Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
6/11/2015
Forced Air Cooling
Forced Air Cooling
Jim Thompson P.E.
Bio. & Agricultural Engineering Dept.
UC Davis
Product Temperature during
Forced Air Cooling
1/2
15/16 Cool
80
70
20
60
10
50
Air
0
30
0.0
1.0
2.0
3.0
4.0
90
30
Peaches at 0.5 cfm/lb.
40
3/4
7/8
15/16 Cool
100
5.0
Hours of Cooling
Divide Airspace Between Coolers
°C
7/8
Temperature (｡F)
Temperature (｡F)
90
3/4
Temperature (｡C)
1/2
100
Forced Air Cooling
Peaches at 0.5 cfm/lb.
30
80
70
20
60
first fruit
50
mass average
last fruit
10
40
30
0.0
1.0
2.0
3.0
4.0
0
5.0
Hours of Cooling
4
Temperature Variation During Forced Air Cooling
Uncooled centers
Large diameter ‐ melons
Airflow
Block warm air from uncooled fruit from affecting fruit that has begun cooling.
1
Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
Temperature Variation During Forced Air Cooling
6/11/2015
Temperature Measurement
Airflow
Small diameter ‐ blueberries
Uncooled near air exit
Reversing Airflow Direction
open
closed
Vertical Cooler with Flow Reversal
closed
open
9
Pressure Drop vs. Airflow
5
10
4
Cantaloupe, dia.=6.0
3
2
Peaches, dia.=3.25"
Airflow (cfm/lb)
7/8 Cooling Time (hr)
Effect of
Product
Diameter
Strawberries, >10% vent
Tomato, 10% vent
1
Canatloupe, 6%
Pears, 5% vent
Plums 4% vent
Pears, 2% vent
1
0.1
0.010
Cherries, dia.=1.0"
0
0
1
2
3
4
Air Flow Rate (cfm/lb or m3/s‐MT)
11
0.100
1.000
10.000
Pressure drop across pallet (in wc)
• Vent area should be greater than 5% for airflow > 1 CFM/lb
(m3/s‐MT)
2
Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
Airflow
100
4
Pear
3
75
Grape
2
50
(mm)
Pressure Drop (in.)
Pressure Drop vs. Airflow
25
1
0
6/11/2015
0
0
1
2
3
Air Flow (cfm/lb, m3/s‐MT)
Maximum airspeed
1500 fpm (7.5m/s)
• Interior packaging has a big effect omn pressure drop.
13
14
Evaporator Capacity
Refrigeration Capacity Calculations
• Evaporator
– Capacity for product cooled by each unit
• Compressor/condenser
– Capacity for sum of all evaporators
Refrigeration
capacity limit
Refrigeration capacity Capacity
(tons/1000 lb of (% of product (kW/MT))
maximum)
7/8ths cooling time (min)
Unlimited
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.
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
3
Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
Electricity Use in
Forced Air Cooling
Product
6/11/2015
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
Electricity Use
(%)
36
Fans
30
Lights
16
Walls
14
Lifts
4
19
FA coolers are inefficient at low product throughput rates
Maximize Use of Refrigerated Volume
Energy Coefficient
1.5
1.0
Strawberry cooler A
• Use racks or stack pallets ‐ Consolidate
• Divide storage and refrigerate only space needed ‐ Shut down
0.5
Strawberry cooler B
0.0
0
500
1000
1500
2000
Capacity (lb/month-ft2)
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
• Install efficient lighting.
Lighting Options
HID High Bay Fluorescent
4
Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
Replace 400 W HID lamps with High Bay Fluorescent
6/11/2015
Cold Storages are Hard to Light
Unit cost Use
Payback
($)
(hr/da) (days)
293
6
1360
Location
Outside
Cold Storage
293
16
280
Cold Storage w/
motion sensor
373
4
210
Task Lighting
Reduce Electricity Use in Cold Storage and Forced‐Air Coolers
• Maximize use of refrigerated volume.
• Install efficient lighting.
• Improve refrigeration system efficiency.
Refrigeration System Efficiency
Refrigeration System Efficiency
• Increase suction pressure.
25 to 40% Reduction in electricity use
– Floating suction pressure control
• Decrease discharge pressure.
– Install more condenser capacity
• Speed control for screw compressors.
• Proper compressor sequencing.
• Optimum control of system.
5
Thompson, Jim "Forced-air Cooler Design"
Postharvest Technology of Horticultural Crops Short Course 2015
(c) Postharvest Technology Center, UC Davis
•
•
•
•
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.
•
•
•
•
•
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.
Forced‐Air Cooler Cost
(Tunnel cooler in an existing cold room, no high side) Minimize Heat Gain
•
•
•
•
Install rapid acting doors.
Use high reflectivity exterior surfaces.
Add wall or roof insulation.
Insulate exterior refrigeration piping.
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.
Approximate
Forced Air Cooler Cost (2012 data)
20 –
repair
electricity
22%
capital
14%
lift truck
rental
labor
61%
6/11/2015
New designs
< energy use
<< lower labor cost
≥ capital cost
$/pallet 15 –
(10 hr/day
100 day/yr)
10 –
Standard tunnel
Vertical, double channel
Flow through
5–
6