DOE/ORNL Heat Pump Design Model

DOE/ORNL Heat Pump Design Model

Notes on Mark VII Web Model

The DOE/ORNL Mark VI Heat Pump Design Model (HPDM) was upgraded in 2005/2006 to the Mark VII version in conjunction with ASHRAE Technical Research Project (TRP)-1173 (Shen et al 2006).

This involved major upgrades to the refrigerant property routines as well as the refrigerant-side heat transfer and pressure drop correlations. There was also significant modification to the air-to-refrigerant heat exchanger routines to track the refrigerant glide more properly and to accommodate refrigerant mixtures such as R-407C with gliding temperatures in the two-phase region.

The compressor routines were modified to accept compressor performance maps based on either dew-point temperatures or mean heat exchanger temperatures; the compressor superheat correction algorithms were also revised based on TRP-1173.

Charge inventory calculations for off-design performance were revised per recommendations from TRP-1173 to implement a charge calibration procedure using the condenser subcooled refrigerant fraction.

Included separately online is a presentation on the Mark VII improvements . The presentation gives details of the heat transfer and pressure drop multipliers for R-22, R-410A, and R-407C for rifled tubes compared to smooth tubes and Mark VI assumptions. In addition, system performance results for a sample system of similar nominal capacity are given for the same refrigerants.

Refrigerant Property Modifications

The existing refrigerant thermodynamic properties used in the ORNL HPDM through Mark VI were replaced entirely with the new routines of REFPROP7 from NIST, the current industry standard (McLInden et al, 2002). ORNL used a modified version of REFPROP, version 7.1, provided by NIST (Lemmon 2004) under a separate DOE project. This version was developed specifically to run faster than their general mixture version for the leading commercial mixtures of R-410A, R-407C, R-507, and R-404A by use of pseudo-pure property representations. A number of interface routines were written to enable this property data transition. These were implemented and tested against the full mixture version in the HPDM to check that the results obtained were essentially the same in either case. The only discernable difference was that the model execution time with the pseudo-pure versions was 1-2 seconds while for the mixtures it ranged from about 80 seconds for a two-component mixture such as R-410A to over 200 seconds for a three-component-mixture such as R-407C.

As for the thermophysical property routines, similar faster running versions were not available in REFPROP 7.1. In this case, ORNL continued to use the transport property routines of Geller (2000), for the four leading HFC mixtures, as first adopted in Mark VI, which are valid up to very close to the critical point.

Heat Transfer and Pressure Drop Correlations

As part of this project, the refrigerant-side heat transfer and pressure drop correlations were completely replaced. Based on the literature review conducted in TRP-1173, the best new HFC-capable heat transfer and pressure drop correlations were selected and implemented for smooth and rifled tubes. To implement the rifled tube correlations, new input parameters were added to the ORNL HPDM to allow the rifled tube geometries to be properly specified. While previous heat transfer correlations in the HPDM were generally for annular flow, with some provision for stratified flow in the condenser, the new correlations were fully flow-regime based. This required implementation first of flow-regime maps for the condenser and evaporator followed by flow-regime specific heat transfer calculations. A listing is provided of the correlations used for smooth and rifled tubes for refrigerant evaporation and condensation at the end of this summary.

These routines were first implemented outside of the system model using the EES (Engineering Equation Solver) program and tested against results in the reference papers and other recent correlations where appropriate. ORNL compared the enhanced tube augmentation factors for heat transfer and their pressure drop penalties to those of the smooth tube correlations to check for reasonableness and to further compare with simplified performance multipliers used in earlier ORNL HPDM versions. This process identified some problems with the initial implementations which were resolved by further communications with the authors of the correlations.

Last, these two-phase flow regimes and heat transfer values were evaluated in the ORNL model in 5% quality increments and integrated over the two-phase region in a manner consistent with the integration approach used in the Mark VI version to obtain average two-phase values.

Modifications were also made to the heat exchanger routines to better model the mixture temperature glide effects of refrigerants such as R-407C. In the condenser, an effective specific heat was calculated for the two-phase region from enthalpy change divided by temperature glide. The condenser was adapted from the original cross-flow configuration to also handle counter-crossflow and parallel-crossflow arrangements. In the evaporator, a simpler mean temperature approach was used with crossflow arrangement, in part because of the reduced temperature glide from the counteracting pressure drop effect and in part because of the added complexity in modifying the dehumidification calculations for other configurations.

No changes were required to the air-side correlations for smooth and enhanced fin surfaces. These had been recently upgraded in the latest version of Mark VI and were found by Purdue to be state-of-the-art.

