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 5th

• 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)

• Wshaft = work done by shaft

• Welec = 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. (Lawrence Berkeley Natl. Laboratory); Wray, Craig P. Source: ASHRAE Transactions, v 108 PART 2, 2002, p 965-975

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.)

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

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

-20% -10% 0% 10% 20%Deviation from Full Charge

Nor

mal

ized

Cap

acity

[Ful

l, 95

°F w

et =

34.

0 kB

tu/h

]

100°F (38°C) wet95°F (35°C) wet90°F (32°C) wet82°F (28°C) wet82°F (28°C) dry

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

Undercharge Overcharge

Superheat too large Superheat too small

Poor heat transferPotential coil icing

Slug compressor

Superheat Test

OrificeControl

Compressor

Condenser AirAMB=85F

Evaporator AirTdb=81F, Twb=68F Suction Line

ST=84F

SH=57FSP=51 psig (Tsat=27F)

Charging Chart

Target = 19°F Actual = 57°F Undercharge

5

10

15

20

25

30

35

40

65 70 75 80 85 90 95 100 105 110 115

OUTDOOR DRY-BULB TEMPERATURE [ºF]

TA

RG

ET

SU

PE

RH

EA

T [

ºF]

7674

7270

68

66

64

6260

58

INDOOR WET-BULB TEMPERATURE [ºF]

19

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

Site

House Location

Cooling Equipment

Age [Years]

Condensing Unit

Rated Capacity [Tons]

Evaporator Rated

Capacity [Tons]

A Larkspur, CA 17 3 Unknown B Sacramento, CA < 1 3.5 4 C Sacramento, CA < 1 3 4 D Concord, CA > 15 3.5 Unknown

Field Testing

Temperature Profiles (Site C)

40

50

60

70

80

90

100

110

120

11:30 12:00 12:30 13:00Time of Day

Tem

pera

ture

[°F

]

4.4

10.0

15.6

21.1

26.7

32.2

37.8

43.3

48.9

Tem

pera

ture

[°C

]

AC On

Charge Added

Liquid Line Temperature

Outdoor Temperature

Suction Line Temperature

Results

Site A Site B Site C Site D

Charge Added [oz]

Fraction of Total

15

17%

11

13%

20.5

33%

19

Unknown

EER Increase 19% 7% 20% N/A

Capacity Increase 33% 18% 38% 35%

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)