Transcript Slide 1

Altered Bi-phase Flow Regime in
Supermarket Evaporative Coils:
Laboratory and Field Experiences
David A. Wightman
XDX Innovative Refrigeration LLC, CEO
Arlington Heights, IL USA
847.398.0250 Tel, 847.398.1365 Fax,
[email protected]
Bernard Wendrow, PhD, P.E
XDX LLC, Consultant
Arlington Heights, IL USA
847.398.0250 Tel, 847.398.1365 Fax,
[email protected]
Richard S. Sweetser
EXERGY Partners Corp., President
Herndon, VA USA
703.707.0293 Tel, 703.707.0138 Fax,
[email protected]
William M. Worek, PhD, MS.
Director of the Energy Resources Center of
Illinois
Professor and Head of Department of
Mechanical and Industrial Engineering, UIC
Chicago, IL USA
312.996.5610 Tel, 312.996.5620 Fax,
[email protected]
ASHRAE Winter Meeting Symposium OR-05-16 TC 10.7 9 February 2005
Advances in Supermarket Display Case Technology
Moderator Van Baxter, Oak Ridge National Laboratories
ABSTRACT
 ABSTRACT
 Examining Carnot Cycle common assumptions, it is
generally believed that the evaporator benefits from a
complete stream of liquid entering the coil. The
conclusion from this is that maximum enthalpic capacity
is achieved by sub-cooling or even super sub-cooling the
entering liquid and that anything less than a full column
of liquid might limit the amount of heat transfer that the
evaporator can accomplish.
 The evolution of refrigerant flow testing and mapping is
now providing support for a different view. This paper
will examine the net effect of an altered bi-phase flow
(ABF) regime in evaporative coils, with consideration of a
High Vapor Fraction and Turbulent refrigerant flow
(HVFT).
Abstract
 Two test and verification projects in supermarket medium and low
temperature display cases and storage applications provide
significant support for examining old rules-of-thumb.
 The first involves a deli service case tested under laboratory
conditions. This test demonstrates the effect of evaporator
operation utilizing an ABF regime and compares this to the
operation of a pulse-type electronic expansion valve system. The
results demonstrated operation with the ABF regime permitted
increased compressor suction pressure, improved product
temperature, provided more stable refrigerant temperatures, and
improved case product humidity.
 The second test and verification project involves field retrofitting of
existing direct expansion evaporators to the ABF regime and
measuring the results. The resulting change in refrigerant flow
regime demonstrated improved performance resulting in consistent
and reduced conditioned supply air and product temperatures,
improved oil return, reduced compressor discharge temperatures,
and increased evaporator pressure. Product Quality and Energy
savings were also measurable.
Introduction
 Introduction
 The industry has had a view that liquid upon entry of the evaporative coil is
desirable, and that superheated vapor at the exit of the evaporative coil is
necessary. The common view has been that any improvement in Delta-h
improves overall system capacity and is therefore pinnacle. It is often
commonly held that the heat transfer coefficient at the entry to the coil
cannot be improved enough to profoundly impact capacity at the entry of the
evaporator or provide increased evaporator capacity.
 The superheated passes at the exit of the evaporator have been viewed as
necessary due to the conventional liquid flow pattern that advances and
recedes in the Dry/Direct Expansion (DX) coil, which if regularly extended
toward the refrigerant outlet from the evaporator coil, might extend toward
the compressor inlet, and encroach the compressor inlet during periods of
abnormal refrigerant flow. Although counter-intuitive using this conventional
thought, and while enthalpic capacity is important, research in two-phase
refrigerant flow regimes and heat transfer rates is supporting the position
that improved flow regimes can make more dramatic system-wide
performance increases.
INTRODUCTION
 Refrigerant feed through thermal expansion devices is
widely recognized as providing limited control over coil
performance. What seems to have been overlooked is
the substantial impact this erratic flow pattern has on the
heat transfer coefficient over the entire evaporator and
more specifically that the quality of the refrigerant in the
inlet portion of the coil can affect the heat transfer
throughout the entire coil.
ABF/HVFT FLOW REGIME
 Comparative testing was performed between two systems designs.
 The first system setup uses a conventional thermal expansion valve
(Baseline System) installed following all manufacturer specified
recommendations.
