6/16/2014
Forced Air Cooling
Jim Thompson P.E.
Bio. & Agricultural Engineering Dept.
UC Davis
Forced Air Cooling
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6/16/2014
Product Temperature during
Forced Air Cooling
Temperature (｡F)
90
3/4
7/8
15/16 Cool
30
Peaches at 0.5 cfm/lb.
80
70
20
60
10
50
40
Temperature (｡C)
1/2
100
Air
0
30
0.0
1.0
2.0
3.0
4.0
5.0
Hours of Cooling
Forced Air Cooling
1/2
3/4
7/8
15/16 Cool
Temperature (｡F)
90
Peaches at 0.5 cfm/lb.
°C
100
30
80
70
20
60
first fruit
50
mass average
last fruit
10
40
30
0.0
1.0
2.0
3.0
Hours of Cooling
4.0
0
5.0
4
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Divide Airspace Between Coolers
Block warm air from uncooled fruit from affecting fruit that has begun cooling.
Temperature Variation During Forced Air Cooling
Uncooled centers
Large diameter ‐ melons
Airflow
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Temperature Variation During Forced Air Cooling
Airflow
Small diameter ‐ blueberries
Uncooled near air exit
Temperature Measurement
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Reversing Airflow Direction
closed
open
closed
open
9
Vertical Cooler with Flow Reversal
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5
7/8 Cooling Time (hr)
Effect of
Product
Diameter
4
Cantaloupe, dia.=6.0
3
2
Peaches, dia.=3.25"
1
Cherries, dia.=1.0"
0
0
1
2
Air Flow Rate (cfm/lb
3
4
or m3/s‐MT)
11
Pressure Drop vs. Airflow
Pressure Drop (in.)
4
Pear
3
Grape
2
1
0
0
1
2
3
Air Flow (cfm/lb)
12
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Pressure Drop vs. Airflow
Airflow (cfm/lb)
10
bagged grapes in liner
2
Chilean grapes
1
strawberries
plums, 4% vent
Bagged grapes EPS
Tomato, 10% vent
Pears, 2% vent
0
0.01
0.1
1
Pressure drop across pallet (in wc)
10
Airflow
Maximum airspeed
1500 fpm (7.5m/s)
14
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Refrigeration Capacity Calculations
• Evaporator
– Capacity for product cooled by each unit
• Compressor/condenser
– Capacity for sum of all evaporators
Evaporator Capacity
Refrigeration
capacity limit
Refrigeration capacity Capacity
(tons/1000 lb of (% of product (kW/MT))
maximum)
7/8ths cooling time (min)
None
2.43 (19)
100
150
Average for first 1/2 cooling period
1.80 (14)
74
155
Average for 7/8 cooling
1.45 (11)
60
165
Based on cooling 24 pallets of broccoli with 32°F (0°C) air and 68°F (20°C) initial temperature. Refrigeration load for product only.
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Energy Coefficient
Energy Coefficient = Cooling Work / Electricity Use • Product cooled per billing period (lbs)
• Temperature drop in cooling (°F)
• Electricity use (kWh)
High EC = more cooling for less electricity Forced‐Air Cooler Efficiency
Cooler
Strawberry A
Strawberry E
Strawberry C
Strawberry D
Strawberry B
Grape C
Grape B
Grape A
Season
Average EC
0.68
Oldest facility
0.40
0.33
0.33
0.23
0.49
0.49
0.34
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Electricity Use in
Forced Air Cooling
Product
Electricity Use
(%)
36
Fans
30
Lights
16
Walls
14
Lifts
4
19
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
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FA coolers are inefficient at low product throughput rates
Energy Coefficient
1.5
1.0
Strawberry cooler A
0.5
Strawberry cooler B
0.0
0
500
1000
1500
2000
Capacity (lb/month-ft2)
Maximize Use of Refrigerated Volume
• Use racks or stack pallets ‐ Consolidate
• Divide storage and refrigerate only space needed ‐ Shut down
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Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
• Install efficient lighting.
Lighting Options
HID High Bay Fluorescent
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Replace 400 W HID lamps with High Bay Fluorescent
Location
Outside
Unit cost Use
Payback
($)
(hr/da) (days)
293
6
1360
Cold Storage
293
16
280
Cold Storage w/
motion sensor
373
4
210
Cold Storages are Hard to Light
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Task Lighting
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
• Install efficient lighting.
• Improve refrigeration system efficiency.
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Refrigeration System Efficiency
• Increase suction pressure.
– Floating suction pressure control
• Decrease discharge pressure.
– Install more condenser capacity
• Speed control for screw compressors.
• Proper compressor sequencing.
• Optimum control of system.
Refrigeration System Efficiency
25 to 40% Reduction in electricity use
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•
•
•
•
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
Maximize use of refrigerated volume.
Install efficient lighting.
Improve refrigeration system efficiency.
Minimize exterior heat gain.
Minimize Heat Gain
•
•
•
•
Install rapid acting doors.
Use high reflectivity exterior surfaces.
Add wall or roof insulation.
Insulate refrigeration piping.
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•
•
•
•
•
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
Maximize use of refrigerated volume.
Install efficient lighting.
Improve refrigeration system efficiency.
Minimize exterior heat gain.
Minimize fan electricity use.
Fan Electricity Use
• Reduce fan speed near the end of cooling?
• In storage, reduce airflow when evaporators operate a partial capacity.
– Fan cycling
– Slow motor speed.
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Forced‐Air Cooler Cost
(Tunnel cooler in an existing cold room, no high side) repair
electricity
22%
capital
14%
lift truck
rental
New designs
< energy use
<< lower labor cost
≥ capital cost
labor
61%
Approximate
Forced Air Cooler Cost (2012 data)
20 –
$/pallet 15 –
(10 hr/day
100 day/yr)
10 –
Standard tunnel
Vertical, double channel
Flow through
5–
18