I had a hangover today. My brain was punishing me somewhat, so I decided on revenge and served it some likewise treatment. Today we looked at the ventilation system for Vanguard's accommodation.
The process gets a bit technical, so I will try writing this blog backward. Start with the proposed outcome, then progress to a solution.
Yacht ventilation systems: The requirements
On a sailing yacht, ventilation is about creating a steady spray-free draft to cool interior spaces and, if lucky, some HVAC when the generator is available. Our problem is different.
Conserve energy
Minimise risk of down-flooding
Happy wife, happy life.
My ever-loving wife, Serbrina, gets hot when she sleeps and does not like it. So if I am to avoid receiving low-level grief "ad nauseam," her sleeping facilities must be cold. We have spent the last 12 years in Charleston and Savannah on the US East Coast. A semi-tropical climate with winter in the 50s (deg.F) and summer in the high 90's. Summer relative humidity is typically 85%, so it's hot and annoyingly sticky. Our livelihood was housebuilding. This necessitated a healthy appreciation of basic HVAC and hygrometry principles (Heating, Ventilation, and Air Conditioning). Before I wax lyrical and bore anyone rigid; how does that affect today's issue?
The Short Story.
Cool the air once, use it twice. Make-up air flows to where the crew occupies. The saloon HVAC works hard and dehumidifies the air as designed. The cabin HVAC works efficiently by cooling the saloon's already-dry air.
Excuse the hand sketch! The line diagram shows how the saloon takes air from outside then hands off to the cabins as needed. Cabin fans only run when cabin is occupied.
Suppose technicalities are not your sweet spot, then probably best to stop reading at this juncture.
Yacht Ventilation Systems: The Engineer's Story, Three Facts I'd Like to Introduce.
Latent heat of water evaporation at atmospheric pressure: 2200 kJ/kg (water).
Enthalpy of dry air at atmospheric pressure: 1 kJ/kg.deg.C
Enthalpy of water vapor at atmospheric pressure: 1.86 kJ/kg.deg.C
Using the above numbers, it takes something like 220 times the energy to reduce the temperature of 1kg 100% humid air from 100 to 0 deg.C than it does to reduce the temperature of 1kg dry air by 100 deg.C. That is a lot of energy. Hurricanes happen because of condensation as all that 2200 kJ/kg of energy is released into the upper atmosphere. On a large yacht such as Damen or Sarp Yachts, fresh air make-up will be dehumidified before flowing to the chillers. We do not have the space to do that so the next best solution is to use the Saloon chiller units as the de facto dehumidifier.
In practical terms, then:
Heating dry or humid air by 1 deg.C does not need much energy.
Cooling dry air or humid air that remains above its dew point (100% humidity) takes a bit more energy, but it's still not much.
Cooling 100% humid air below its local dew point is energy intensive.
The illustration shows how energy content of dry and saturated air change with temperature. We need to stay on the blue line as much as possible to maximize system efficiency for a given cabin temperature.
So we need to heat and cool Vanguard's interior spaces. The local Webasto installer has completed those calculations, and I accept that. But we also need to provide fresh air into these spaces, and there are rules around that supply. Referring back to our homebuilding experience, we looked at the International Residential Building Code, or IBC for short. The most commonly used version is dated 2018 with new ones released every 3 years or so.
Yacht Ventilation Systems: Outside Air Requirements
2018 IBC Guidelines for ventilation of residential spaces.
Note: 1 Cubic Foot per Minute (cfm) = 0.03 m^3/min
0.35 air changes per hour, calculated based on the total floor area
15 cfm (cubic feet per minute) per person
Kitchens: 100 cfm (intermittent) or 25 cfm (continuous)
Bathrooms: 50 cfm (intermittent) or 20 cfm (continuous)
We have HVAC air handlers in each conditioned space, but we still require outside makeup air similar to the above guidelines. We also need to minimize the risk of down flooding in case of a broach, be space and especially energy efficient.
Yacht Ventilation - Solutions for Temperate Climates
The HVAC is not required, relying instead on natural ventilation, either in-line fans at the helm station or simply leaving doors/hatches open when appropriate. Cabin vent fans will ensure local circulation when occupied.
Yacht Ventilation - Solutions for Cold Climates
Turn the heating on, and in all else, the above also applies.
Yacht Ventilation - Solutions for Hot Humid Climates
Condition the incoming air once, and use it twice; use follows how cabins are occupied. Remember 2200 kJ/kg to condition/dehumidify (water vapor) and 1-2 kJ/kg to cool (air) after that. You cannot cool air that is 100% humid without first removing the water to reach the local, lower dew point. Introduce only dehumidified air to cabin spaces to minimize mold and mildew risks. Use existing extraction fan systems to drive air through the hull.
The air inlet to the saloon will be fan (twin) assisted and restricted to a single location above the main helm. Air will flow aft across the saloon and out or into the cabins. Note the galley has its own independent extractor unit to limit propagation of cooking smells. (no bacon smell when seasick!)
Cabin air will draw from the saloon (not from outside). The saloon air will be free of spray and salt and already dehumidified by the saloon HVAC. It will thus also protect from any down-flooding risk.
Air will be extracted from the cabins using the bathroom exhaust systems, but they operate only in an occupied cabin.
Air will enter the cabins with the passive transfer ventilation relying on a slight pressure gradient induced by the bathroom extractor fans (exhausts). The exhaust ducts are backflow protected.
The laundry area has its airflow system utilizing the combined washer and dryer with an external vent.
Well, that's the theory; let us see if it works!
Chris Leigh-Jones
We sketched out our system, and eventually, the head returned to its normal fuzzy state, free of conditions otherwise chemically induced. None of us live or work in a vacuum, so I'd be interested in any feedback available for the collective mind and experiences of our readers.