 The second system combines thermal expansion valve in conjunction with
the ABF/HVFT Flow, that varies the vapor fraction of the refrigerant and
creates turbulent flow through a mechanically induced fluid process
(ABF/HVFT System).
 Each test and verification project is monitored to measure the effect that this
improved flow regime can have upon cooling rates, compressor work,
temperature differences, evaporator efficiency, control of superheat, and in
other significant observations.


TEST GUIDELINES MET
All Laboratory and Supplemental Field Thermocouples and Sensors are certified as matched, and
have been certified together using standards having traceability to the NIST and were
manufactured in accordance with the guidelines set forth by ISO 9001. All infrared readings were
used as confirmation of thermocouple readings and are not reported.
Figure 1: Baseline System and ABF/HVFT System Test Schematics
Distributed Enthalpy: Hypothesis
 The hypothesis being tested is that entering an
evaporator coil with a high vapor fraction enables
the novel and highly efficient flow regime to be
achieved throughout the evaporative coil.
 Furthermore, annular flow at the outlet of the
evaporator coil in the ABF/HVFT System
communicates very efficiently with the superheat
sensing bulb, whereas the conventional vapor barrier of
superheat in Baseline has extremely poor heat transfer
and cannot communicate well.
Distributed Enthalpy: Hypothesis
 The ABF/HVFT System has been operated throughout
the study in multiple and single pass circuiting,
air/ventilated and gravity feed coils, and high, medium
and low temperature applications. Reduction in
superheat exiting from the evaporator coil is
accomplished with minimal liquid and is very tightly
controlled with little fluctuation as verified on a glass tube
evaporator test stand. July 12-15, 2004 Wightman, Wendrow, Sweetser;
International Refrigeration and Air Conditioning Conference at Purdue.
 Lower superheat can mean greater surface exposure to
the refrigerant and result in higher evaporator pressures.
 Lower superheat allows for a denser refrigerant,
boosting compressor capacity and lowering compressor
outlet superheat. Energy is reduced in each case.
Observations
The ABF/HVFT System fed evaporator with a distributed enthalpy
and with low degree superheat is as indicated. Note:
 A uniformity of evaporator tubing temperature was experienced
 The uniformity in temperature results in a uniformity in frost formation.
 The uniformity in tubing temperature impacts air and product
temperatures
 The ABF/HVFT Flow was found to force oil return of oil formerly logged
in the evaporator
 The higher suction pressure is consistent with our expectations, being
higher due to better utilized surface area and improved heat transfer.
 Compressor Conditions improved with the ABF/HVFT
Baseline Product Tem perature and Evaporator Pressure Correlation
5
Degrees Celcius and Bar
4
3
2
1
0
-1
Ch-25.Product rear
Ch-26.Product middle
Ch-27.product front
Ch-28 Product rear
Ch-29.Product middle
Ch-30.Product front
Ch-52.Suction pressure evap out (ave = 3.52)
ABF HVFT Product Tem perature and Evaporator Pressure Correlation
5
4
3
2
1
0
-1
Ch-25.Product rear
Ch-26.Product middle
Ch-27.product f ront
Ch-28 Product rear
Ch-29.Product middle
Ch-30.Product f ront
Ch-52.Suct ion pressure evap out (ave = 3.70)
Baseline Evaporator and Air Tem peratures
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
Ch-31 Air on (ave = 1.23)
Ch-32.Air Off (ave = -2.87)
Ch-33 Evap coil temp (ave = -5.55)
Ch-34.Evaporator out pipe (ave = -1.06)
Ch-35.Evaporator in pipe (ave = -7.05)
ABF HVFT Evaporator and Air Tem peratures
12
10
8
6
4
2
0
-2
-4
-6
-8
-10
Ch-31 A ir on (ave = 0.81)
Ch-32.A ir Of f (ave = -3.69)
Ch-33 Evap coil t emp (ave = -5.38)
Ch-34.Evaporat or out pipe (ave = -4.83)
Ch-35.Evaporat or in pipe (ave = -6.72)
Baseline Mass Flow and Liquid Tem perature
120
100
80
60
40
20
0
Ch-50.Liquid pipe temp (ave = 25.76)
Ch-51.Mass Flow (ave = 27.59)
ABF HVFT Mass Flow and Liquid Tem perature
120
100
80
60
40
20
0
Ch-50.Liquid pipe temp (ave = 25.46)
Ch-51.Mass Flow (ave = 27.83)
Baseline TXV Coil Performance
Return Air
Supply Air
Coil refrigerant
outlet
Coil refrigerant
inlet #2
Coil refrigerant
inlet #1
Visualization
 A laboratory project was designed to
demonstrate a flow visualization using a
glass tube evaporator to better understand
the bi-phase refrigerant flow
characteristics Comparing the flow regime
in a system using “direct expansion” and
the ABF/HVFT.
