Generator temperature in an enclosed compartment 6

[cayrick]

Insights please, on an air cooled mobile Honda Generator (5kw) housed in a weatherproof concrete and metal box ( new installation)

. Installed in a temperate climate, with an average ambient temperature range of between 68F-85F year round. I installed a small vent fan of approx 850 cfm. Most small generator mfrs. do not publish operating environment temperature limit info, presumably because they do not want generators installed in compartments, and so if they have info, they intentionally withold it. Published info might also become problematic if there are warranty issues. I have installed a propane conversion kit and run it on propane only rather than gas. My concern recently, while running it for over an hour, the compartment interior reached an equilibrium delta T of 50 deg F under load. In other words the outside temp. was 75 F and the interior of the compartment reached 125 F.
In my mind this set off alarm bells as I am concerned about alternator and engine damage. I am looking for insights from people who have more knowledge than myself in this area. Similar installations are commonly found in Youtube videos where people showcase their personal generator installations in plastic garden sheds. These are done typically by folks with a limited technical background and nobody ever talks about or considers interior operating temperatures to be an issue, and operate on the principle "if it ain't broke don't fix it". Should I be looking at a larger capacity fan? What delta T should I be shooting for?

 

[TugboatEng]

You'll need to get some airflow through there. Fans are the best option but draw a lot of power. One silly thought is to put an uptake on the box and use the exhaust as an eductor to induce draft. Obnoxiously you'll need an inlet louver and it will need to be baffled if you're interested in keeping the sound in the box.

 

[EnergyProfessional]

It also depends on how long you run it at what load. I bet the manufacturer recommendation is to have it located outdoors and NOT in an enclosure. Anything short of that will either make it outright stop working, or at least diminish its life. Their exhaust system is designed to be outside as well. Many people win the Darwin award by running these in enclosed spaces. And you don't want exhaust re-circulation.

The only reason people get away with this is that such generators often don't run at nominal load and don't run for long. If you want to go against what the generator was designed for, at least get a higher enclosure so the normal convection can cool all parts. And increase the fan flow rate. You basically operate 50°F above ambient. I would also determine that delta under full load.
Average temperature is meaningless. On average Jeff Bezos and I each have $60 billion......
You need to design this for the maximum ambient temperature assuming you also want to cover power outages during hot days.

 

[TugboatEng]

 

[MintJulep]

850 cfm gave you a 50 degree F temperature rise above the outdoor temperature.

Working backwards, you therefore need to vent away almost 46,000 BTU/hr.

If you want to keep the box temp to 95 on that 75 outside day then you need 2,125 cfm.

Of course, if the power goes out when it's hot outside the box temp also gets hotter, and the generator won't like that.

Also, just because some piece of paper associated with the fan says "850 cfm" does not mean that you actually get 850 cfm when you bolt it to your box.

 

[EnergyProfessional]

You also need to connect that fan to the generator. And if you keep the openings the same, you need to add quite some fan to get 3 times the airflow. Hopefully the generator is large enough just to run that fan.......

 

[itsmoked]

It's probably safe to say that the heat rejection will be about 2 times the electrical load. 3,000W then 6,000W will need to be dissipated from your compartment.

A great deal of that heat is the exhaust. If you can guide it out of the enclosure on it's own or get it very close to the the existing outlet vent (so there is no mixing with the air in the enclosure) you will see the maximum heat rejection ability of your venting system.

I always have to ask why not just use a EU7000? Then all you need is rain protection. You could use an EU7000 as your dining table. They also use about 1/2 the fuel.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

I don't understand where your air inlet is for the combustion air and the vent air for the radiator?

Can you draw this a bit better?

I doubt the generator is inteded to work in ambient air much above 40C - say 105 to 110 F

YOur issue may be more that there isn't enough cooling air as it won't be designed to flow air through louvres

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cayrick]

Thanks for the excellent replies and suggestions. I will elaborate a little further. I have a metal hood on each end of the compartment. One end is intake and the other end has the extraction fan. I ran an exhaust pipe out of the rear of the compartment and the length inside the compartment is short (12") and the exit hole is sealed and the pipe inside is covered with insulating material. Combustion air comes through the air intake which is shown in the photo in my original submission. I installed a propane conversion kit for the gen set and it functions well.
I have been confused by the lack of published info on acceptable temperature levels inside the compartment. My gut says a rise of 50 F in the compartment interior won't cut it and likely a bigger exhaust fand is required. Please if you have any further suggestions to offer please pass them along.

 

[miningman]

Way back in the day , I was running a 1981 Dodge half ton that had been converted to propane. One of the recommendations was to use the lowest temperature thermostat that one could find. I found one rated at 160 degrees F to replace the standard 185 degrees . The only downside was a lack of interior heat in minus 25 weather. You could try this to see what effect it has on your local running conditions

 

[TugboatEng]

Air cooled engine

Heat rejection is equal to the power output of the engine. 5kW in this case. Pick an acceptable rise, say 10 degrees F and size a fan to move the amount of air that 5kW can heat 10 degrees. The math is relatively simple, you'll need to look up the enthalpy of air.

But... Sizing the fan will be challenging. Most fans list a flow rate but that is at the most ideal pressure ratio. You observed performance will be substantially lower than that number. Try to buy a fan that has a fan curve available. Mouser has good option.

 

[LittleInch]

I strongly suspect your 850 cfm fan is doing much less than that.

It looks like the vent is about 1 ft2?

So at 850 cfm, that's 14 ft/sec. That's quite fast. With the losses in the enclosure you might be down to 3 to 400.

Double up on the fans and inlet hoods is my suggestion.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MintJulep]

Pipe the combustion air intake directly to outside also. Because as you have it now it is fighting against the (anyway too small) ventilation fan.