For the flow control routines, no improvements were made for this project. The short-tube orifice equations used in the HPDM are the latest correlations from Payne. For the capillary tube model, ORNL had recently updated the equations for R-410A in ARTI-21CR/605-50015-01 (Rice 2005) based on ASHRAE work of Wolf and Bittle (1995). These are used instead of the ASHRAE recommended generalized correlations for R-410A (Bittle et al 1998) based on problems with the transport properties used in development of these correlations (as noted in Rice 2005).

Compressor Model Modifications

Provision was made to use compressor maps based on either dew point or mean (mid-point) heat exchanger temperatures for refrigerant mixtures such as R-407C. Many of the compressor maps for mixtures were originally developed based on mean (mid-point) heat exchanger temperatures rather than dew-points (compressor inlet and exit saturation temperatures). This implementation allows either to be used. For the mean temperature implementation, the condenser exit subcooling must also be specified as input to define the evaporator inlet saturation temperature for the mean evaporator calculation.

Next, near the end of the TRP-1173 project, the compressor superheat correction algorithms were also revised based on Purdue recommendations. This involved essentially removing the compressor power correction. ORNL also added as new optional input parameters the adjustment factors used for the mass flow rate correction and the option to include a power correction factor in the future. These revisions were not made until near the end of the TRP-1173 work so as to not interfere with the final testing of the ORNL model by Purdue. With these revisions, the final model should give better agreement with the test results in cases where the inlet to the compressor was at saturation temperature or at exit qualities less than 100%.

Improved Charge Inventory Analysis

Last, as part of the project, ORNL worked with Purdue to improve the off-design predictions of the model by modifications to the refrigerant charge inventory calculations. This involved implementing new inputs and adjustment equations for the refrigerant charge calculations in the condenser subcooling region. By use of a suitable calibration approach developed in TRP-1173, based on lab data on system performance changes over a range of charge levels, the tracking of condenser subcooling levels and thus system off-design performance predictions over a range of ambients and charge levels was significantly improved. No improvements in refrigerant void fraction models were made as this effect was seen to be small relative to the benefits from the charge correction approach that was proposed. ORNL continues to use the Hughmark void fraction method in our charge calculations.

ORNL implemented this charge calibration approach along the lines proposed by Purdue. By determining the offset in heat exchanger length needed to match the experimental subcooling change resulting from a given change of refrigerant charge, a mass per unit HX length ratio k is determined. This ratio is used along with a reference subcooling length to apply the charge correction factor at other off-design conditions. Also added per Purdue recommendations was an unaccounted charge value. In summary, from one charge variation test, a k-factor, a reference subcooling fraction, and an unaccounted charge value are determined.

These new inputs were added to the Mark VII Web version in the input section dealing with “Component Dimensions - Refrigerant Charge Calculation” as Charge Adjustment Factors @ Calibration Point.

Once all the new properties and correlations were implemented, the new program was installed on the Web for use by Purdue in TRP-1173. The Web version was used by Purdue in comparing the ORNL model predictions to their test data for R-410A and R-407C unitary air-conditioning systems.

Summary of Refrigerant-Side Heat Transfer and Pressure Drop Correlations Used in Mark VII HPDM Upgrade

Evaporative heat transfer correlation for smooth tubes: Thome [1]

Evaporative flow regime map for smooth tubes: Thome and Hajal [2], Kattan [10]

Condensation heat transfer correlation for smooth tubes: Thome, Hajal, and Cavallini [3,9]

Condensation flow regime map for smooth tubes: Hajal, Thome, and Cavallini [4]

Evaporative heat transfer correlation for rifled tube: Cavallini, Del Col, Doretti, Longo, and Rossetto [5,8]

Condensation heat transfer correlation for rifled tubes: Cavallini, Del Col, Doretti, Longo, and Rossetto [6]

Pressure drop correlation for both smooth and rifled tubes: Choi, Kedzierski, and Domanski [7]

Heat Transfer and Pressure Drop References

[1] J.R. Thome, On Recent Advances in Modelling of Two-Phase Flow and Heat Transfer, Proc. 1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, HEFAT2002, 8-10 April, 2002, Kruger Park, South Africa, Vol. 1, Part 1, pp. 27-39.

[2] J.R. Thome, J. El Hajal, Two-Phase Flow Pattern Map for Evaporation in Horizontal Tubes: Latest Version, Proc. 1st International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, HEFAT2002, 8-10 April, 2002, Kruger Park, South Africa, Vol. 1, Part 1, pp. 182-187.

[3] J.El Hajal, J.R. Thome, A. Cavallini, Condensation in Horizontal Tubes, Part 1: Two-Phase Flow Pattern Map, Int. J. Heat and Mass Transfer, 2003, 46, pp. 3349-3363.

