Studying - cost of a heat exchanger?

[stoppit]

Hi all
I'm new so I hope you'll be able to help out and don't find this question beneath contempt!
I'm a chemist retraining in chemical process engineering and I'm faced with a very abstract maths problem. Here goes:

"The cost of a heat exchanger is estimated to be $10,000. Estimate the cost of a heat exchanger that is twice the size"

I don't know what is meant by size - area, duty, geometry? I think this is the point, the exercise is about using rules of thumb and empirical models to get start-point estimates to complex problems instead of working through a lengthy modelling exercise.

Being a chemist I suspect I'm missing something obvious here so.... Where do I start?!

I hope someone out there can shed some light!

Thanks

 

[dcasto]

Heat exchangers can be estimated several ways, I like the cost per pound method versus the "size".

To ratio up or down costs, the general formula as presented in my books at school and others says this:

Cost of scale = cost of base * (size scale/size of base) ^.8 or 1.74 times as much for the unit twice as large.

 

[SNORGY]

A variation of that is the "0.6 Rule". I think it's the same as dcasto's expression with a different exponent.

When I used to trend costs for equipment back in the mid-nineties, in the case of S&T heat exchangers, a good "estimate" for cost was $25.00 CDN per square foot of bare tube surface area. For aerial coolers it was closer to $40.00-$45.00 CDN per square foot of bare tube surface area.

Regards,

SNORGY.

 

[rmw]

And, it depends on the context of your study from which came the question. if your Hx is built with 90% exotic tubing and channel materials encapsulated in 10% mild steel shell, then the answer would have to be factored accordingly. If it is steel tubes in a steel shell with a steel tubesheet, then that is another ratio which produces altogether another answer.

rmw

 

[stoppit]

I had a think about it and here's what I came to -

If the duty doubles, the area doubles (?)

If the length of a shell/tube exchanger doubles, the area doubles

So assuming cost/area is a sensible way to estimate, the cost will just be x2 i.e. $20k

Right?!

 

[SNORGY]

badyaker:

One would almost think that, but in my opinion, it wouldn't quite work that way. Consider, for example, a 24" diameter NEN S&T exchanger doubling in length (and hence area) from 15 feet to 30 feet between tubesheets. Much of the materials and fabrication costs would go into the tubesheet rolling and the tubesheets themselves, and these costs are higher than a few extra baffles and longer tubes. So, you would lose the economies of scale in the smaller unit; conversely, you would gain some economies of scale in the larger unit. However, it wouldn't be 1:1.

Regards,

SNORGY.

 

[MJCronin]

As a chemist going into chemical process engineering, I believe you are making a mistake.

As an ancient mechanical engineer that has wandered into process equipment design, you should know that I am getting out....too many slick salesmen, MBAs and BS artists selling services and hiring temporary staffs.

Your question sound too much like a homework problem.....

"Twice the size" means twice the surface.....to me.

Suggest that you consult the text "Plant Design and Economics for Chemical Engineers" -- Peters & Timmerhaus Chapter 14

-MJC

 

[SNORGY]

That stated...it does appear to contradict the "$/ft^2" methodology. That particular methodology was derived in house at a place I used to work from plots of historic Purchase Order data. I never checked the data in detail but when I used the methodology in estimates my numbers always seemed to be pretty good.

Regards,

SNORGY.

 

[stoppit]

Thanks very much for your responses guys. I'm going with simply x2 on the grounds of cost/area as an initial crude estimate.

@MJC - it is a homework problem!

 

[zaigrata]

Hi badyaker,

it's the same as dcasto's expression
The way this should be solved is:

if you are looking to find the cost for heat exchanger B while you already have the cost of heat exchanger A = $ 10 000:

cost h.e. B = cost h.e. A (size of B / size of A)^exponent

the exponent as a rule of thumb is either 0.6 or 0.7 as suggested above by SNORGY

particularly for heat exchanger from Peters / Timmerhaus:
heat exchanger, shell-and-tube, floating head, c.s 10-40 m2 (100-400 ft2) =0.6
heat exchanger, shell-and-tube, fixed sheet, cs 10-40 m2 (100-400 ft2) =0.44

so in your case:
cost = 10 000 (2)^0.6
cost = 15 157

this is an example of the economy of scale