Heat Transfer Calculations

[dhill11]

Hello,

I work with a vessel manufacturers and we are frequently asked by customers to perform heat transfer calculations.(i.e. if the vessel you are making us has 500Lt of water in it @ X°C, how long will it take to either heat it up or cool it down or how much energy (W or kW) is needed to heat it, etc.)
I was wondering if someone had a set of hand-written calculations that I could possibly work with? Or some advice on where to get an example that I could use again.

The vessels are usually stainless or an exotic alloy with dimple, limpet or flooded jacket and usually have agitators.
I know this is a big ask but I would appreciate it.

Many thanks in advance,
dhill11

 

[IRstuff]

Are you saying that your company, which has been in business some number of years, has never done the calculations, nor made any tests?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[dhill11]

Hello IRstuff,

We would normally get the help of a third party, or the customers engineering house would do the calculations on their behalf.
The engineering house might just ask us what is the jacketed area on the vessel or stipulate that the vessel needs so many m2 of jacket.
The company I work for is not particularly large so trials have never been done.

 

[MortenA]

Heating s liquid in a vessel require energy for:

a) heating the liquid [sic]
b) once the liquid is warmer than the ambient - compensate for heat loss from outside wal to ambient
c) if the vessels are open or vented - evaporation

a) Should be simple enough (dT*Cp*vol*density) - thats the result in kJ - and then your client must tell you the time for heating, divide by time in seconds and you have the effect in kW
b) More complicated - but unless is MUCH hotter (or heating is very slow) it wont be that much compared to the energy required for heating.

c) Could be a significant amount - have no clue right now

b) will depend on geometry and ambient condition (exposure to wind etc).

Best regards

Morten

 

[chicopee]

First of all how is the vessel heated before the water content is heated? and secondly when water is to be cooled thru vessel wall, what is on the external surface of the vessel? is it water, air or other medium?

 

[TrippL]

Sounds like you need to hire a professional engineer. Maybe only to consult you on individual projects or hire on staff. It is not difficult to plug numbers into an equation, the difficult thing is knowing what equations to use. Since I'm sure your company makes vessels for many different applications, every situation is going to be different. Many different things can have major effects on the heat transfer characteristics of a particular application. That is why we have engineers. Point is, there are no set equations to account for every possible scenario. Consult or hire a licensed and experienced engineer.

 

[TrippL]

This post reminded me of this classic story:

There was an engineer who had an exceptional gift for fixing all things mechanical. After serving his company loyally for more than 30 years, he happily retired.

Several years later the company contacted him regarding a seemingly impossible problem they were having with one of their multi-million dollar machines. They had tried everything and everyone else to get the machine fixed, but to no avail. In desperation, they called on the retired engineer who had solved so many of their problems in the past.

The engineer reluctantly took the challenge. He spent a day studying the huge machine. At the end of the day, he marked a small "x" in chalk on a particular component of the machine and proudly stated, "This is where your problem is." The part was replaced and the machine worked perfectly again.

The company received a bill for $50,000 from the engineer for his service. They demanded an itemized accounting of his charges. The engineer responded briefly:

One chalk mark -$1
Knowing where to put it $49,999

It was paid in full and the engineer retired again in peace.

 

[IRstuff]

I agree, you need to get some local expertise to come up with models of your various configurations that will at least approximate their true behaviors. The basic equations are extremely simple and can be run in Excel, but knowing what to model and what to ignore requires detailed understanding of your equipment, which is not something that can be readily done over the internet in a few forum postings.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Jmoore1]

Doing a heat transfer calculation like this is pretty simple for someone that knows what there doing. Most Mec'es and Chem'es take a basic of this in school...but this doesn't always transfer well to actual practice. There are alot more unknowns to solve for to get a true prediction using these equations as IRstuff pointed out. Any Mec/Chem fresh out of university could give you a number, but I gaurantee someone with experience in the area would give you a very different one using the same equations.

 

[tesser]

There might be some useful information and calculations at the Spiraxsarco website:

http://www.spiraxsarco.com/resources/steam-engineering-tutorials.asp

 

[ginsoakedboy]

You can download a good textbook on heat transfer here:

http://web.mit.edu/lienhard/

http://www/ahtt.html.

You can start learning from the book.

And, heed the advice in posts before mine.

 

[dhill11]

Thank you all for the help. I am very much appreciative for all the advice and links.
dhill11

 

[dbecker]

dhill,

You will need to know the mass of water, the heat input in BTU/s or Watts..

You will need to know, roughly, the surface area exposed to the water where the heat input is being generated (heating coils? I dont know what you use). You will need to assume some things like convection coefficient which will vary with time.

You may not be able to get away with lumped capacitance method because water is a poor conductor of heat and will not change in temperature uniformly. Look up on the internet for lumped capacitance and you will see why.

Getting your heat transfer coefficient will be the challenging part because it depends on fluid motion due to natural convection (I am assuming you dont have an agitator in there somewhere). Anyway as others have said, the calculations can be performed but there is not ONE equation that will do it. You need to make assumptions and this is where experience and judgement comes into play.