International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004
ABF/HVFT Coil Performance
Return Air
Supply Air
Coil refrigerant
outlet
Coil refrigerant
inlet #2
Coil refrigerant
inlet #1
Field Application Results
 IN A DELI SERVICE COUNTER, THE PRE-RETROFIT TEMPERATURES
WERE MONITORED CONSISTENTLY ABOVE 4.44°C (40°F) AND
TEMPERATURE FLUCTUATED GREATLY BETWEEN DEFROST
PERIODS. THE POST-RETROFIT MONITORING OF THE OPERATION
OF THE SAME CASE INDICATES TEMPERATURES CONSISTENTLY
BELOW 4.44°C (40°F), AND TEMPERATURES THAT ARE STABLE
BETWEEN DEFROSTS.
 IN A DISPLAY FREEZER, TWO DEFROSTS PER DAY EXISTED PRE-
RETROFIT. TEMPERATURES EXCEEDED –6.67°C( +20°F) DURING
DEFROST, WHILE OPERATING RANGE WAS MONITORED BETWEEN –
17.8°C TO –20.6°C (-0°F TO -5°F). POST-RETROFIT IN THE SAME
CASE NOW DEMONSTRATES THAT DEFROSTS ARE REDUCED TO
ONE PER DAY, DEFROST PEAKS ARE 5.56°C (10°F) COLDER, AND
SHELF TEMPERATURES ARE REGULARLY NEARER TO –23.3°C (10°F). RACK SUCTION PRESSURE WAS ADJUSTED SLIGHTLY
HIGHER WHILE MAINTAINING COLDER THAN PRE-RETROFIT
CONDITIONS.
 IN THE PRE-RETROFIT OPERATION OF A DOOR FREEZER CIRCUIT,
TWO DEFROSTS PER DAY AND TEMPERATURES ON FREEZER SHELF
THAT FLUCTUATED SLIGHTLY ABOVE –20.6 °C (-5°F), WITH A 1.67°C
(3°F) DEGREE SWING WERE MONITORED. IN THE POST-RETROFIT
OPERATION OF THE SAME CIRCUIT, DEFROSTS ARE REDUCED TO
ONE PER DAY, DEFROST PEAKS ARE NOW 5.56°C (10°F) COLDER,
AND SHELF TEMPERATURES ARE REGULARLY –23.3° TO –24.4°C (10°F TO -12°F).
Defrosts are two per day Pre-Retrofit. Temperatures
on shelf regularly range between
-14.44°C and -11.67°C (6°F and 11°F).
Defrosts Post-Retrofit are reduced to one per day. Note the retrofit prior to the
15th, where temperatures plunge to below – 26.1°C (-15°F).
Ambient Temp to kW comparison for Rack D for case study #1-1K20
100
95
90
85
80
75
70
65
60
kW
55
50
TEMP
KW
45
40
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
Improving compressor C.O.P.s through floating head pressures does
reduce overall power consumption. This demonstrates the effect
that flooded condenser low-ambient controls have upon system
power usage. The ambient temperature is shown dropping more
than 11°C (19.8°F), power consumption remained relatively constant
because the compression ratio reduction was minimal.
Explanation of Capacity Increase
Inside Boiling Heat Transfer Coefficient Vs. Vapor Quality
Conclusion
 The ABF/HVFT (Altered Bi-phase) and HVFT (High Vapor Faction and
Turbulent) flow at the evaporator inlet and extending this flow throughout
the evaporator to safely minimize superheat.
 Field applications repeatedly out performed the Baseline DX evaporator,
which has demonstrated that the existing evaporator surface area can
repeatedly be utilized more efficiently.