 

[TugboatEng]

It's standard practice to use the combustion air as part of the enclosed space ventilation. Use the fan as a supply and let the engine contribute to the ventilation as part of the exhaust.

 

[EnergyProfessional]

Combustion air should come directly from outside unless the space is VERY WELL VENTIALTED with almost no dT to ambient. If the combustion air is heated up, it will be more likely to overheat the engine.

Indoor generators have huge fans and are liquid-cooled and the fan blows all the radiator air directly outside. The above generator is air-cooled and will marinate in its own hot air.

The generator in this thread is not designed to be inside. The best you can do is to have it under a tarp-tent or some other protection from rain with all 4 sides open. There is a reason the manufacturer doesn't publish data for indoor use, because they don't want you to use it indoors. if you want toe sides enclosed (for noise or theft protection) you need huge airflow. The generator likely would need all its power to just run the fan.

 

[Rogue909]

Thats a nice box for the setup. I'm assuming the paneling in the walls/doors is there to minimize noise. What is the purpose of the generator?

Is this generator to run during emergencies only? In which case the simplest solution may just be to open the doors/crack the top and let it be open to the ambient. If you really want to keep the doors close a louvered side on both ends would allow a pseudo cross breeze.

If it's going to run constantly they do have off the shelf generators that are water cooled. The radiator could be upsized and moved outside the unit. Honda EV6010. I realize the generator in the picture looks fairly new so the idea of buying another may be off-putting.

 

[cayrick]

This is strictly for emergency backup power for extended power outages. It supplements existing backup power which consists of a battery bank and inverter/ charger connected to utility power. Batteries are good for two to three hours max. So for extended outages this will take over. I have never ever had an outage that drained my batteries in 14 years but who knows in the future. I have no need for A/C and appliances where possible are propane (stove & dryer) for example. So this powers a few lights and tv's. It is possible that it never gets used at all but that's the price for being anal.
Yes, the compartment is lined with sound barrier and sound absorbant material.
At this point it would be difficult to separate combustion air from inlet air. I have taken the advice of one poster, and today installed a larger fan a variable speed 14 in fan rated at 2400 cfm or roughly 3 times that of the existing fan.
The biggest problem for me in building this compartment is to find info on a max. air temperature surrounding the generator. Here are some sources I found regarding air temperature
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https://www.stamford-avk.com/sites/stamfordavk/files/AGN088_C.pdf

Here the ambient temperate max. is 40 C
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https://www.csemag.com/articles/design-generator-rooms-for-optimum-performance/

- here they speak of generator room temp of 104 F
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https://www.championpowerequipment.com/wp-content/uploads/2017/08/100331-om-english.pdf

p18 indicates a max ambient air temp of 40 C or this number of 104 F agaom.
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https://electricalnotes.wordpress.c...entilated-air-and-heat-generation-of-d-g-set/

The author of this article says the generator room temp. should be 8-12 C above outside air temperature.
-Champion Generators responded to an email requesting info on the temperature rise in the compartment of one of their packaged standby generators. Champion claims that they design for a compartment temperature of 5 C above outside ambient.
- Honda did not answer my request for information
This project for me arose after looking at youtube videos where amateur backyard engineers would simply buy a Rubbermaid or Suncast plastic garden shed and pop in any old fan. Later, as I jumped into this project I realized that people were putting in 8 or 10 inch shuttered attic fans but ignoring how hot it gets inside the box and acting as if temperature doesn't really matter. My original fan was 12 in. and the new replacement is 14. I bought a remote thermometer and on the generator itself I installed a bimetallic thermal cutout switch in the event of a fan failure.
I watched one video where a poster installed a 122F (50 C) thermal shutoff switch. It is then I began questioning myself and my install after a one hour run time where compartment temperatures reached 125F where they appeared to stabilize while temperature outside the box was only 75 F. This is when alarm bells went off.
I understand that the temperature issue is with the head temperatures in the engine as well as the alternator windings.
The recommendation to separate combustion air from ventilation air is interesting but physically with the compartment already fabricated and the difficulty in sealing off the air filter would be quite difficult.
I will now see how this new fan switch plays out but I expect it will appreciably drop internal temperatures.
I attempted to upload a photo showing my engine exhaust. The exhaust inside the cabinet is leas that a foot in length and has a heat shield wrap around it so I am not very concerned with exhaust heat being dumped into the compartment. I really appreciate all the suggestions on this forum and will post the results. Thanks for all the help and suggestions.

 

[EnergyProfessional]

If this 2400 cfm fan is just like a free-standing propeller fan... note that cfm rating is for free flow. not for flow through a box and louvers like yours. that would be significantly less.

 

[cayrick]

True enough and the point is well taken. This is an after construction mod so there is not a whole lot I can do without investing a lot of new $$ and labor into it. The way I rationalize it is like this.
- I get significantly increased cooling air flow now.
- This is a backup to a backup and is for emergency use only so it will rarely if ever be operated continuously. My battery backup in 8 years has never ever failed me.

- Local ambient temperature here is temperate with little variation so this is a factor working in my favor.
- The inlet air is free flowing convection air. If I am challenged with air temperatures, an inexpensive way to increase air flow is to put a free standing fan on the ledge of the inlet air or on the floor to force more inlet air into the compartment. Champion generators states that 40C or 104F is the max ambient operating temperature for their air cooled generators. Since Honda ignored my request for information, I assume the Champion info translates into Honda operating temperature ranges.
- This will be my target temperature range and if I cannot achieve it, or at least approximate it,then low operating times and very few operating cycles should compensate for it.

 

[TugboatEng]

I'm telling you, some creative exhaust ducting can be used to improve air flow through the enclosure. The exhaust can be used to power an eductor. This won't be all that you need but utilize the engine as much as you can to ventilate the space.