There are so many considerations when it comes to ER ventilation. I've been digging into this over the past year in preparation to making some improvements to mine. In no particular order, here are some of the considerations that I have learned.
- Every engine has specs for its ambient heat rejection. That's the amount of heat that it puts into the surrounding space. It does not include the heat taken out through the cooling system - just what it dumps into the ER space.
- Every engine also has specs on it's max operating environment. Other equipment in your ER will have such specs too. Typical limits are 130F max ER temp, and/or max 30F temp rise from outside ambient to ER ambient.
- A hot ER is not only stressful on equipment, it is stressful on you if you need to get in there to do something. 120F or 130F is dangerously hot and can quickly lead to heat stroke and other nasty things.
- To take the heat out of the ER, there needs to be air movement from outside, through the ER where it picks up heat, then out again. There are formulas that you can find that will tell you how much air flow is required to remove the engine heat, and what temp rise will result.
- Your engine itself is a pretty good exhaust fan, pulling air in from the outside where it passes through the ER and picks up heat, then gets sucked out of the ER through the engine intake and out the exhaust. The engine specs will tell you how many CFM of flow this will be. Note that this is only true if the engine pulls its intake air from the ER. If it has dedicated ducting to pull air from outside then it will not air in ER venting.
- In some case, the engine air flow may be enough to keep the ER sufficiently cool. In other cases additional air flow is required.
- Air will enter and exit your ER through vents of some sort. They may be big, small, or anything in between.
- The vents may be large enough to provide sufficient cooling via convection. Or you may have (or need) mechanical air flow assistance in the form of either exhaust or intake blowers.
- If your intake vent area is too small, then a vacuum can develop in the ER. This will be exacerbated by any exhaust fans. If there is too much restriction, you won't get the needed air flow. Also, the more vacuum, the more air will be pulled from other sources like wire chases, conduits, voids behind walls, etc. Air will get drawn in from all sorts of places following the paths of least resistance.
-If your exhaust vent area is too small, then pressure can develop in the ER. This will be exacerbated by intake fans. The more pressure there is in the ER, the more air will get pushed out through all sorts of voids and passages throughout the boat. Exhaust restrictions can also severely limit the flow of exhaust air.
- Fans all have a flow rating that is normally in free air, i.e. no intake or exhaust restrictions. Flow drops quickly when there are restrictions, and fan specs usually have a chart showing this. It can have a significant impact. I encountered a 1200 CFM fan once that was exhausting through a hose to an outside grate. The restriction from the hose had reduced the actual air flow down to about 350 CFM. That fan was WAY less effective than anyone thought.
- Ideally you want very little intake or exhaust flow restriction (Delta-T shoots for 0.5" H2O), with a slight vacuum in the ER. The slight vacuum ensures that any ER and bilge smells get pulled into the ER and out the exhaust fans rather than getting pushed into the boat's living spaces.
- If you have an automatic fire suppression system in the ER, it should be wired up to shut off all fans in the event of a fire. That is to contain the fire suppressant so it can do its job, and also to cut off sources of combustion air. Fancier system even have louvers that close off vents.