[4] J.R. Thome, J.El Hajal, A. Cavallini, Condensation in Horizontal Tubes, Part 2: New Heat Transfer Model Based on Flow Regimes, Int. J. Heat and Mass Transfer, 2003, 46, pp. 3365-3387.

[5] A. Cavallini, D. Del Col, L. Doretti, G.A. Longo, L. Rossetto, Refrigerant Vaporization Inside Enhanced Tubes: A Heat Transfer Model, Heat and Technology, 1999,17(2), pp. 29-36.

[6] A. Cavallini, D. Del Col, L. Doretti, G.A. Longo, L. Rossetto, Heat Transfer And Pressure Drop During Condensation Of Refrigerants Inside Horizontal Enhanced Tubes, Int. J. Refrigeration, 2000, 23, pp.4-25.

[7] J.Y. Choi, M.A., Kedzierski, P.A. Domanski, A Generalized Pressure Drop Correlation for Evaporation and Condensation of Alternative Refrigerants In Smooth And Micro-Fin Tubes, NISTIR 6333, NIST.

[8] G. Censi., D. Del Col , L. Rossetto, Vaporisation of Refrigerants in Horizontal Microfin Tubes, Proc. of XXI National Conference on Heat Transfer, Udine, Italy, pp. 307-312, 2003.

[9] D. Del Col , A. Cavallini, J.R. Thome, Condensation of Zeotropic Mixtures in Horizontal Tubes: New Simplified Heat Transfer Model Based on Flow Regimes, Journal of Heat Transfer, March 2005, Volume 127, Issue 3, pp. 221-230.

[10] Kattan, N., Thome, J.R. and Favrat, D. (1998). Flow Boiling in Horizontal Tubes. Part 1: Development of a Diabatic Two-Phase Flow Pattern Map, J. Heat Transfer, Vol. 120, No. 1, pp. 140-147.

General References

R. R. Bittle, 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.

V. Z. Geller, B. V. Nemzer, and U. V. Cheremnykh, 2000. “Thermal Conductivity of Mixed Refrigerants,” 14^th Symposium on Thermophysical Properties, June 25-30, 2000, Boulder, Colorado.

V. Z. Geller, D. Bivens, and A. Yokozeki, 2000. “Viscosity of Mixed Refrigerants R404A, R407C, R410A, and R507A,” 8^th International Refrigeration Conference at Purdue University, West Lafayette, Indiana, USA, July 25-28, 2000, pp. 399-406.

E. A. Lemmon, 2004. Personal Communication regarding RefProp 7.1, NIST, January

M. O. McLinden, S. A. Klein, E. A. Lemmon, and A. P. Peskin, 2002. NIST Reference Fluid Thermodynamic and Transport Properties—RefProp, Version 7.0.

W. V. Payne and D. L. O'Neal, June 1999. “Multiphase Flow of Refrigerant 410A Through Short-Tube Orifices,” ASHRAE Transactions, Vol. 105, Part 2.

W. V. Payne, 1997. A Universal Mass Flowrate Correlation for Refrigerants and Refrigerant/Oil Mixtures Flowing Through Short-Tube Orifices, Ph. D. Dissertation, Texas A&M, May

C. K. Rice, 2005. Investigation of R-410A Air Conditioning System Performance Operating at Extreme Ambient Temperatures up to the Refrigerant Critical Point, Final Report, ARTI-21CR/605-50015-01, ORNL/TM-2005/277, December 2005.

B. Shen, E. A. Groll and J. E. Braun. Improvement and Validation of Unitary Air Conditioner and Heat Pump Simulation Models for R-22 and HFC Alternatives at Off-Design Conditions (1173-RP), Final Report, Purdue University, June 2006.

D. A.Wolf, 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.

TRP-1173 Project Report and Related Papers

B. Shen, J. E. Braun, and E. A.Groll. “A Method for Tuning Refrigerant Charge in Modeling Off-Design Performance of Unitary Equipment (RP-1173)”, HVAC&R Research, ASHRAE, Vol. 12, No. 3, July 2006, pp. 429-449

B. Shen, J. E. Braun, and E. A. Groll, “Modeling of Compressors and Expansion Devices With Two-Phase Refrigerant Inlet Conditions”, 2006 Purdue International Refrigeration and Air Conditioning Conference, Purdue University, July 17-20, 2006.

B. Shen and J. E. Braun, “Some Modeling Improvements for Unitary Air Conditioners and Heat Pumps at Off-Design Conditions”, 2006 Purdue International Refrigeration and Air Conditioning Conference, Purdue University, July 17-20, 2006.