A Better Understanding Of Superheat

April 29, 2005
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While there is a lot of talk these days about the coming of 13 SEER, there are more current concerns regarding existing 12-SEER units.

"There are estimates that many 12-SEER air conditioners are actually running at 6 SEER," said Rey Harju, president of Fieldpiece Instruments. "Other estimates say that technicians overcharge air conditioners most of the time.

"I don't know if there are any scientific studies on this, but after talking to many technicians about superheat, our general feeling is that most air conditioners in service do not have the optimum refrigerant charge."

Harju said part of the problem could be traced to an understanding - or misunderstanding - of superheat. "Few technicians measure superheat often enough, probably because it is too difficult."

Superheat Defined

Technical information from Fieldpiece provides a description of superheat.

"Measuring superheat is important because it can prevent damage to the air conditioner and make it run more efficiently. Superheat is the difference between the boiling point temperature of the refrigerant in the evaporator coil and the actual temperature of the refrigerant gas as it leaves the evaporator.

"After boiling, the refrigerant continues to warm up. The number of degrees it ‘warmed up' after boiling is called the superheat. Under the worst-case conditions (low load), the refrigerant in the evaporator boils near the end of the evaporator coil. To make sure liquid doesn't enter the compressor under the worst case condition of low load, A/C manufacturers publish charts indicating what the superheat should be at a given indoor wet bulb measurement and outdoor air temperature.

"If the superheat is too high, the air conditioner will be inefficient. If the superheat is too low, you risk flooding the compressor.

"You can change the superheat by adding or taking away refrigerant. Measuring superheat is the best indication on a fixed orifice system of the proper refrigerant charge and operating conditions. If everything else is working properly and the actual superheat is too high, add refrigerant. If it is too low, remove refrigerant. Using superheat to determine proper charge enables an air conditioner to deliver the best reliability and the best efficiency."

The ASH3 superheat accessory from Fieldpiece.

Measuring Superheat

Proper tools then become an important part of the equation.

"To measure superheat, a technician has to take a pipe temperature reading, take a pressure reading, convert pressure to temperature to get the boiling point or read the temperature scale on the gauges, write all that down someplace, and subtract the boiling point temperature from the pipe temperature," Harju said.

"That's all too complicated and time-consuming for many busy HVACR technicians," said Harju.

There are now several products on the market designed to make measuring superheat faster and easier. One displays the actual superheat from suction line pressure and suction line temperature measurements. An example of this kind of product is the ASH3 superheat digital multimeter (DMM) accessory head from Fieldpiece.

Harju said the superheat accessory head enables digital multimeters to measure both suction line pressure and suction line temperature simultaneously and calculate the actual superheat. It can be used in conjunction with a pipe clamp thermocouple (such as the Fieldpiece ATC1). Such a spring-loaded thermocouple clamp is designed to take suction pipe temperature.

"It firmly holds the thermocouple junction up against the suction line temperature measurements," Harju said.

He noted that the ASH3 accessory head "has a ‘stable indicator' LED which is important in measuring superheat because it tells the user that the reading is stable." The product can be used with a data logger to record measurements over time, and works with R-22 and R-410A.

The other product measures the parameters needed to determine the target superheat, indoor web bulb, and outdoor dry bulb temperature. An example is Fieldpiece's ARH4, which connects to most DMMs and displays both web bulb and air temperature.

"The ideal is for a technician to easily and quickly measure superheat," Harju said, "and avoid the urge to ‘wing it.'"

For more information, visit www.fieldpiece.com.

Publication date: 05/02/2005

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