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Cap Tube Air Conditioners

It is often said that only two things should be inside any refrigeration system — refrigerant and oil — very little refrigerant.  The main cause of compressor failures is lack of lubrication.  How refrigerant is handled, or we should say mishandled, in a system is the main cause of lack of lubrication because all refrigerants are lousy lubricators!

Cap Tube Systems

Cap tube systems predominate in the air conditioning field up through as large as 7-1/2 ton systems.  Low cost was a factor for the switch to cap tubes from TXV’s, but another reason manufacturers of refrigeration systems embraced the cap tube as an expansion device was the ability to reduce the amount of refrigerant needed in a system.  As an example, a 5-ton air conditioning split system with a TXV would need about 15 lbs. of R-22 to operate.  The same system with a cap tube-metering device needs only 3 to 4 lbs. of R-22.

Every effort should be made to keep excess refrigerant out of a system.  Excess refrigerant getting into the crankcase of a compressor will soon lead to compressor failure.  The less refrigerant in a system, the less chance of excess refrigerant getting in the compressor crankcase causing lubrication failure.

From the OEM standpoint, a cap tube costs much less than a TXV.  No expensive pump down system is needed.  Starting components can be eliminated, since cap tube systems automatically pressure balance.  Less expensive, low starting torque motors can be utilized in compressors for cap tube systems.  Cap tubes require much less refrigerant.  Cap tube systems are lower weight and have a smaller physical size than TXV systems.  With precharged or evacuated line sets, the OEM has much tighter control of an installed system.  There is less expensive maintenance of cap tube systems.  There are less electrical components to fail, and since a cap tube has no moving parts, there is nothing to fail, except possible clogging of a cap tube, a rare occurrence, as long as the refrigeration system has never been opened.

This isn’t to say cap tube systems are any better than TXV systems, or for that matter, any worse.  When one truly understands them, they are no more difficult to service properly than an expansion valve system.

A major manufacturer of compressors conducted a study on residential air conditioners that had been serviced at one time or another.  They found that 85% of the units were overcharged! 

Since most of these systems were cap tube systems, designed to have as little refrigerant as possible in them, the addition of excess refrigerant guaranteed compressor failure at an accelerated rate.

The refrigerant charge of a cap tube system is critical.  Only enough refrigerant is needed to maintain a liquid seal at the entrance to the cap tube for it to function properly.  Any more will result in exceeding the refrigerant-to-oil ratio shortening compressor life.

Let’s review the relationship of refrigerant and oil.

R-22 is heavier than oil.  Excess liquid refrigerant that enters a compressor will settle to the bottom of the crankcase, the oil will float on top of the refrigerant.  There is always some refrigerant in the oil, but excess refrigerant will separate out of the oil.  When the compressor starts, the first liquid to be pumped to the bearings will be refrigerant, not oil.  The excess refrigerant will also instantly vaporize, or boil, due to the decreased crankcase pressure, causing foaming, even if the oil absorbed the excess refrigerant without separating.  This foaming action carries the oil out of the crankcase, and it may be many minutes before the oil returns in enough quantity to again lubricate the compressor.  Excessive wear, even catastrophic failure, occurs.

During the off cycle of cap tube systems, the refrigerant will bleed through the cap tube to the low side of the system.  Cap tube systems do not employ pump-down and don’t need pump down if properly charged.  When properly charged, the amount of refrigerant that will migrate to the crankcase will be small, small enough to be controlled by a crankcase heater.  Obvious conclusion; all compressors should have operating crankcase heaters, regardless of their location or ambient temperature conditions.  Most instruction manuals for homeowners warn them to turn on the power to their air conditioners for 24 hours before actually running them.  This energizes the crankcase heaters and gives them enough time to warm up the crankcase and drive off migrated refrigerant accumulated in the crankcase over the extended winter off period.  It is not a bad idea to put a warning label on the air conditioner disconnect as a reminder.

Overcharging a cap tube system also results in liquid refrigerant flood back, the refrigerant washing the oil off the moving parts.  Liquid flood back differs from “slugging.”  Slugging a compressor happens when a solid mass of liquid is suddenly introduced into the suction port of the running compressor and will probably result in the instant destruction of the compressor.  Liquid is non-compressible, and a slug of liquid will break the valves, shatter pistons, snap rods, etc.

