Objectives Finish up discussion of cycles Differentiate refrigerants Identify qualities of a good...
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Transcript of Objectives Finish up discussion of cycles Differentiate refrigerants Identify qualities of a good...
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)