Engine room blowers

Our boat has four blowers in the engine room. The two forward ones blow air in, and the two aft ones suck air out.

To me it would seem more efficient to have all of them sucking air out and just let the air find its own way in. But what do I know.

I rarely use them since they make a horrendous amount of noise.

Is there a good reason to leave them in their current configuration, or should I change them so all four exhaust air from the engine room?

You probably will get more airflow leaving them as is than changing them so all four exhaust. But I have to ask: why do you need to run the blowers at all? Three possibilities:

1. You have a gasoline engine and you need to exhaust the air in the engine room before you start.

2. Your engine room runs too hot in normal operation and you need the blowers to keep it down to reasonable temps. Most engine manufacturers are ok with engine room air at 120 deg F. FWIW a well designed engine intake vent system is all you need to keep a diesel down to 120.

And your blower installation may block off intake air from the diesel when the blowers aren't running and when they are running they are taking air away from the diesel's air intake.

3. After shutting down your engines, you want to cool down the engine room quickly to keep it from heating up adjacent people areas.

So the only reason that is valid for a diesel engine is the last one.

David

Diesels use a tremendous amount of air for efficient combustion. It would not be a good idea to have them sucking air out while running. However if you are just using them to scavenge hot air out after shutdown then it would be ok.

I have two intake without blower and on the other side one blower intake and one blower blowing out (totally ridiculous in my sense). I want to change this to 2 free flow intake and 2 blower exhaust just to keep engine compartment (for willy it is compartment not room) relatively cool and provide fresh air.

L

Greetings,
Mr. ww. I don't understand what you're asking. What sort of improved efficiency are you hoping to achieve? Cooler ER?



I just did a very rough calculation of the 2 "blowers" (Ford Lehman 380 cubic inch) in our ER. At 1750 RPM they are consuming about 790 cu ft of air per minute (total) that must come in through the air intakes.

I use a large squirrel cage exhaust and dedicated large intakes. The cooler the engineroom, the more efficient your diesels run. Excessive heat also can shorten the life on many things in the usual engineroom.

wwestman said:

Our boat has four blowers in the engine room. The two forward ones blow air in, and the two aft ones suck air out.

To me it would seem more efficient to have all of them sucking air out and just let the air find its own way in. But what do I know.

I rarely use them since they make a horrendous amount of noise.

Is there a good reason to leave them in their current configuration, or should I change them so all four exhaust air from the engine room?

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No suggestions here pertaining to how many blowers nor if they should all pull in or blow out. The blowers on our boat were noisemakers until I removed them from their mounted positions and let them float, relying on the hoses for support. I found that the noise radiated throughout the entire boat when they were physically attached. Something that you may want to consider.

Before you change things around, see if you can find a manufacturer's requirement for square inches of air inlet. Most engine manufacturers list requirements for free air openings. If you're going to run an exhaust fan (as I do) while underway. You need to provide additional free air openings for this also. An easy mistake to make is assuming that the opening for the 4" fan is going to allow the same amount of air in as the fan blowing it in. While I don't have the number, you will probably need to increase the 13 sqin. opening of the fan to 100 sqin. for free air. Ask any engine manufacturer and they will tell you the engine can't compete with the exhaust blowers, you need to supply enough free air for both.

Ted

To answer some of the posts:
Diesel engines, so blowers not required.
The blowers were installed by the builder.
The only time I have used them was trying to cool things down after running engines before going into the engine room.
Have never used them while underway, since we are in the PNW heat does not seem to be a big issue.

The source of the question was thinking about evacuating smell/smoke/anything else out of the ER the fastest, and which would work best. 2 pushing/2 pulling or 4 pulling and doubling the volume of air being moved.

I may try the unmounting idea as a way to reduce noise. Maybe hang them from bungee cords to keep them in the same relative place.

Lepke said:

I use a large squirrel cage exhaust and dedicated large intakes. The cooler the engineroom, the more efficient your diesels run. Excessive heat also can shorten the life on many things in the usual engineroom.

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That makes sense... except that the engines are already dragging a huge amount of air into the ER just as it is running. I’m not sure how much cooler the exhaust fan will keep the ER while it is running. I’d be really interested in knowing if you have any actual measurements of the ER temps with and without that blower running?

