But in the world of HVACR, there are more than just those familiar refrigerants. Some refrigerants have more specialized uses, and those due for phaseout still need alternatives.
One of those refrigerants is HCFC-124, used in certain chiller applications and “accepted as a major refrigerant for air conditioning in high ambient temperature applications,” stated Beixiong Zhang, vice president of Engineering for Lintern Corp. of Mentor, Ohio.
Zhang is in search of an R-124 alternative and recently published a paper on the topic. “Unfortunately, HFC refrigerants thermodynamically similar to 124 have not been developed yet,” he wrote. He said that “there is no official definition for the temperature range designated as ‘high ambient.’
“Conventionally, it refers to an ambient temperature range of 122 to 194 degrees F. R-124 is capable of covering this range. However, in most industrial high-ambient applications, it would be satisfied if the air conditioning devices could be operated up to 158 degrees. For our discussion, it is focused on the performance up to 176 degrees condensing temperature.”
While noting that no ideal match currently exists of available HFCs, Zhang said most current research, and testing is focusing on HFCs R-134a, -236fa, and -227ea.
He noted that all refrigerants in his study have the following conditions:
Refrigerant capacity is evaluated from the difference of enthalpies between vapor with quality of 1.0 at evaporating temperature and liquid with quality of 0.0 at condensing temperature.
Superheat is not included in the capacity calculation. Pressure drops along lines are neglected.
Temperature, PressureZhang said, “The critical temperature governs the upper boundary of temperature range for the application. R-124 and -236fa have critical temperatures considerably higher than the condensing temperature of 176 degrees. It implies a potential suitable for even higher condensing temperature. R-236fa has a lower pressure than 124.
“It should be pointed out that when the saturate temperature is lower than 29.4 degrees, the corresponding saturate pressure of 236fa will drop below the atmospheric pressure. R-134a and -227ea have about the same critical temperatures, which does not leave much room for a condensing temperature over 176 degrees. The critical pressure of 227ea is lower than that of 134a.”
Zhang said the level of maximum condensing pressure “is one of the most important factors that limit the operational ambient temperature.” He noted that 124, 236fa, and 227ea “all have mild condensing pressures.”
He further commented, “Although the condensing pressure of 134a at 176Þ (2.63 MPa, or 381 psia) is quite higher than the others, it is still practical. If the system is carefully designed to control the delta-T between the condensing temperature and ambient within 18 degrees, all the four refrigerants discussed should allow the system to operate at 158 degrees ambient.”
Compression Ratio“The compression ratio is an important parameter affecting the performance of a real compressor,” Zhang noted.
“The volumetric efficiency is determined by the type and structure of the compressor; and, it is also a strong function of compression ratio. A high compression ratio will cause significant drop on volumetric efficiency, hence reducing the actual capacity of a compressor.
“The compression ratio is defined as the ratio of condensing pressure to evaporating pressure. If compressed between the same interval of evaporating temperature and condensing temperature, a refrigerant with higher condensing pressure does not imply a higher compression ratio.
“R-134 has the lowest compression ratio. R-124 and -227ea have higher compression ratios, which are almost identical.
“The worst one is 236fa, which has a considerably high compression ratio of 8.699. It demands a high-quality compressor with perfect volumetric efficiency to maintain the actual delivered capacity with that high compression ratio.”
Cooling CapacityThe cooling capacity of refrigerants describes the cooling potential from 1 kg of refrigerant cycling under specified conditions, according to the author.
“It indicates the efficiency of the refrigerant. An ideal refrigerant should have high cooling capacity, thus less refrigerant needs to be cycled for a given load.
“Comparing the cooling capacity, we find that in a cycle between 45 degrees of evaporating temperature and 176 degrees of condensing temperature, the 134a has the highest one, and 227ea has the lowest one.
“If we take the cooling capacity of 124 as unity, then the 134a, 236fa, and 227ea will have cooling capacities of 1.160, 0.855, and 0.395, respectively. Comparing 134a and 227ea to get an equal cooling effect, the mass flow of 227ea has to be about three times that of 134a.”
Performance CoefficientTextbooks tell you that the coefficient of performance (COP) is a measure of energy efficiency, defined as the ratio of cooling capacity to isentropic compression work.
In the comparison of the refrigerants in his report, Zhang noted that the 45 degrees of evaporating temperature is identical for all cases.
He added, “124 and 134a have very close COPs at condensing temperatures of 140 and 158 degrees. At 176 degrees, however, the COP of 134a is slightly lower.
