New
instrumentation assures accuracy
By Adolfo Wurts
With increasing
energy costs and growing demands for more efficient cooling systems, the need
for accurate superheat measurements has become more important. Unfortunately,
many technicians have either forgotten what they were taught of superheat
in school or think it’s too much trouble to perform. They learn the
procedure early in school and practice it there, but they find it more difficult
in the field, largely because of forgotten techniques or ineffective measuring
tools. This lack of superheat measurements results in air conditioners that
are either under charged or over charged and in danger of compressor failure.
Superheat is defined as the difference between the temperature the refrigerant
boils at a given pressure in the evaporator and the temperature of the refrigerant
gas as it leaves the evaporator. In a worst case scenario with low indoor
heat load and the air conditioner still running, if overcharged, the refrigerant
in the evaporator would remain in liquid form in the coil and back up into
the compressor, steadily destroying it. In a properly tuned system, the refrigerant
continues to boil and exit the evaporator as a gas even under the worst possible
conditions.
A distinction must me made between systems which employ a TEV/TXV and those
which have a “fixed restrictor”/ “flow-rater”. The
superheat on a TEV/TXV should always be within a certain range regardless
of the condition. On a TXV/TEV system which is starved, the superheat will
go up above the range. But on an overcharged TXV/TEV system, the superheat
will not go below the range. For this reason, to properly charge these systems
you must charge to the subcooling. However, on a fixed restrictor the superheat
varies with the inlet to the evaporator wet bulb and out-door dry bulb. When
the correct measurements are taken, the technician learns how much heat the
refrigerant gas has picked up from the evaporator coil and gains insight into
how the unit is operating. A large temperature difference (superheat) indicates
that the refrigerant has turned to a gas a longer distance from the compressor
than a lower superheat. With this information in hand, the technician can
add or subtract refrigerant to meet a target superheat number.
The four key measurements
In reality, using superheat measurements to determine the correct refrigerant
charge is not that difficult and the savings in energy and potential repairs
are significant. All that the technician needs is a basic knowledge and some
modern measuring tools. The process requires only four measurements.
First, to find out the target superheat for a fixed restrictor system you
will need to measure two parameters
1. Outdoor air temperature taken from the air that is going into the condenser
coil.
2. Indoor wet bulb temperature taken by a wet bulb thermometer that has been
soaked in water and held in front of the indoor return grill or, better yet,
just in front of the evaporator coil.
Air conditioner manufacturers display the target superheat temperature on
or near the unit in the form of a chart. The target on the fixed restrictors
varies with the indoor wet bulb and outdoor dry bulb measurements. On the
required superheat charts the indoor wet bulb temperature is usually listed
across the top and the outdoor dry bulb along the side with the ideal target
superheat displayed where the two intersect.
When the technician has determined the target temperature, he needs only two
more numbers: The boiling point (saturation point) of the refrigerant at the
pressure in the evaporator and the suction line temperature.
The boiling point is easy. You can measure the pressure at the evaporator
with your pressure gauge and in most cases read the boiling point right on
the gauge. If the boiling point for the refrigerant you are working with is
not on the gauge, then you will have to look it up on a pressure temperature
chart.
To determine the temperature of the refrigerant in the suction line pipe,
all you need to do is measure the temperature of the pipe.
When you have the two, to get the actual superheat, subtract the boiling point
off of your gauges or charts from the actual evaporator superheat temperature.
If the actual superheat is lower than the target, remove refrigerant and if
higher, add refrigerant. Always let the system stabilize and check again after
adding or subtracting refrigerant.
The difficulty of getting accurate results
Unfortunately, as many technicians can attest, it sounds a lot easier than
it is. The problem is the three temperature measurements. They’re not
as easy to take as it would seem. While instrument suppliers have spent a
lot of time developing better instruments, many have missed problems faced
by field service technicians in the real world. These three temperature measurements
are good examples.
Outdoor air temperature
When measuring the outdoor temperature, for example, temperatures can vary
considerably in the area around the condenser. The only reliable place to
take the outdoor temperature is right in front of the condenser coils and
the thermocouple should remain there for a full minute to ensure accurate
measurement. Holding the thermometer there for the whole minute can be a problem.