My guess is that the mfg designed the system so it didn't steal combustion. 2 in - 2 out would seem to provide balance for the blower system.

wwestman said:

The source of the question was thinking about evacuating smell/smoke/anything else out of the ER the fastest, and which would work best. 2 pushing/2 pulling or 4 pulling and doubling the volume of air being moved.

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I certainly don’t know the answer, but I can guess...

The first question, unless I missed it, do you have open air vents besides the 4 blower vents? If you do, how big are the vents? If the vents are of an equal area to the 4 blower vents that you have then I would think that if all 4 exhausted, you would likely increase your air exchange when the engine is not running.

OTOH, if your blower vents are 4”, then if your open vents are less than 50” sq.in. I think that switching all to exhaust would actually reduce your air exchange. The reason is that if your trying to exhaust through 50” sq.in yet have a smaller intake than that, your blowers will be less effective. Now, if your passive vents are around 50” sq.in then you might get better exchange if you reverse one of the flans blowing into the space, so you would have 3 in and 1 out.

I have one blower fan. I would like to run it for 20-30 minutes after engine shut down, but it is really loud and annoying. My ideal situation would be to replace it with a quieter fan and put it on a timer.

I have a blower that exhausts and draws from up high in the ER. So it is there for cooling after a run. I don’t bother since the ER doesn’t really get that hot. I don’t need it to evacuate smells either.

I agree that, unless you have a sealed positive-pressure engine room (and that brings its own problems), there should be absolutely no need for blowing air IN to the ER. I have hi-volume (not squirrel-type) blowers x2 OUT that draw from the top of the ER (where obviously the hottest air accumulates). I run my blowers underway (why not?) and when I get back to the dock and on shore power, I leave them on while we tie up, wash off and pack up. On my long list of 'get to it one day' projects is an idea for a simple timer so the blowers can be left on for a longer period and/or controlled by a t/stat. Unfortunately, my batteries & electronic regulators are in the ER so I like to get temps down as soon as possible. At anchor, I'll generally let the blower on the lee side continue to run for around 40min after the pick hits bottom. Not sure this does much other than make me feel I'm doing something !

We have 8 x 6 inch PVC tubes on foredeck inside vented lockers forcing cool air in.
We have 2 x 24v Davies Craig radiator fans about 1ft in dia. sucking warm air out, up two decks and out the funnel.
We have ducting from the funnel to a box with air cleaner above the engine for the turbo to suck cool clean air.

John Deere lists the minimum ventilation area at 93 sqin. for my 135 HP 4045 engine. The maximum air intake is 346 cfm. The ventilation blower pushes 200 cfm. I have around 200 sqin. of ventilation area feeding the engine room. With the blower running underway, the near ceiling temperature is a little over 100 degrees. Without the blower during the summer, it can exceed 120 degrees. Even though the engine room ceiling is well insulated, running the blower while underway and for a half hour after shutdown, helps reduce the temperature in the saloon above.

Ted

Electronics, battery charger, solar controller, hydraulics, batteries all are happier cooler. Engine room vents are to keep the temperature in there manageable and therefore extend the life of these components. Obviously, as long as there is sufficient combustion air the engine doesn't care. Cooling it off faster after use is probably pointless but you need to get air out as well as in so your current system is most efficient. Unless the onlt thing in your engine compartment is the engine, keep the blowers and use them as long as they are running. Most blowers are noisy so spend your time making them quieter if you are concerned.

Seems to me that the amount of air needed to supply the engines can be measured by the flow of exhaust out the hoses at the stern. If you want excess air in the ER for cooling, your area of intake vent must exceed the area of exhaust by some significant factor.
Your present setup of 2 in and 2 out, if that is all you have, ie no other openings into or out of the ER, will deprive the engine of its necessary air supply, so I would use the fans only for cooling the ER after shutdown.

I run the blower for about a half hour after turning off the engines to "cool" down the ER for the reasons stated. I wonder how much it helps, but figured it can't hurt.

