AirConditioningCalculation
Air Conditioning Calculation
Summary
Calculating air conditioning is more complex than heating. Air conditioning can be supplied through forced air or a water-based chiller. Three methods of calculation are illustrated, including a provided spreadsheet.
Method
Air conditioning can be supplied through forced air or a water-based chiller. AAmpere (amp), SI unit of electrical current chiller circulates chilled water through a water distribution system to the cabins, to cool them in summer. All pipes should be insulated to prevent condensation. (Similarly, if you opt for forced air, the ducts should be insulated.)
The heat exchanger can be water-air or water-water. A water-air exchanger would have to work against the heat in the engine room, so it makes more sense to use a water-water heat exchanger with a keel cooler as a heat sink. This is overall more efficient (the temperature differential is higher with water), and avoids generating extra heat in the engine room.
Calculating air conditioning is more complex than heating and so the answers are more varied. The below table gives three sets of estimates to illustrate the issue.
- Column A gives a series of BTUBritish Thermal Unit values derived from the buyenergyefficient.org web site [1].
- Column BBeam is based on an expert rule of thumb of 14 BTU per cubic foot, plus an extra 1000 BTU for good measure.
- Column C uses the spreadsheet calculator.
Except for the last two data points, methods A and C are in good agreement, but make your own judgement.
The spreadsheet calculator is adapted from Air Conditioning Your Home [2], published by the Energy Office of Natural Resources Canada (NRCAN) and available from its web site. It appears to fall within the general range of the other methods, based on area alone. Most rules of thumb are designed for single rooms, or two rooms joined. The author'sSecond calculator considers numerous more factors:
- Number of occupants
- Area of each accommodation
- Area of windows and degree of sun exposure
- Energy efficiency of windows
- Shading of windows
- Degree of insulation in the boat
- Heat gain through the engine room bulkhead
- Heat gain from ACAlternating current machinery in the accommodation
- Heat gain from DCDirect current machinery in the accommodationHeat gain from DC lights in the accommodation
Several approximations were made in adapting the NRCAN model. For example, houses have a fixed position, allowing us to calibrate the different heat gain from windows facing any compass quadrant. Boats are mobile, allowing windows to face any direction at any time. The calculator assumes the worse case, with one full side of the boat having maximum southern sun exposure, the other minimum, i.e., it is moored east-to-west.
The degree of insulation is set with the KKelvin, SI unit of thermodynamic temperature factor in the heating calculation. The factor for heat gain through engine room bulkheads is a pure guess. The heat gain from AC and DC equipment is factored at 3.4 - 4.3, while NRCAN suggests 3.0 for AC appliances in a house.
| Recommended Cooling Capacity (BTU/hBritish Thermal Units per hour) | |||
|---|---|---|---|
| Area (ftFoot2) | Method (ft) | ||
| A[3] | B[4] 14 BTU/ft2 |
C[5] Calculator (K=0.7) |
|
| 100 – 150 | 5,000 | 3,100 | 3,465 |
| 150 - 250 | 6,000 | 4,500 | 5,775 |
| 250 - 300 | 7,000 | 5,200 | 6,930 |
| 300 - 350 | 8,000 | 5,900 | 8,085 |
| 350 - 400 | 9,000 | 6,600 | 9,200 |
| 400 - 450 | 10,000 | 7,300 | 10,395 |
| 450 - 550 | 12,000 | 8,700 | 12,705 |
| 550 - 700 | 14,000 | 10,800 | 16,170 |
| 700 - 1,000 | 18,000 | 15,000 | 23,100 |
Related Pages
Calculating Heating Requirements
Calculating Ventilation Requirements