“It reflects that the efficiency of the refrigerant at that condensing temperature starts to drop. R-236fa and -227ea both have lower COPs. Especially for 227ea, the COP at 176 degrees of condensing temperature is only 51% of that of 124; i.e., to obtain the same amount of cooling, the energy consumption has to be doubled if you replace 124 with 227ea. For 236fa, the energy consumption increases 18.4% over that of 124 at 176 degrees of condensing temperature.”
Compressor SizeOnce again, a check of a textbook shows that the required compressor displacement for 1-kW cooling capacity with a volumetric efficiency to be unity is determined by the refrigerant’s cooling capacity (kJ/kg) and the density of suction vapor (kg/cu m).
A lower cooling capacity of refrigerant and lower density of suction vapor will require larger compressor displacement, and vice versa.
Once more, Zhang worked from that given to see how these alternative refrigerants operate as compared with 124.
“Under the condition of compression between 45 degrees of evaporating temperature and 176 degrees of condensing temperature, as comparison, if we take the compressor displacement required for unit cooling capacity for 124 as unity, the displacement for 134a will only be 0.6120, and for 236fa and 227ea it will be 1.645 and 1.768, respectively.
“In other words, the displacement requirement for 124, 236fa, and 227ea is respectively 1.634, 2.688, and 2.889 times that for 134a.”
He added that this comparison is based on the assumption of an ideal unity volumetric efficiency. He said, “As real compressors, the volumetric efficiency is less than one and is strongly dependant on the compression ratio. Since all the 124, 236fa, and 227ea refrigerants have higher compression ratios, the actual displacements required for them are even bigger.”
Wet CompressionIf liquid is generated during the compression, it is called “wet compression.” Zhang noted that wet compression is not wanted because of its destructive impact on the compressor.
“For a refrigerant with wet compression, additional superheat is required at suction status to ensure a complete dry compression process. Generally speaking, for a given unit, a higher superheat on the suction vapor will result in the reduction of cooling capacity.
“For an isentropic compression, 124 has mild wet compression, 236fa and 227ea have significant wet compression, and 134a does not have any wet compression. Compressed from 45 degrees of evaporating temperature to 176 degrees of condensing temperature, to avoid being wet during the compression, 236fa requires at least 27.74 degrees superheat at the suction point, and 227ea requires at least 29.18 degrees superheat.”
Zhang indicated that these are extremely high degrees of superheat. “This increases the application difficulty and also reduces the system performance. In practice, for safety reasons, a superheat several degrees higher than the minimum requirement is always necessary, which results in an actual superheat even higher.”
ConclusionsAs a replacement of 124 in the ambient temperature range up to 158 degrees, Zhang said 134a should be the first choice, “provided that the condensing temperature can be controlled under 176 degrees and the maximum condensing pressure of 2.63 MPa can be handled.”
He added that 134a has the highest refrigerant capacity, lowest requirement on the volumetric efficiency of compressor, and requires the smallest compressor. Also, there is no wet compression.
“The compression will always be dry as long as the suction status is dry. Besides, the thermal expansion valve (TEV)for 134a is one of the most available TEVs on the market. However, limited by its lower critical temperature and higher pressure, it is not recommended to be applied in any situation yielding a condensing temperature over 176 degrees.”
Regarding 236fa, Zhang contended that that refrigerant allows operation at condensing temperatures over 176 degrees because of its high critical temperature. “Its pressure is low. However, it might be too low to keep positive all the time.
“Compared to 134a, the compressor for 236fa has to be as big as 2.688 times in displacement. The COP for 236fa is lower than 134a, which yields higher power consumption and a larger motor. The high compression ratio for 236fa makes the real size of the compressor even bigger. Its significant wet compression further complicates the application. Finally, a TEV for 236fa is not available and needs to be developed.”
Turning to 227ea, Zhang said it has very low efficiency. “Its critical temperature is approximately the same as 134a, and that determines a similar upper boundary of condensing temperature as 134a; i.e., the condensing temperature should not exceed 176 degrees.
“If operated between 45 degrees of evaporating temperature and 176 degrees of condensing temperature, compared to 134a, the cooling capacity of 227ea is as low as 34 percent of 134a. To deliver the same amount of cooling, 227ea requires a compressor as big as 2.889 times in displacement; and the power consumption of 227ea (or the size of motor) is as much as 1.87 times.
“The wet compression of 227ea is even worse than 236fa. A TEV for 227ea is not available. The only merit for 227ea is its lower condensing pressure. However, the overall performance of 227ea does not demonstrate it as an attractive refrigerant.”
For more information, contact Lintern Corp., 8685 Station St., Mentor, Ohio 44060.
Note: The original material had temperatures in Celsius, which were converted to Fahrenheit in the text for U.S. publication.
Publication date: 02/03/2003