Objectives Finish up discussion of cycles Differentiate refrigerants Identify qualities of a good refrigerant Compare compressors Describe expansion valves

Homework Assignment 3

Administrative Reminder Oral Presentations Monday December 5 th 6 – 8:30 pm Location TBA Annotated bibliography due 10/25 (two weeks) No homework for a week+

Compressor Workhorse of the system Several types – all compress gas with varying degrees of efficiency Far from isentropic (our assumption earlier) W shaft = work done by shaft W elec = electric power requirements

Reciprocating Compressor Figures 4.4, 4.6

Rotary Compressors Higher efficiency, lower noise and vibration Cylinder rotating eccentrically in side housing

Scroll Compressors One scroll is fixed The other scroll “wobbles” inside compressing refrigerant Often requires heat transfer from refrigerant to cool scrolls

Scroll Compressors Constant displacement Higher efficiency, but harder to manufacture Close tolerance between scrolls Ugly to analyze – see text for details

Screw Compressors Rotating meshed screws One or two screws

Summary Many compressors available ASHRAE Handbook is good source of more detailed information Very large industry

Expansion Valves Throttles the refrigerant from condenser temperature to evaporator temperature Connected to evaporator superheat Increased compressor power consumption Decreased pumping capacity Increased discharge temperature Can do it with a fixed orifice (pressure reducing device), but does not guarantee evaporator pressure

Thermostatic Expansion Valve (TXV) Variable refrigerant flow to maintain desired superheat

AEV Maintains constant evaporator pressure by increasing flow as load decreases

Summary Several compressor options Dramatically impacts energy use of air conditioner Expansion valves make a big difference in refrigeration system performance Trade-offs Cost, critical refrigerant amount

Real Data An evaluation of superheat-based refrigerant charge diagnostics for residential cooling systems Siegel, Jeffrey A.Siegel, Jeffrey A. (Lawrence Berkeley Natl. Laboratory); Wray, Craig P. Source: ASHRAE Transactions, v 108 PART 2, 2002, p Abstract: Although refrigerant charge has an important influence on the performance of residential cooling systems with fixed orifice metering devices, there has been little research to quantify the effects of incorrect charge or design new diagnostics for evaluating charge level. The most common diagnostic for charge level in these systems is the superheat test. In this paper, we examine three superheat technologies/techniques. Two of the diagnostics are appropriate for detecting incorrect charge, one is not. Additionally, measurements at four houses indicate that it is important to measure the condenser air entering temperature with a high degree of accuracy. Measurement of the wet-bulb temperature in the return plenum and the suction line temperature are equally important but seemingly easier than measuring the condenser air temperature, as several measurement technologies yielded similar results for these quantities. The importance of refrigerant charge to energy use and capacity of residential cooling systems, the limitations of the superheat test, and the variations in the test method results and interfaces necessitate the development of a standard method or methods to determine refrigerant charge level. (8 refs.)Wray, Craig P.

Motivation Demonstrate importance of refrigerant charge Evaluate technologies for assessing refrigerant charge with superheat method Stimulate discussion on new technologies and approaches for measuring charge levels

Refrigerant Charge Ref: Farazad and O’Neal, 1988

How Much of a Problem? Proctor (2002) studied 13,258 residential central air conditioners 55% of residential units out of specification 42% undercharged by more than 10% 13% overcharged by more than 10% Commercial air conditioners showed similar patterns

Superheat Thermodynamic metric Difference between refrigerant temperature in suction line and its saturation temperature UnderchargeOvercharge Superheat too largeSuperheat too small Poor heat transfer Potential coil icing Slug compressor

Superheat Test Orifice Control Compressor Condenser Air AMB=85F Evaporator Air T db =81F, T wb =68F Suction Line ST=84F SH=57F SP=51 psig (T sat =27F)

Charging Chart Target = 19°FActual = 57°F Undercharge

Caveats Orifice-controlled systems TXV-controlled devices are much less sensitive to charge level Small sample size Imperfect truth method Insufficient time to do gravimetric procedure

Test Houses

Field Testing

Temperature Profiles (Site C)

Results

Summary Refrigerant charge matters a lot Particularly with orifice (i.e. cheap and simple) expansion valves Superheat is desirable Prevents slugging of compressor with liquid Manufacturer provides target superheat Function of evaporator entering t* (for air) and outside temperature –Why?

Next class Heat exchangers (evaporators and condensers)