Calculating the Ideal Volume of Water/Ice (PCM) Box
* One ton (12,000 BTU/hr) AC freezes 2000 lbs of ice
in 24 hours. Ideally, a 12,000 BTU/hr window AC can cool about
500-600 sq ft under normal conditions.
* A 6,000 BTU/hr AC (½ ton) freezes 1000 lbs of ice
in 24 hours. Ideally, a 6,000 BTU/hr AC can cool about
250-300 sq ft under normal conditions.
Let’s assume a 6,000 BTU/hr AC (½ ton) runs from 2PM to 8PM (6Hours)
per day. That means, the AC does not run for 18 hours per day
(24-6=18).
To estimate how many pounds of water a 7 cubic feet chest freezer can
freeze in 18 hours, we need to consider the freezer's cooling capacity
and efficiency.
Step 1: Freezer's Cooling Capacity
A typical chest freezer has a cooling capacity of about 400 to 1000 BTU/hr, depending on the model.
For this calculation, let's assume the freezer has an average cooling rate of 750 BTU/hr (common for a mid-range unit).
Step 2: Energy Required to Freeze Water
To freeze water at 32°F:
Latent heat of fusion of water = 144 BTU/lb
Energy required to freeze 1 lb of water = 144 BTU
Step 3: Calculate Freezing Energy in 18 Hours
Energy produced by the freezer in 18 hours:
750 BTU/hr×18 hrs=13,500 BTU
Step 4: How Many Pounds of Water Can Be Frozen?
Now, we can calculate how much water can be frozen with the 13,500 BTU of energy:
13,500 BTU / 144 BTU/lb=93.75 lbs of water
Conclusion:
A 7 cubic feet chest freezer with an average cooling rate of 750 BTU/hr
can freeze approximately 94 lbs of water (1.5 cubic feet) in 18
hours, under ideal conditions.
Assumptions:
Our assumption was that our 6,000 BTU/hr AC (½ ton) runs from 2PM to
8PM (6Hours). That means, the AC does not run for 18 hours
(24-6=18).
Now let’s calculate what size (BTU/hr) AC can freeze 94 lbs of water in 6 hours.
To determine the required BTU/hr AC capacity, we need to calculate the
total heat removed during the freezing process. The AC needs to remove
13,600 BTU/hr to freeze 94 lbs of water in 6 hours, requiring a 3,000
BTU/hr AC.
Here's the breakdown:
Latent Heat of Fusion: It takes 144 Btu to freeze 1 pound of water.
Total Heat to Remove: 94 pounds of water * 144 Btu/pound = 13,536 Btu.
BTU/hr Calculation: 13,536 Btu / 6 hours = 2256 BTU/hr.
AC Capacity: Since the AC needs to remove the heat in 6 hours, you need an AC with a capacity of at least 2256 BTU/hr.
Standard AC Sizes: Standard AC sizes are in increments of 1,000 BTU/hr.
Therefore, a 3000 BTU/hr AC would be a good choice for this application.
Below table compares the two (freezer vs. window AC)
|
7 cubic feet freezer
|
3000BTU/hr Window AC
|
14 cubic feet freezer
|
6000BTU/hr Window AC |
BTU/Hr
|
750
|
3000
|
1500
|
6000
|
Watts/hr
|
220
|
879
|
440
|
1760
|
Runs# of Hrs/Day
|
18
|
6
|
18
|
6
|
Insulated Ice-Box Volume CuFt
|
1.5
|
1.5
|
3
|
3
|
Cooling area SqFt
|
125 to 150
|
125 to 150 |
250 to 300
|
250 to 300
|
Example Refrigerators & Freezers
| Model |
Capacity |
Freezer
Size |
Cost |
Notes |
Whirlpool
WRT311FZDB |
20.5 cu. ft. |
6.08 cu. ft. |
$808 |
More than enough
space for a 4 cu. ft. ice box. |
Frigidaire Chest
Freezer |
7.0 cu. ft. |
Entire unit can be
converted |
$180 |
Can be repurposed
as a cooling unit in summer. |
Alternative Configuration
- A 6.5 cu. ft. PCM reservoir (Insulated Ice-Box) would be equivalent
to a 9,750 BTU/hr AC, which can cool approximately 425
sq. ft.
- Instead of purchasing a dedicated 9,750 BTU/hr AC,
a portable freezer can be used both for cooling in summer and food
storage in winter.
Additional Enhancements
- More powerful water pump for better circulation.
- Low-viscosity antifreeze for improved cooling
efficiency.