Liquid flood back is a constant return of refrigerant particles to a running compressor, not large enough to be a “slug,” but large enough to wash oil off critical moving parts.  Overcharged cap tube systems will suffer from liquid flood back.  Too much refrigerant will pass through the cap tubes to be completely boiled off in the evaporator.  An accumulator could be added to prevent liquid flood back, but it should not be necessary in properly charged cap tube systems.

To be able to service the refrigeration part of a cap tube system, one must have an in-depth understanding of the terms saturation, superheat, and sub-cooling.  If one knows the meaning of these refrigerant states, where they will be found, and how variations in them affect a system, one can diagnose and properly correct system failures.

Saturation

Saturated refrigerant vapor can only occur when liquid and vapor are present together.  Saturation happens when any small addition of heat to a liquid causes the liquid to boil or a small removal of heat causes a vapor to condense.  The temperature and pressure at this point is the “saturation point,” the point at which liquid and vapors are present together.

There are only two places in the system where saturated refrigerant should be found.  One is the condenser, the other the evaporator.

In an air-cooled condenser, after the first bend or two, the hot refrigerant vapor begins to condense to a liquid.  Vapor pressure at that point is the pressure needed for the refrigerant to condense to a liquid at that temperature.  Evaporator saturation is found where the capillary lines enter the evaporator to within a few bends of leaving the evaporator.

Most systems do not have pressure taps at these points.  Since temperature and pressure correspond at the saturation point, it isn’t necessary to get gauge readings to determine pressure.  Taking the temperature and then using a T&P chart will tell you the pressure at those points.  (Conversely, if one can get pressure readings they could be converted to the actual temperature of the liquid refrigerant.)

The condensing temperature on an air-cooled system can never be cooler than the air blowing across it.  The refrigerant temperature should be 15°F to 30°F warmer than the air temperature (review Info-Tec 7 for condenser splits.)  Thus, a temperature close to entering air temperature indicates an undercharge, a restriction, or poor pumping compressor.  Too high a temperature indicates low airflow, poor fin/tube bond, restriction, or even the wrong refrigerant in the system.

For the evaporator, once again, the refrigerant temperature will never be as warm as the air it is cooling.

Saturated refrigerant temperature should be 10° to 20°F cooler than the leaving air.  Too wide a differential could be caused by undercharge, insufficient refrigerant feed, bad air or refrigerant distribution, excess oil in circulation, bypassing air, poor fin/tube bond.

On air conditioners that are air-to-air systems, saturated condenser temperatures will be warmer than entering air, but not too warm.  Saturated evaporated temperatures will be cooler than leaving air, but not too cool. 

Saturation temperatures are seldom used in servicing an air conditioner, superheat and sub-cooling being the important measures, but knowing what saturation temperatures are and what they mean may help in servicing some tough jobs.

Sub-Cooling

Sub-cooled liquid is at a temperature below its saturation point.  Sub-cooled liquid can only be found where no vapor exists.  It is obtained by taking sensible heat away from the liquid refrigerant.  The efficiency of a cap tube system depends a lot on sub-cooling.

Sub-cooled refrigerant will be found only in one place, from the last couple of bends of a condenser to the cap tube.  A liquid line is almost always sub-cooled.

In an air-cooled condenser, the superheat is rejected in the entering few bends, and then the majority of the rest of heat rejection is in the form of latent heat as the refrigerant vapor is condensed into liquid.  After the entire amount of refrigerant is liquid, any more heat rejection is sub-cooling.

Sub-cooling, then, is cooling a refrigerant below its boiling point.

Accessories and the line itself create friction in a liquid line as refrigerant flows through it.  This causes a pressure drop and, if there is no or little sub-cooling, the refrigerant will boil, resulting in “flash gas” in the liquid line.  These bubbles lower system capacity.  Sub-Cooling should be measured at the entrance to the cap tube.  To get sub-cooling convert gauge pressure to saturation temperature using a T&P chart.  Take the actual temperature of the liquid line.  Subtract the actual temperature from the saturation temperature.  The difference is sub-cooling.

