Glide Effect on Performance

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Evan Ward
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1 Glide Effect on Performance ASHRAE, January 2016 Bachir Bella Regis Leportier* ASERCOM

2 Azeotropic Mixtures The azeotropic (refrigerant) mixtures are usually binary mixtures that behave like a pure fluid A single temperature defines either the evaporation or the condensing pressure. p h diagram for azeotropic mixture evaporating pressure: p 1 (t 1d ), condensing pressure: p 2 (t 2d ) ASHRAE, Jan. 2016, 2

3 Non-Azeotropic or Zeotropic Mixtures Non-azeotropic (refrigerant) mixtures, exhibit a temperature variation during constant pressure phase change. Glide" is widely used to describe the temperature change during process. p h diagram for Non-azeotropic mixtures evaporating pressure: p 1, condensing pressure: p 2 ASHRAE, Jan. 2016, 3

4 Non-Azeotropic or Zeotropic Mixtures The temperature at which condensation starts is called the dew point, as condensation progresses, the temperature falls to bubble point. During the evaporation process, the temperature changes from the temperature at the inlet of the evaporator t 1e to the dew point temperature t 1d. Superheat occurs after evaporation is complete, raising the temperature to t 1 (suction temperature at the compressor inlet). Compressors are rated according to this cycle, with the evaporating and condensing pressures expressed as dew point temperatures. ASHRAE, Jan. 2016, 4

5 Non-Azeotropic or Zeotropic Mixtures Which Reference Temperature? The question, to which temperature along each change of state process should be used to define the evaporating and condensing temperatures. A mean temperature may be defined for purposes of analysis to represent the actual system performance or for comparing blends with pure refrigerants. Compressor standards agree for using dew point temperatures because they allow for a clear correlation between pressures and temperatures. ASHRAE, Jan. 2016, 5

6 Performances Declaration Dew Point Protocol Evaporating and condensing temperatures are defined as the dew temperatures t 1d and t 2d. A single temperature defines the compressor inlet (evaporation) pressure and it is independent of the condensation process. Superheat is easily calculated as the difference between compressor suction temperature and evaporating temperature. Liquid sub-cooling is however still calculated with respect to the bubble point. ASHRAE, Jan. 2016, 6

7 Mid-point Protocol For given discharge pressure, t 2d and t 2f are fixed t 2m = (t 2f + t 2d ) / 2. Average evaporating t 1m between inlet t 1e and dew t 1d t 1m = (t 1e + t 1d ) / 2 t 1e changes with the condensing pressure or the extent of sub-cooling t 1e = f(p 1, p 2, sub-cool) Superheat can also be misinterpreted when using midpoint data Hence, any performance related to mid-point temperature could create a misunderstanding if insufficient information is given ASHRAE, Jan. 2016, 7

8 Economizer Application When an economizer cycle is applied the mid-temperature t 1m depends on the outlet temperature t 10 of the economizer liquid sub-cooler. Hence the mid-temperature is changing with the sub-cooling at the same evaporating and condensing pressure. Thus, referring to mid-temperature introduces a further complication for economizer application. ASHRAE, Jan. 2016, 8

9 Compressor Performances According to EN12900 and ARI 540 Standards The polynomial equation reported below is used either by EN as well by ARI 540 standards to generate the performances of the compressor: where: X is the refrigerating capacity (mass flow), Absorbed power, Current S is the evaporating temperature at suction dew point D is the condensing temperature at discharge dew point, ASHRAE, Jan. 2016, 9

10 Cooling Capacity Variation Glide Effect on Performance Effect of mid or dew reference on Cooling Capacity Experimental Data Refrigerant R407C -10 C / 45 C 10% 8% Cooling Capacity (Mid vs. Dew) 0K Subcooling 6% Capacity is approximately 5 % Higher for mid-temperature reference 4% 2% 4.7% 5.6% 5.6% No appreciable difference in the COP 0% 5/50 /15 C -10/45/20 C -10/45/0 C The system designer may properly interpret the data from the appropriate definition, but a casual observer may conclude that the compressor delivers less capacity when dew point definitions are used, although this is not the case. ASHRAE, Jan. 2016, 10

11 Recommendations of using Mid-point temperature Condensing mid temperature: t 2m = (t 2f + t 2d ) / 2 Sub-cooling: Dt sub = t 2f t 5 Evaporating mid temperature: t 1m = ( t 1e + t 1d ) / 2 Gas Superheat at the compressor inlet t sh = (t 1 t 1d ) ASHRAE, Jan. 2016, 11

TOLERANCES AND UNCERTAINTIES IN PERFORMANCE DATA OF REFRIGERANT COMPRESSORS JANUARY 2017

TOLERANCES AND UNCERTAINTIES IN PERFORMANCE DATA OF REFRIGERANT COMPRESSORS JANUARY 017 111 Wilson Blvd, Suite 500 Arlington, Virginia 01 USA +001 (703) 54-8800 Published by: TABLE OF CONTENTS SECTION