 While 4% to 6% steady state efficiency improvements have been
experienced, transient operation demonstrates energy reduction or
capacity increase. ABF/HVFT evaporator is a viable means of improving
heat transfer and overall evaporator efficiency. Additional work is underway.
 The refrigeration industry has focused upon high side savings for reduction
in energy consumption, and has addressed air-side concerns in its
approach to improve evaporator performance. The bi-phase region of a
refrigeration system is not well understood, and advancement in our
industry hinges on improvements in this area.
 Altered Bi-phase Flow or ABF regime at the evaporator inlet the extension
of this flow throughout the evaporator to safely minimize superheat as
demonstrated has repeatedly out performed the conventional DX
Evaporator, is a viable means of improving heat transfer and overall
evaporator efficiency.
REFERENCES
 *






Report Dated November 1999 – Test Comparison Between
Conventional Refrigeration and XDX®, Underwriter’s Laboratories,
1999
*
ASHRAE Handbook, 1993, ASHRAE, Atlanta , GA.
*
A Basis for Rational Design of Heat Transfer Apparatus, 1915,
Wilson , E. E., Trans. ASME, Volume 37
*
Industrial Refrigeration Handbook, 1988 - 1995, Wilbert
Stoecker
*
Report on the Recent Studies of Flow Boiling in Horizontal
Tubes, The Journal of Heat Transfer, Volume 120pp 140-165 (1998)
*
Flow of Fluids Through Valves, Fittings, and Pipes, Technical
Paper No. 410, 1998 Crane Co
*
ASHRAE Paper, July 2002, Improved Performance
Characteristics of a Fin-Tube Heat Exchanger Using High Vapor
Fraction and Turbulent (HVFT) Entering Fluid, Wightman, Wendrow,
Sweetser, Worek
International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004
ACKNOWLEDGEMENTS
 The authors thank Wilbert F. Stoecker,
ASHRAE Fellow, Professor Emeritus
University of Illinois at Urbana-Champaign
for his ongoing contributions.
 The authors would also like to thank
Michael Micak of Carrier Commercial
Refrigeration for his project observations
and contributions.
International Refrigeration and Air Conditioning Conference at Purdue, July 12-15, 2004
Baseline Product Tem perature and Evaporator Pressure Correlation
5
Degrees Celcius and Bar
4
3
2
1
0
-1
Ch-25.Product rear
Ch-26.Product middle
Ch-27.product front
Ch-28 Product rear
Ch-29.Product middle
Ch-30.Product front
Ch-52.Suction pressure evap out (ave = 3.52)
ABF HVFT Product Tem perature and Evaporator Pressure Correlation
5
4
3
2
1
0
-1
Ch-25.Product rear
Ch-26.Product middle
Ch-27.product f ront
Ch-28 Product rear
Ch-29.Product middle
Ch-30.Product f ront
Ch-52.Suction pressure evap out (ave = 3.70)
Evaporator temperature uniformity allows
for frost to build more uniformly across
the coil and can therefore
reduce defrost frequency or duration
by not causing a restriction in air-side velocity.
Table 5 Theoretical Evaporator Model – Calculated Tube Segments
Refrigerant
Quality Range
Length of
Segment
Total
Evaporator length
BTU/min
/length
(vapor percentage)
(feet)
(feet)
Average
BTU/min/ft
Sub-cooled
1.52
1.52
– 4.04
0.0 to 0.1
5.41
6.93
3.88
20.99
Strat-Wavy
0.1 to 0.3
5.05
11.98
8.31
41.97
Intermittent
0.3 to 0.8
5.77
17.75
18.19
104.96
Annular
0.8 to 0.85
0.44
18.19
23.61
10.39
Annular
0.85 to 0.9
0.46
18.65
23.07
10.61
Annular
0.9 to 0.98
1.24
19.89
13.51
16.75
Ann/Strat-Wavy w/ dry-out
10°F Superheat
2.58
22.47
2.13
5.52
Vapor
Liquid
Evaporator
BTU/min
22.47
Flow*
211.19
Table 6 Theoretical ABF/HVFT Evaporator Model – Calculated Tube Segments
Refrigerant
Quality Range
Length of
Segment
Total
Evaporator length
Evaporator
BTU/min/length
(vapor percentage)
(feet)
(feet)
BTU/min/ft
0.3 to 0.8
5.77
5.77
18.19
0.8 to 0.85
0.44
6.21
23.61
0.85 to 0.9
0.46
6.67
23.07
0.9 to 0.98
1.24
7.91
13.51
1°F Superheat
.275
8.19
2.24
Flow*
104.96
Annular
10.39
Annular
10.61
Annular
16.75
Ann/Strat-Wavy w/ dry-out
.616
Vapor
Evaporator
BTU/min
8.19
143.326
ABF/HVFT Evaporator Coil
The inside heat transfer coefficient is found from the
Dittus and Boelter equation and is equal to 72 Btu per
hour per sq. Ft. Per degree F. (408.9 W/m2K).
