Research: Solving The Moisture Load
That’s what happened the morning of July 1, when papers were presented at 8 a.m. on the “Effects of Unitary Cycling on Unitary System Performance.” The session was standing room only, and for good reason: The researchers were discussing moisture loads from unitary air conditioners.
The studies themselves quantify things that have been talked about already. They confirm that system run times are an important contributing factor to the latent load and that the evaporator coil starts evaporating moisture after refrigerant stops running through it.
The news here was that all of these factors have been (and in some cases are still being) painstakingly measured in the lab and the field. The incoming data are matching up well with the mathematical models. And in a precompetitive technology consortium, equipment manufacturers are starting to examine new designs to address problems pinpointed by the models.
Once an operational effect has been measured, it can be modified.
The researchers specifically mentioned rapid-cycling system designs similar to those found in automotive A/C.
The Big DisconnectHugh I. Henderson Jr., Member ASHRAE with CDH Energy Corp., Cazenovia, N.Y., presented “Understanding the Dehumidification Performance of Air-Conditioning Equipment at Part-Load Conditions: Background and Theory.”
“Why is part-load dehumidification performance important?” Henderson asked, then quickly replied, “Because it affects space humidity levels at part-load conditions, where most operation occurs.”
Relative humidity (rh) levels higher than 60 percent can add to IAQ problems, he noted. Most hourly building simulation models do not consider humidity, he continued, “so they do not accurately predict space humidity levels or potential IAQ problems.” A couple of programs do take latent loads into account. These include a version of the “EnergyPlus” program to be released next spring, IHAT/DOE2, EPRI/SST, and FSEC 3.0.
Studies from the Air-Conditioning and Refrigeration Institute (ARI) have shown that rated or nominal sensible heat ratios (SHRs) have been the same over the years for higher and lower SEERs — but not so for effective sensible heat ratios. According to Henderson, “Part-load latent degradation can create this disconnect.”
Newer equipment may use larger coils and higher suction temperatures to achieve the same SHR (remaining near 0.75 SHR). In short, “Steady-state performance is unchanged, but part-load characteristics may be affected.”
And, indeed, they are. According to this and previous research, the latent capacity of a cooling coil degrades at part-load conditions. Part of the goal of the project is to “determine control approaches and equipment configurations that will minimize degradation and provide better humidity control.”
How Degradation OccursEarlier studies looked at part-load efficiency degradation; researchers in the latent load study said they were able to make some of the same part-load assumptions:
An additional assumption, said Henderson, is that off-cycle evaporation is an adiabatic process, with virtually no heat exchange.
If the compressor cycles on and off and the fan stays on, he pointed out, the unit starts up and reaches steady state; then it shuts off, but the sensible effect continues. The coil becomes an evaporative cooler, even though no refrigerant is flowing through it.
However, it takes time to build up to useful moisture removal — it takes 10 to 20 minutes of operation, he said, for enough moisture to build up on the coil so that it falls off. “If things do stay off for a while, that mass of moisture will return to the airstream,” Henderson said. Not enough moisture will build up and fall off, so the moisture becomes entrained back in the supply airstream, he noted.
That is latent load degradation. The original latent load of the ambient air is degraded by the added moisture off the coils.
“In 1996, we didn’t know the way moisture falls from the coil,” said Henderson. So, the researchers worked out equations to determine the rate at which moisture will fall off the coil at part-load conditions. Then those equations were compared to results from field data collected on a 3-ton residential water-to-air heat pump. The models worked.
Earlier studies from the University of Illinois examined the retention of moisture on coils, but these were specific to automotive A/C. Nevertheless, based on these studies, the researchers verified that moisture retention:
From this, the researchers improved upon an off-cycle evaporation model. There is “good agreement between the model and lab data.” According to the most recent study, the model can “predict the evaporation trend for any airflow and set of entering conditions.” This, in turn, led to a more detailed part-load model for residential and commercial unitary applications.
“I think there’s good news and bad news,” Henderson told The News.
He said a large (10-ton), single-stage commercial unit is an example of a system that would show a high degree of LHR degradation. However, the results bode well for two-stage units and some modulated units, he said. It also shows the importance of not oversizing equipment.