Twin Perkins 4.236 naturals here with no ER fans, just passive ventilation through 2 vents forward and 2 vents aft. The ER is roughly 12'x20'. I monitor outside and ER temps at the lower helm and the delta normally runs 20*F to 25*F. The most delta I've ever noted is 29*F.

This boat/engine pairing has been operated successfully for 41 years so I guess it's OK.

Not sure if this applies to low HP engines, but I read somewhere that if your ER hatch is noticeably harder to open at cruising speed, the engines are not getting enough air and creating a vacuum in the ER.

FlyWright said:

Twin Perkins 4.236 naturals here with no ER fans, just passive ventilation through 2 vents forward and 2 vents aft. The ER is roughly 12'x20'. I monitor outside and ER temps at the lower helm and the delta normally runs 20*F to 25*F. The most delta I've ever noted is 29*F.

This boat/engine pairing has been operated successfully for 41 years so I guess it's OK.

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Be careful when you use the term ER !

L

Thread creep here, sorry.


I'm currently dealing with a leak on one of my fuel tanks. I eventually got it stopped by pumping the tank dry, but for a few days I was changing diapers every 8 hours.



I've never used my blower prior to this, but I ran it 24/7 while I had fuel leaking into the ER. It helped keep the small down a lot. I'm hoping it kept it form permeating the cabin. It was nice to have when I needed it.

There really are two aspects to ER venting. The first is providing combustion air the engine, and the second is providing cooling to the equipment space.


The engine intake specs will tell you how much combustion air is required, and often how much vent area is required. That's good, but it's only part of the problem, and typically the easy part.


Also in the engine specs is a value for the rejected ambient heat. That's how much heat the engine dumps into the surrounding space, in our case, the ER, and it can be a lot. A 4 cylinder 4 liter Deere 160hp dumps 17kw of heat into the ER while running. It's a little less at lower power loads, but not much. A 12 liter 425hp Deere dumps 43kw into the ER. That's a really big heater running all the time you are underway, and it takes a lot of air flow to get it out.


Pretty universally equipment manufacturers quote one of two (and some both) specs for allowable surrounding temps. 130F is the absolute max, and they want no more than 30F over the outside ambient temp. Those two become one in the same when it hits 100F outside.


Some boats with smaller engines and lots of natural air flow are fine without mechanical ventilation. But with bigger engines, you need to move a LOT of air to keep the ER temps reasonable.


Part of the cooling air flow can also double as the intake air. The engine intake is pulling hot air out of the ER just like an exhaust fan, and both get replaced by cool outside air drawn in. The rest needs to be exhausted by mechanical ventilation.

Two fans in series is normally used to provide adequate (whatever is decided by whomever is designing it) flow in a relatively high resistance application, like ductwork. Each fan moves the same amount of air, of course, but operates higher on its flow rate vs static pressure curve.

It's also used where you want a neutral pressure in between the fans, but that's usually a consideration where you're moving lots of air (like a building) and the thing that could be negatively affected is relatively small, like a gas or oil burning device where you don't want to pull or push air through a chimney with the fans. Or the doors of a store where you don't want to pull a vacuum that people struggle to overcome.

But in an engine room, just putting adequate ventilation intake area (taking into account the effective reduction of that area by the louvers, which can be substantial) is cheaper but requires lots more area (loosely speaking) than forcing intake through a smaller area with a fan or blower.

So it could be that the designers didn't want to use large intake grills, or there's intake/exhaust ductwork, or whatever. You'll almost always move more air with both blowing out, as long as there is low static pressure across the intake grill or through the intake ductwork. Which I'm guessing is the reason for having intake fans vs just large intake openings - they didn't want large openings.

Those little solar vent fans, for what they're worth, can be reversed for intake or exhaust, but they move so little air, you'd be better off using two exhaust with the intake air leaking in from everywhere, rather than one in and one out. But higher air flows using real fans or blowers may require boosting on both ends, if lots of intake duct/grill area isn't feasible.

I'd be curious to know how much of a vacuum is pulled in the typical engine room with it/them at speed, with only the normal vent openings working, with or without fans running. Or with intake fans only running, assisting in bringing in combustion air, and air also drawn in backwards through the exhaust fans. Or with exhaust running, pulling a vacuum against what's already a vacuum from engine induction air.