The inside area of an 11mm ID tube is 0.11338ft2/ft of
length (0.03456m2/m).
The following diagram illustrates the sub-cooling
path:
air  65°F
refrigerant

40°F 
35°
 50°F
Δtlm = [(65-40) – (50-35)]/logm(25/15) – 19.6
(3)
q = 72(0.211338) (19.6) = 160 Btu/hr per foot of tubing
(153.8 W per meter of tubing)
(4)
Then the length of tubing required to supply the energy to heat the
refrigerant from 35°F to 40°F is found as follows:
Length – 4.04 (60)/160 = 1.52 feet (0.463 m)
(5)
FROM 0.85 TO 0.9
Evaporative heat = (2.25-2.125) (84) = 10.5 Btu/min
(744.35) (0.11338) (16.4) = 1384.1 Btu/hr per foot of tubing = 23.07 Btu/min./ft.
L=
10.5
23.07
=
0.46 feet
 From 0.85 to 0.9
air  65°F
refrigerant

41°F 
40°F
 50°F
For 1°F Superheat, t = 40+1 = 41°F
Superheat = cpm(Δt) = 0.2182 (2.5) (41-40) =
.55 BTU/min.
Δtlm
=
(65-41) - (50 – 40)
l ( 2 )
n
4
1
0
= 14
0.8
755
=
16°F
 Examining Standard 210-74/ASHRAE
Standard 51-75 Laboratory methods of
testing fans for rating. Concerning Air
straightness.
Baseline TXV Fed Glass Evaporator
evaporator
pressure
discharge
temper
ature
inlet
evaporat
or tubing
surface
reading
at 2
o’clock
inlet
evaporat
or tubing
surface
reading
at 4
o’clock
midpoint
evaporator
tubing
surface
reading at
2 o’clock
midpoint
evapora
tor
tubing
surface
reading
at 4
o’clock
outlet
evapora
tor
tubing
surface
reading
at 2
o’clock
outlet
evaporat
or tubing
surface
reading
at 4
o’clock
kPa (psi)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
110.3 (16)
58.89 (138)
2.83 (37.1)
0.61 (33.1)
0.56 (33)
-2.89 (26.8)
8.50 (47.3)
8.39 (47.1)
113.8 (16.5)
60.3 (141)
-6.39 (20.5)
-6.67 (20.0)
.011 (32.02)
-4.13 (24.57)
0.04 (32.0)
-3.26 (26.13)
111.5 (16.25)
59.44 (139)
2.944 (37.3)
0.28 (32.5)
1.06 (33.9)
-2.56 (27.4)
0.94 (33.7)
-0.22 (31.6)
ABF / HVFT Flow Regime Device
Fed Glass Evaporator
inlet
evaporator
tubing surface
reading at 4
o’clock
midpoint
evaporator
tubing surface
reading at 2
o’clock
midpoint
evaporator
tubing
surface
reading at 4
o’clock
outlet
evaporator
tubing
surface
reading at 2
o’clock
outlet
evaporator
tubing surface
reading at 4
o’clock
evaporator
pressure
discharge
temperature
inlet
evaporator
tubing surface
reading at 2
o’clock
kPa (psi)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
°C (°F)
182.7 (26.5)
47.78 (118)
2.33 (36.2)
1.72 (35.1)
4.06 (39.2)
2.22 (36)
3.61 (38.5)
1.22 (34.2)
186.5 (27)
49.44 (121)
6.72 (44.1)
5.89 (42.6)
46.1 (40.3)
3.61 (38.5)
2.67 (36.8)
2.33 (36.3)