IMPROVEMENT AND VALIDATION OF UNITARY AIR CONDITIONER AND HEAT PUMP SIMULATION MODELS FOR R-22 AND HFC ALTERNATIVES AT OFF-DESIGN CONDITIONS (1173-TRP)
(Phase II of ASHRAE 859-RP, "Capacity and Power Demand of Unitary Air Conditioners and Heat Pumps Under Extreme Temperature and Humidity Conditions")
Within ASHRAE Research Project-859, -Capacity and Power Demand of Unitary Air Conditioners and Heat Pumps Under Extreme Temperature and Humidity Conditions", the accuracy of three public domain simulation models in predicting the performance of unitary equipment at extreme cooling conditions was investigated. It was concluded that the models indicated large errors in predicting performance in cases where the units were subjected to high temperatures and off-design airflow rates or refrigerant charge levels.
In cases where the airflow rates were modified to degrade system performance (reduced airflow rates at high outdoor temperatures), the computer models over-predicted the actual cooling capacity by up to 20% and underpredicted the compressor power consumption by up to 12%. The average deviation in predicting the cooling capacity at high ambient temperatures and off-design airflow rates was about 16% as compared to 5% at design airflows. LeRoy (1998) also showed that the models tended to overpredict sensible capacity and underpredict dehumidification.
The models had large errors in predicting the absolute charge in the system. Even when the models were tuned at the design condition for measured superheat and subcooling, the effects of off-design charge levels on system performance were inadequately represented.
These results were all obtained in tests with R-22 in split and packaged systems. These models have begun to be applied to the design of R-410A and R-407C systems but with correlation developed for R-22. Correlation developed for HFCs need to be implemented and the accuracy of the system models tested. The performance trends of these refrigerants under wide-ranging conditions will differ to some extent from R-22 because of lower critical temperatures, different property variations with ambient, and zeotropic mixture effects.
Work by Farzad et al (1994) and Orth (1995) show additional results using existing void fraction models for R-22 and R-134a systems. Robinson et al (1994) and Kattan et al (1995) provide additional data for charge levels and void fraction models for selected HFC mixtures.
ADVANCEMENT TO THE STATE-OF-THE-ART (JUSTIFICATION)
The design of high efficiency equipment for R-22 alternatives that performs comparably and does not compromise equipment reliability would benefit significantly from validated simulation models that can accurately predict off-design refrigerant conditions and system performance trends. The increased condenser size in higher efficiency designs relative to the evaporator makes good charge management even more important. Charge models that can predict at what conditions a system will begin to floodback refrigerant to the compressor or operate at excessive superheat, subcooling, or pressure ratios are important to avoid damaging operating conditions. Extremely mild ambients with reduced indoor air flow (Heflin and Keller 1992) can be as or more important in this regard than are extremely high ambients. The accurate prediction of capacity and power at extremely high ambient conditions is important in determining equipment sizing and power demand requirements. Also, by testing at such wide-ranging conditions, the simulations are better proven to be fundamentally applicable and can be used with more likelihood of success on the wide variety of indoor and outdoor hardware configurations on the market.
The project consists of the following tasks.
To two public domain air-conditioning models, the contractor shall work with program developers to
Once the model improvements are incorporated, the contractor shall test the improved models first against selected air-conditioning data for R-22 split and packaged systems with short-tube orifice or TXV flow controls from the Phase I project. The overall performance prediction accuracy of models with and without tuning, as done in Phase I, shall be assessed and, to the extent possible with available data, the contribution of each major component model to these errors shall be determined. Compare the performance of the improved models to those used in Phase 1.
The following component model testing procedure is recommended. For the compressor and flow control models, the contractor shall compare the agreement in mass flow rate (and compressor power input) at the design cooling conditions between experiment and model at measured inlet and exit conditions, as appropriate. For heat exchangers, the contractor shall compare the predicted capacities and saturated exit temperatures, before and after design point tuning, at measured evaporator superheat and condenser subcooling levels to experimental results. In this case, the contractor shall use the compressor model tuned from the earlier comparisons at the design ambient to match the insitu refrigerant flows at equivalent conditions.