We have 2 passive intake vents, and one blower exhaust vent, powered by a quite aggressive and power hungry 12v fan. The ER is noticeably cooler with it on. The exhaust intake is cunningly positioned near the air cooled eutectic fridge and freezer compressor,so I run it in conjunction with the compressor when on the hook,with the battery charger to feed the exhaust fan.

I have the same set up. Two in at the front of the ER and two out at the rear. However my external vents are in the side walkways of the boat - meaning that there may not be as much suction through boat movement. If that is your design, I would suggest there may be logic in the set up.

Additionally my 16KW generator is interlaced to the blowers, unless one set of blowers is on (we use the out) the genny will not run.

AS noted almost NO boat has a sealed engine room so pushing air in simplypushes engine room and bilge smells into the cabin.

Of course a small exhaust leak could be a killer not just a stink..

Diesel Engine room Blowers

As others have said, the blowers are not required by ABYC or USCG. However, most diesel boat builders today install them to remove excess engine room heat during heat soak at engine S
shut down. . Many electronics do not like high heat environment. Magnasine inverters limit the ambient to 77 degrees. a temperature you will not see in the engine room till the engine is dead cold. It has been th epractice there for to add blowers. some blowing air in and some taking air out, to reduce the engine room heat as quickly as possible for the sake of elecronics, and also to reduce the load on conditioned living spaces as well.

Here is what ABYC has to say on engine room ventilation.
H-32.5 GENERAL REQUIREMENTS

32.5.2 Removal of Fixed Gaseous Fire Extinguishing System Discharge - DC-powered ventilation shall be provided to remove the discharge from fixed gaseous fire extinguishing systems (see ABYC A-4, Fire Fighting Equipment).

32.5.3 Combustion Air – ventilating provisions and openings to the machinery space provided for supplying combustion air shall accommodate at least the sum of the maximum air requirements specified by the engine manufacturer(s) for each propulsion and auxiliary engine(s) in that space.

NOTES: 1. Consult the engine manufacturer(s) for combustion air requirements, including inlet air temperature and restriction/depression.
2. Due to the characteristics of diesel fuel and the closed nature of the diesel engine fuel system, neither mechanical nor natural ventilation, as prescribed for gasoline powered vessels, is necessary to remove diesel fuel vapors.

32.5.4 Additional Uses of Ventilation - power or natural ventilation may be needed to control compartment temperature. Power ventilation may also be used in the machinery space for odor control and personnel comfort while servicing equipment.

32.5.5 Ambient Temperature - for design purposes, the ambient temperature of machinery spaces is considered to be 122°F (50°C), and of all other spaces, is considered to be 86°F (30°C).NOTE: Temperatures in machinery spaces after engine shut-down may exceed this design ambient temperature.

32.6INSTALLATION

32.6.1Non-metallic materials used for ventilating components installed below deck shall be capable of continuous exposure to a temperature range of -22°F (-30°C) to 185°F (85°C) without failure.

32.6.2 Non-metallic ventilating components shall be installed at least nine inches (229 mm) horizontally from or below, or 18 in (460 mm) above, any surface capable of reaching a temperature of 200°F (93°C) under normal operating conditions.EXCEPTION: Ventilation system components designed for use in higher temperature locations provided the component manufacturer’s maximum temperature rating is not exceeded.

32.6.4External openings for ventilation intakes and exhausts shall be arranged to minimize entry and recirculation of engine exhaust fumes.

32.6.5External ventilation openings shall be located and oriented to prevent entry of fuel vapors. In no instance shall the ventilation openings be closer than 15 in (381 mm) from the diesel tank fill and vent openings (see FIGURE 3).

I read somewhere that it takes about 1500 cubic feet of air (at room temps) to combust a gallon of diesel fuel. If I had a boat with 4 fans, I would have 3 pulling fresh air in, and one taking some heat out. Understand that as air heats, the volume increases, so the engine heated air will be higher volume than when it came in. The only saving grace would be that the engine(s) would be consuming a significant amount of air in the process of generating all that heat.

I would never make a diesel engine scavenge for air or fuel, and expect decent performance. When it has to pull too hard for air or fuel, you're going to have problems.