Example:  R-22 Air-Cooled System 

Gauge reading of head pressure:  260 lbs.

260 lbs. is 120°F.

Actual measured temperature of the liquid line at entrance to the metering device 110°F.

120° - 110° = 10°F sub-cooling. 

Obviously, on air-cooled systems, the ambient air temperature, or entering air temperature of the condenser is the limit of sub-cooling ability.  Also, condenser design will cause variations in sub-cooling.  Most cap tube systems should have sub-cooling from 10° to 20°F, 15°F being average.

Superheat

Superheat is the temperature of a vapor above its saturation point.  Superheated gas can only be found where no liquid is present.  Superheat is the systems condition indicator.  Superheat is found only in two places in an operating system; entering and leaving the compressor.

Proper suction superheat is extremely important.  A liquid cannot be compressed (slugging) and floodback will damage a compressor.  Therefore, only superheated vapor should be returning to the compressor.  This insures no liquid refrigerant is entering the compressor’s suction port.  Conversely, the compressor needs to be cooled by returning refrigerant.  If refrigerant returns to a compressor too superheated, the compressor will overheat and be damaged.

Suction superheat will vary on an operating system, but should never be less than 10°F or more than 30°F.

Discharge superheat is a combination of suction superheat, heat of compression, motor heat, and friction.  A deviation of any of these factors will result in either more or less discharge superheat.

The range of discharge superheat is from 40° to 80°F.

If an R-22 air conditioner has a head pressure of 260 lbs., the saturation temperature (from a T&P chart) is 120°F.  If the discharge line temperature, measured about 6” from the compressor, is 180°F, discharge superheat is 60°F.  Thus, by knowing the head pressure, one can predict what the discharge temperature should be in a properly operating system. Too low a discharge superheat usually means low suction superheat and indicates floodback.  Too high can indicate high suction superheat, compressor electrical problems, or compressor lubrication problems.

By understanding the three refrigerant states, saturation, superheat, and sub-cooling, and applying this knowledge, servicing the running cap tube air-cooled air conditioner can be easily done.

In summary, proper suction superheat is critical for long compressor life.  Discharge superheat is the indicator of compressor operating conditions.

A review of Info-Tec 7, pages 4 and 5, will complete this Info-Tec.

Addendum

Occasionally, someone will attempt to apply a pump-down system to a cap tube system (probably a commercial refrigeration man who knows that the only way to prevent refrigerant migration to a compressor crankcase is by using a pump-down system).

Do not apply pump-down to cap tube systems.

First, they should not need pump-down.  Remember, a properly charged cap tube system contains a very small amount of refrigerant relative to its size, so the oil to refrigerant ratio is high.  As a percentage, the desirable refrigerant in oil will be about 10% to 12%.  A crankcase heater that raises the temperature of the oil about 25° to 40° F above ambient will keep the percentage to an acceptable level.

Pump-down employed on TXV systems respond quickly.  When the liquid line solenoid closes, the TXV will sense a rapid rise in superheat and the TXV will open fully, allowing liquid refrigerant trapped between the closed solenoid and TXV to be vaporized and quickly “pump out.”  With small bore cap tubes, the refrigerant flow will be slow, making the compressor run with high superheat at the suction port for too long a time before reaching the cut-out setting.

The differential setting of the low-pressure control would have to be beyond low-pressure control capability.  Cap tube system compressors are low starting torques.  The high and low side system pressures must almost balance before the compressor can start and run.  If the cutout setting was, say 30 lbs., the high side pressure at cut-out will vary according to ambient, but let’s say it is about 200 lbs.  On a call for cooling, the solenoid opens, and refrigerant flows to the low side, raising the low side pressure until the cut-in setting is reached and the compressor starts.  Because the compressor can’t run until high and low side pressures are almost equalized, a low-pressure control with a differential setting of at least 125 lbs. is needed.

There are some other reasons why pump-down is not used, but suffice to say trying to apply pump-down to a system that does not require it is not worth the trouble or cost.

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