In parallel with task 2, the contractor shall obtain test data with R-22, R-407C, and R-410A at the maximum, rated, and minimum cooling ambient conditions specified by ARI 210/240 (115/80/67, 95/80/67, 82/80/67, and 67/67/57) over a range of indoor airflows and humidities for selected split-system or packaged systems with short-tube orifice control. (These units should be selected in consultation with the PMS members.) Indoor airflow rates should range from the ARI maximum rated value of 450 cfm/ton to a minimum of 150 cfm/ton (the latter to include worst case indoor airflow conditions with highly restrictive ductwork). Indoor humidity values should range from dry coil conditions of about 20% indoor relative humidity (representing an arid climate) to high humidity of about 80% indoor relative humidity (representing startup conditions in humid climates). (These extreme indoor relative humidity tests correspond to the dry coil and condensate removal tests, respectively, that are listed in ASHRAE Standard 37-1988.) Last, tests with 20% and 40% low charge and 20% high charge should be conducted at all ambient test conditions.
The heat exchangers should be sufficiently instrumented with thermocouples at return bends to identify the fractions of coils in superheated, two-phase, and subcooled conditions (within two tube passes). This information is important in determining the proper operation of the heat exchanger models and charge prediction, especially in the condenser (Orth, et al, 1995). Both refrigerant and air-side capacity determinations should be made where the refrigerant remains single-phase leaving the heat exchangers and the agreement should be within the relevant ASHRAE testing standards (37-1988, 116-1995). For cases where the refrigerant is two-phase leaving the condenser or evaporator, the air-side capacity measurements shall be used to determine exit qualities.
The contractor shall assess the overall accuracy ranges of model(s) for these HFC alternatives and identify which major component models contribute most to the overall errors. The contractor shall provide guidelines as to how models should be used to obtain the observed accuracy levels. Operating conditions and heat exchanger configurations where detailed tube-by-tube heat exchanger models are needed for improved accuracy shall be identified. Recommendations for future improvements in correlation or models needed to further narrow these accuracy ranges for off-design performance predictions, especially at extreme conditions, shall be made. Finally, the contractor shall assess the need to perform similar testing and model validation in the heating mode.
LEVEL OF EFFORT
The level of effort is anticipated to be three man-month of the Principal Investigator, ten man-months of the Research Assistant, three man-months for the first subcontractor, and three man months for the second subcontractor (see Scope - Task 1) with the total project to be completed within 20 months.
CRITERIA FOR CONTRACTOR SELECTION
The PMS will select the contractor based on the following criteria:
ARI, 1994, Standard for Unitary Air-Conditioning and Air-Source Heat Pump Equipment, Standard 210/240.
ANSI/ASHRAE 37-1988, Methods of Testing for Rating Unitary Air Conditioning and Heat Pump Equipment
ANSI/ASHRAE 116-1995, Methods of Testing for Rating Seasonal Efficiency of Unitary Air Conditioners and Heat Pumps
Farzad, M. and D. L. OíNeal, 1994, "The Effect of Void Fraction Model on Estimation of Air Conditioner System Performance Variables Under a Range of Refrigerant Charging Conditions", International Journal of Refrigeration, Vol. 17, No. 2.
Heflin, Chris and F. Keller, 1992, "Performance and Reliability Degradation of a Residential Unitary Air Conditioning System Resulting from a Mismatch of a 1992 Air Conditioning Unit with a 15-20 Year Old Evaporator", Proceedings of the 1992 Intíl Refrigeration Conference -- Energy Efficiency and New Refrigerants, Volume 1, David R. Tree, editor, July 14-17, 1992, Purdue University.
Kattan, N., J. R. Thome, and D. Favrat, 1995, "Measurement and Prediction of Two-Phase Flow Patterns for New Refrigerants Inside Horizontal Tubes", ASHRAE Transactions, Vol. 101, Part 2.
LeRoy, Jason T., E. Groll, and J. E. Braun, May 1997, Capacity and Power Demand of Unitary Air Conditioners and Heat Pumps Under Extreme Temperature and Humidity Conditions, Final Report, ASHRAE Research Project-859.