The results didn’t indicate any real regional differences based on climate.
“It’s a problem everywhere,” Henderson told The News. “People always suspected the constant-fan problem.” Constant-fan operation is practically never used in homes in hot, humid climates anymore, he said. At least, it shouldn’t be.
Lab And Field Work“Understanding the Dehumidification Performance of Air-Conditioning Equipment at Part-Load Conditions: Test Results” was presented by Don B. Shirey III, Member ASHRAE, with the Florida Solar Energy Center (FSEC), Cocoa, Fla. (The entire two-part paper was authored by Henderson, Shirey, and FSEC’s Richard Raustad; the project is funded by the U.S. Department of Energy.)
Shirey explained that once they had figured out a reliable working model, they tested up to 10 direct-expansion (DX) and chilled-water coils in various configurations at the FSEC lab, with different drybulb and wetbulb conditions and running times. They were typically on 45 to 60 minutes, then off for a similar period. The fan was always on.
In all, 28 parameters were monitored at 15-second intervals. The parameters included coil temperatures at various locations, air dewpoint leaving and entering the cooling coil, airflow, evaporator outlet and liquid line refrigerant pressures, condensate, and electrical consumption.
Actual lab results confirmed the evaporative cooling effect, Shirey said. It took about 17 minutes for the first moisture to fall. The off-cycle evaporation rate followed the theoretical trends.
The researchers then surveyed specifications for 500 commercial and residential air conditioning units. The goal was to determine the range of common coil geometries and variations by equipment type.
They found that the typical DX A/C coil is three rows, 15 fins-per-inch (fpi), with some variations by equipment type and size.
The real-world data conditions to date have matched the lab results. They are still analyzing and collecting this field data, Shirey said. The project is scheduled to end in September, but it probably will be extended through the end of the year. Future work probably will focus on using the data to create better coil configurations and control approaches.
Rapid CyclingA description of a possible design solution, now being examined by some manufacturers, was given by Clark Bullard, Ph.D. and Fellow ASHRAE, from the University of Illinois at Urbana-Champaign, Urbana, Ill., in the paper, “Rapid Cycling for Control of Capacity and Humidity.”
In addition, “New Defaults for the Cyclic Degradation Coefficient Used in Rating Central Air Conditioners and Heat Pumps” was presented by Brian P. Dougherty, Associate, of the National Institute of Standards and Technology, Gaithersburg, Md.
Bullard works at the university’s Air Conditioning Research Center, a consortium for precompetitive research. Bullard said it was established in 1989, after the Montreal Protocol changed a lot of known parameters for air conditioning systems.
According to Bullard, compressor and unitary equipment manufacturers have interest and participation in rapid cycling technology development. Likewise, “OEM and heat exchanger manufacturers are all interested in figuring out how to fool the system that it is getting a steady flow of refrigerant,” Bullard said, instead of the pulsed flow it actually would be getting.
As mentioned previously, the technology already exists, but it has primarily been used in automotive applications.
What is rapid cycling? Every 10 seconds or so, Bullard stated, the compressor is turned on and off. Can it reach the same efficiencies as current variable-speed unitary products? No. The question is, how to maintain efficiency — specifically, how to minimize losses on the refrigerant side, Bullard explained.
Researchers have measured evaporator cycling losses and condenser cycling losses, he reported. “The 3¼8-inch tubes on the evaporator dried out too soon,” and the condenser collected more moisture. A big factor was the heat exchanger, and lack of thermal mass in the outdoor coil compared to the indoor coil.
While it has lower efficiencies than variable-speed systems in the field, “It is still very close to [the efficiency of] a variable-speed unit with an inverter,” he said.
Industry also needs to determine how rapid cycling should be controlled. Should the compressor pulse every couple of minutes, for instance, or every 10 seconds? “How do we control it in such a way that the evaporator and condenser don’t know that the compressor is pulsing?” Bullard asked.
This technology has been worked on at the center for the last couple of years, he told The News. The manufacturers were aware that the information would be presented at the ASHRAE seminar.
How long until the technology might make it to commercialization? “We don’t ask,” said Bullard, “and they don’t tell!”
Publication date: 07/28/2003