by the lower condenser temperature, the liquid temperature in the bottom of the condenser will be low, causing liquid subcooling in the condenser to be increased.

Less refrigerant circulated means less work for the entire system to perform, so the amp draws of the compressor will be lowered.

If the system is set up for this reduced condenser air entering temperature, the head pressure can be designed to “float” or change with the changing ambient temperature. This will give lower head pressures and in-creased efficiencies. however, a TXV that is properly matched to handle reduced pressure drops across its orifice may need to be incorporated into the design.

 

[MintJulep]

Imok,

You are of-course correct. I was thinking only in terms of the refrigerant pressure-temperature-enthalpy diagram, not in terms of the full system as I should of been.

 

[rmw]

Mint and Imok, if you are still following this thread, I have a question that I think fits into the topic here. While I could have (mostly) answered the original question although not nearly as eloquently as you two did there is one thing that I have never been able to get my mind around regarding the refrigerant cycle. I am a degreed, licensed ME and I can read freon tables and P-T-E diagrams. I own 2 gage sets and they are well used.

Over the years I had the joy (agony) of keeping up a series of large motor coach size buses for a church where I used to live and by far, I'd say 50% of the total maintenance required by those buses was on the A/C system-not the heating system, the Freon side.

My involvement with the system included measuring and setting superheat at the evaporator outlets (there were two for the coach plus one for the drivers area), replacing and/or setting compressor unloader valves, replacing several whole compressors, as well as several valve decks in those compressors, clutches, seals, dryers, various pressure transmitters, switches, whole condensers, service valves, hoses and fittings. Plus the whole air handling side, fans, motors, motor brushes, duct work, etc. There weren't many parts of the system that didn't have my hands on it at some point over that time. I have an automotive certification so I could legally handle "the stuff". It was R-500 on the last bus and R-12 on the first several that I dealt with over 25 years.

The buses had an accumulator or receiver tank for the condensed liquid down stream of the condenser and per the bus manual, the proper way to fill the system (short of actually just weighing out the 35 lb. charge) was to charge until the liquid was above the lower sight (bullet) glass and below the upper glass. I typically tried to fill it until it just danced into the upper window in order to try to stay ahead of the leaks that the system inevitably had-get one fixed and another appeared shortly. The old hand pat test especially of the dryer which was immediately downstream of the receiver tank could give a reasonable assurance that there was liquid in the bottom of the receiver all the way to the dryer (the sight-glass wasn't able to be seen except by the use of a mirror-but that too was a check that was available too if I wanted to go to the trouble.)

The tank was handy in periods of non use because most of the charge could be pushed over into the high sided; condenser and tank and stored there until released back into the system by use of the service valves.

Now my question is this: When the tank was just slightly low, as in when the system flow fluctuated somewhat you could see that liquid would dance into the lower glass from time to time indicating that the true level was just below the glass, the system would know that and would perform poorly. Charging it until the level was just above the lower glass made all the difference and, of course, charging it until it appeared in the upper glass was real good.

Since the liquid lines all the way to the evaporators where the throttling valves were located were full of liquid since there was liquid in the lower part of the tank below the lower sight glass, how did the system know that the level where it needed to be?

The gages never read any different for the differing levels but you sure would know it in the bus when trying to get down the road on a hot summer day. It suffered when the level wasn't right.

The system didn't like overcharging either, which I tried to do when I knew that I had a leak working against me. The suction service valve was located on the front side of the compressor and required that the condenser be unbolted and swung (it was hinged) away from its mounting bracket to get to the valve so you didn't want to do that too often.)

I never understood how the system knew what the status of that tank was, but boy it did. No significant heat transfer took place there so that wasn't it. (It was located in the engine compartment so the delta-T wasn't there to help sub-cool the condensate to any extent.)

Is there an explanation? I have been trying to rationalize this for 25 years.

rmw

 

[MintJulep]

By definition, the condition of refrigerant in a receiver tank is saturated.

You don't mention a sub-cooler circuit in the condenser coil, nor a suction/discharge heat-exchanger, so I will assume there is neither. (At one time is was common to run the liquid and suction lines in buses together - in a pear-shaped insulation - to serve as a suction/discharge exchanger)

So, saturated liquid leaves the receiver and heads to the TXV. The length of the liquid line is probably on the order of 20+ feet, and there is probably a 10 foot elevation change.

The liquid's pressure drops and picks up heat along the way (the insulation probably was never that good, and certainly didn't get better over the years). Enough pressure drop and heat gain, and now you have some of the liquid flashing to gas before it gets to the TXV.

The extra weight of the column of liquid in the receiver can be enough to make a difference.

 

[rmw]

OK, I'm cogitating on that but while I do, I'll give some more details. The tank and the evaporator(s) are at essentially the same elevation if not even that the evaporators were slightly (probably less than a foot) above the C/L of the tank. As was the dryer. But your guess of 20 ft of distance is pretty spot on and heat gain is a definite possibility as the 10 feet or so of copper tubing that left the dryer (I don't remember it having much insulation if any) before it transitioned to rubber hose and entered the utilities "tunnel" located under the center aisle passed through the engine compartment and over the transmission and drive axle. The drive axle area was a major area of exit of engine heat (from the air inflow through both the radiator fan and the condensor fan) which wasn't able to exit straight down below the bus.

The condensor did not have a sub cooling section that I could detect although it was hot gas in at the top and liquid out at the bottom so unless there was any subcooling effect on the liquid that pooled at the bottom of the condenser before it was expelled by the pressure to the receiver, it was as you state, saturated. The suction line was in no way associated with the liquid lines in the sense of a heat exchange purpose.

One of the design flaws of the Eagle Bus (and I am a die hard Eagle fan) was that the hot engine compartment air exiting the wheelwells at the drive axle was sucked right back into the intakes of the condenser and the radiator. It could be plainly seen when running in a slight mist where there was a lot of spume coming off the wheels. That had to have put quite a penalty on the condensing temp/pressure of the system that was marginal at best. Especially when the coolest air over the road on a hot summer day was hovering around 100F anyway.

Once I realized this I developed a profound respect for the designers of the MCI (Motor Coach Industries) bus that Greyhound used, which to an Eagle fan is the same as a Ford to a Chevy enthusiast. But they got theirs right in that their condenser was located forward of the drive axle and its source of cooling air had never been near the engine compartment.

Thanks for you reply.

rmw

 

[soiset]

Ow, you guys hurt my head. Thank you very much for your thorough responses. I'll need some time to digest them before I can respond.

Chris