LeRoy, Jason T., August 1997, Capacity and Power Demand of Unitary Air Conditioners and Heat Pumps Under Extreme Temperature and Humidity Conditions, Master's Thesis, Purdue University.
LeRoy, Jason T., E. Groll, and J. E. Braun, 1998, "Computer Model Predictions of Dehumidification Performance of Unitary Air Conditioners and Heat Pumps Under Extreme Operating Conditions", ASHRAE Transactions, Vol. 104, Part 2, pp. 771-786.
Orth, L. A., D. C. Zietlow, and C. O. Pedersen, 1995, "Predicting Refrigerant Inventory of R-134a in Air-Cooled Condensers", ASHRAE Transactions, Vol. 101, Part 1.
Robinson, J. H. and D. L. OíNeal, 1994, "The Impact of Charge on the Cooling Performance of an Air-to-Air Heat Pump for R-22 and Three Binary Blends of R-32 and R-134a", ASHRAE Transactions, Vol. 100, Part 2.
Related Papers from Texas A&M Energy Lab
Payne, W. Vance and D. L. O'Neal, January 1998, "Mass Flow Characteristics of R407C Through Short-Tube Orifices", ASHRAE Transactions, Vol. 104, Part 1A, pp. 197-209.
Payne, W. Vance and D. L. O'Neal, June 1999, "Multiphase Flow of Refrigerant 410A Through Short-Tube Orifices", ASHRAE Transactions, Vol. 105, Part 2.
Related Reports and Papers from Iowa State Engineering Research Institute
Wolf, D. A., R. R. Bittle, and M. B. Pate, 1995, Adiabatic Capillary Tube Performance with Alternative Refrigerants, ASHRAE RP-762, Final Report, Engineering Research Institute, Iowa State University, ERI-95413, May.
Bittle, R. R., D. A.Wolf, and M. B. Pate, 1998, "A Generalized Performance Prediction Method for Adiabatic Capillary Tubes", HVAC&R Research, Vol. 4, No. 1, January, pp. 27-44.
Related University of Illinois ACRC Reports
Graham, D. M., T. A. Newell, and J. C. Chato, December 1997, Experimental Investigation of Void Fraction During Condensation, ACRC-TR-135, Air Conditioning and Refrigeration Center, University of Illinois, 81 p.
Wilson, M. J., T. A. Newell, and J. C. Chato, July 1998, Experimental Investigation of Void Fraction During Horizontal Flow in Larger Diameter Refrigeration Applications, ACRC-TR-140, Air Conditioning and Refrigeration Center, University of Illinois, 158 p.
Yashar, D. A., T. A. Newell, and J. C. Chato, July 1998, Experimental Investigation of Void Fraction During Horizontal Flow in Smaller Diameter Refrigeration Applications, ACRC-TR-141 Air Conditioning and Refrigeration Center, University of Illinois, 114 p.
Kopke, J. R., T. A. Newell, and J. C. Chato, August 1998, Experimental Investigation of Void Fraction During Refrigerant Condensation in Horizontal Tubes, ACRC-TR-142, Air Conditioning and Refrigeration Center, University of Illinois, 93 pp.
Graham, D. M., H. R. Kopke, M. J. Wilson, D. A. Yashar, J. C. Chato, and T. A. Newell, January 1999, An Investigation of Void Fraction in the Stratified/Annular Flow Regions in Smooth, Horizontal Tubes, ACRC-TR-144, Air Conditioning and Refrigeration Center, University of Illinois, 19 p.
Newell, T. A. and R. K. Shah, "Refrigerant Heat Transfer, Pressure Drop, and Void Fraction Effects in Microfin Tubes", 2nd International Conference on Two-Phase Flow Modeling and Experimentation, Pisa, Italy, May 23-26.
Yashar, D. A., M. J. Wilson, H. R. Kopke, D. M. Graham, J. C.
Chato, and T. A. Newell, August 1999, An Investigation of Refrigerant
Void Fraction in Horizontal Microfin Tubes, ACRC-TR-145, Air Conditioning
and Refrigeration Center, University of Illinois, 21 p.