VentilationCalculation
Ventilation Calculation
Summary
Ventilation is required to replenish oxygen removed by people and sources of combustion, and to dilute odours and pollutants. The most effective method is an integrated HVACHeating, ventilation, air conditioning system with air distribution and local controls in each cabin. But without pass-throughs for air ducts an integrated ventilation/humidification system is impossible. AAmpere (amp), SI unit of electrical current water system for distribution minimises the scope of pass-throughs in water-tight bulkheads but like many design decisions it forces trade-offs and the use of local ventilation.
Method
Fresh air ventilation is required to replenish oxygen removed by people and sources of combustion, and to dilute odours and pollutants. Local exhaust ventilation is required in heads and the galley to remove airborne odours before they spread through the boat. From a ventilation viewpoint, the most effective method is an integrated HVAC system with air distribution and local controls in each cabin. Such a system can include an air-to-air heat exchanger to precondition the temperature of the air and recover energy, and a humidifier/dehumidifier to control levels of indoor moisture. Humidity control is especially important in hot humid climates where unconditioned ventilation can deliver 1 pound (lbPound weight) of water per cubic foot (ftFoot^3) of intake air.
Excess humidity causes condensation on windows and water pipes. It can blister paint, rust metal and warp wood, and cause electrical faults. Dust mites, fungus, mildew and mould thrive in humid conditions, aggravating allergies and sometimes damaging lungs. Insects like clothes moths, cockroaches and fleas also like high humidity.
People prefer a relative humidity of 30 to 50%percent and find anything much higher to be very uncomfortable.
Unfortunately choosing in favour of a water system for distributing heating and air cooling has further consequences. A water system minimises the scope of pass-throughs in water-tight bulkheads but like many design decisions it forces trade-offs. It makes an integrated ventilation/humidification system impossible.
The alternative to running fairly large air vents the length of the boat is local ventilation in the main zones of the boat. This is far from ideal. In both summer and winter the air intakes will be working against the air conditioning and heating systems, respectively, and deck-mounted dorades for intake and return air are multiple hull openings. The ventilation system must be designed carefully to minimise these risks of water entering.
Humidity control is also difficult with local ventilation; although it may be possible to incorporate small electronic dehumidifiers into the vents. Electronic dehumidifiers use small peltier heat pumps but consume a fair bit of electrical energy. For small vents, mechanical dehumidifiers don’t scale down, and desiccated dehumidifiers are overly complex.
If you plan to spend your time in hot humid climates, you should consider a solution that incorporates a dehumidifier.
Ventilation Rates
Ventilation rates can be expressed in several ways:
- Cubic feet per minute (CFMCubic feet per minute) or litres per second (Llitre/sSecond) of outside air brought into the boat
- CFM per person: CFM/p
- CFM per unit floor area: CFM/ft2
- Air changes per hour: (ACHAir changes per hour)
Standards for ventilation differ, and have varied over time subject to lobbying, energy efficiency doctrines and the emergence of sick building syndrome. A reasonable yardstick is somewhere in the range of 0.5-1.25 ACH or, more precisely, 1.0 ACH translating to around 1.66 CFM per 100 cubic feet of cabin volume. You can double check this to ensure at least 15 CFM/p.
For example, assume a boat having 6,000 cubic feet of volume and berths for five people. Using 1.0 ACH this yields 99.6 CFM and 15 CFM/p yields 75 CFM.
Maximum air velocity in ventilation ducts and vents should not exceed 2.6-3.3 ft/s (feet/second) (0.8-1.0 mMetre, SI unit of length/s) to minimise noise and differentials in air pressure. In comparison, air ducts for combustion systems can run as high as 40-66 ft/s (12-20 m/s).
Working an Example
Let’s work a complete example. Assume a salon of 1280 cubic feet. At 1.0 ACH this requires 21.3 CFM:
CFM = Volume * ACH/60 minutes
The corresponding vent area with a velocity of 2 ft/s is:
Vent Area = CFM/(Velocity * 60 seconds) = 21.3/120 = 0.18 sq ft = 25.6 sq in
Close enough.
In this case, we could put a 5- x 5-inch intake vent at one end of the salon and a vent of the same size at the other end with an exhaust fan driving 2 ft/s.