Fieldpiece has added an alligator clip to a beaded k-type thermocouple (model
ATA1 so it can be easily fastened directly onto condenser grill and stay in
place as long as needed. It works with any meter that accepts a K-type thermocouple.
More accurate temperatures can be achieved by paying attention to subtleties.
With the most common K-type thermocouple thermometers, there is a temperature
reference junction inside the meter (the “cold junction”) that
is monitored by a thermometer inside the meter. Both of reference junction
and the thermometer need to be at the same temperature to ensure an accurate
reading. Some meters employ an adapter, with the reference junction inside
this adapter. Any difference in temperature between the external reference
junction in the adapter and the internal thermometer inside the meter will
show up as an error. Simply by holding this adapter in your hand you can alter
the reading that the multimeter displays. A solution developed at Fieldpiece
places the reference junction and the thermometer inside the meter on a ceramic
substrate so the reference junction and thermometer will be at the same temperature
regardless of the ambient temperature.
Wet bulb indoor air measurement
Indoor wet bulb temperature measurement also presents a problem. The “wetting”
material is difficult to find and also to affix on a thermocouple. Technical
articles, manuals and educational texts suggest such things as moistened toilet
tissue and paper napkins. One major instrument manufacturer suggests a piece
of cotton shoe lace, thus sending technicians on a fruitless quest since modern
shoe laces are made of synthetic materials and blends. Fortunately, some meter
manufacturers, including Fieldpiece, make a thermocouple made especially for
taking wet bulb measurements, (model ATWB1). It includes a permanently attached
“sock” which holds considerable water, has a large surface area
for evaporation, and has intimate thermal contact with the thermocouple. Here,
again, an alligator clip on the thermocouple makes a valuable contribution
to accuracy by enabling the technician to clip the instrument directly on
the indoor return grill or directly to the evaporator coil on the incoming
air side. It can be left in place until it completely cools down in the air
stream. It’s easy and reliable and can be used with any thermometer
using K-type thermocouples.
Suction line measurement
The last measurement presents an even bigger source for potential errors.
The suction line temperature cannot be measured with a simple pocket thermometer
because the thermal contact with the pipe is not good enough and the thermal
contact with the environment is too good. The resulting temperature would
be somewhere between the pipe temperature and the air surrounding it. The
trick is for the technician to find a way to measure only the pipe temperature.
One way involves the use of a standard beaded thermocouple with a Velcro strip.
Cutting back the insulation on the thermocouple about an inch, the technician
wraps the whole inch of bare wire around the pipe and holds it in place with
the Velcro. Another method requires the technician to push the beaded thermocouple
under the pipe insulation. For this to work, the insulation must be dry and
fit tightly. In both of these methods, the amount of thermal contact between
the thermometer and the pipe is still unknown and can vary with how snug the
thermometer is to the pipe.
The most effective way to accurately measure suction line temperature requires
a pipe clamp thermocouple such as Fieldpiece’s ATC1 or ATC3 thermocouple
pipe clamps. Both work with any meter that uses K-type thermocouples and provide
excellent thermal contact by squeezing directly onto the pipe and providing
good thermal isolation for the temperature sensor. This style of thermocouple
snaps quickly and easily onto a pipe and maintains a reliable thermal contact.
The problem in determining superheat centers on getting accurate measurements
in the first place. By keeping in mind the pitfalls involved in dry bulb,
wet bulb, and suction line measurements and by having the best tools available
to get accurate results, the modern technician can easily determine the appropriate
refrigerant charge for an air conditioner, given the conditions. Thereby assuring
efficient operation while avoiding serious damage to compressors.
Adolfo Wurts is a
Research Specialist at Fieldpiece Instruments and is a field certified HVAC/R
technician.
Fieldpiece
Instruments, Inc.
580 W. Central, Suite A
Brea, CA 92821
(714) 257-9060 FAX (714) 257-9069
fpinfo@fieldpiece.com
