Design of heat exchanger step :3 : Estimate

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The 3rd Step in Heat exchanger design a preliminary estimate of the size of the exchanger is made,using a heat-transfer coefficient appropriate to the fluids, the process,and the equipment.

Generally we can get reference from various resources listed below, either expected heat flux (W/m2) or range of heat transfer co-efficient. based on this information, we can estimate the heat transfer area required.

But if the case is, you are doing for first time, and have no earlier experience in that type of heat exchanger, then better is make your self ready with all information e.g fluid properties which includes

  1. Dynamic viscosity (for the temperature ranges)
  2. Conductivity (for the temperature ranges)
  3. Density (for the temperature ranges)
  4. Boiling temperature
  5. Specific heat

The grate equation (you can see in my earlier post) for heat duty is:
Q = U A LMTD & Q = m. cp. DT

where
Q = heat duty (kcal/hr)
U = over all heat transfer co-efficient (Kcal/hr.m2.oC)
A = heat transfer area (m2)
LMTD = Log mean temperature (degC)
cp = Specific heat of the fluid to be heated / or to be cooled (kcal/kg)
DT = temperature difference of any one of the fluid (degC)
m = mass flow rate of the fluid (kg/hr)

LMTD is calculated based on the temperature on both the side for both the fluid
it’s simple if you understand the following figure

logarithmic arithmetic mean temperature difference lmtd amtd
& LMTD = (dto – dti) / ln(dto / dti)

Just Plot your temperature, take difference on each side, and do the arithmatics…

Some good refrences (Free google search results 🙂

http://www.chemsof.com/lmtd/lmtd.htm
http://en.wikipedia.org/wiki/Log_mean_temperature_difference
http://www.wlv.com/products/databook/databook.pdf

Thermosiphon Reboiler Design

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During Resent days, I was trying to design thermosiphon reboiler!

After long days of calculation, rewriting calculations on pages! on excel! finally I come to some ‘lesson learn’ which I’ll  be sharing now.

But before that, following are two good references to design a thermosiphone reboiler
1. Applied Process Design Volume-3 by Ludwig
2. Process Heat Transfer – D.Q. Kern

Heat exchanger: step 1: application

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<p>The first step dorm designing a heat exchanger is to understand where it will be used for.</p>
<p>Application is most important criteria and mostly its input from client.</p>
<p>Application demands understanding of functionaries expected, duty needed, properties of both side fluids, system information e.g presuredrop expected, cleaning method of the equipment, hazard category, local laws pertaining to pressure vessel and hazard handling.

Some time the application demand ‘soft’ heat transfer due to the media e.g yeast cooler due to delicate yeast cells are always co-current type which lead to higher surface area and are more costly than counter current heat exchangers; but due to application those are designed that way.

What is heatexchanger?

GoodRead

Yup, just a brief on this subject.

Heat exchangers are devices used to transfer heat energy from one fluid to another, gas to gas or gas to liquid. 

Typical heat exchangers are seen around in most unexpected location. From household appliances as air conditioners, refrigerator etc.  Boilers and condensers in thermal power plants are examples of large industrial heat exchangers. There are heat exchangers in our automobiles in the form of radiators and oil coolers.  Heat exchangers are also abundant in chemical and process industries.

Heatexchanger sizes varies from couple of square mm (heat sink in computer cpu) to length exceeding hundreads of square meters!

From simple study flow to complex flows.

Heat exchanges as they say “comes in all sizes and nearly for all duties”

We will, in my next post discuss about fundamental of heat exchangers.


Heat exchanges… Google way.

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If you Google the word you will get morecthan a billion results. That just shows how it closely associated to industry, to us.

But at the same time ut shows how wast  the subject is.

The purpose of next few blows is to simplify your search by telling you what to ask and where to see.

Figures crossed 🙂

HEAT EXCHANGERS

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Past few months I was thinking of creating a post on heatexchangers. I think now is the time when i can write a bit on this subject.  

There are numerous sites available which will explain verwell about this subject. But as usual they lack the subject matter. What we all are interested in. The core, the ‘how’s part of it.  

In my next few pposts you fand this information.

Take care.

Heads as per ASME : Quick overview

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I was studing some heads for a while, and thougth to summarise the variety in one go…

1.      Flanged Head : Normally found in Vessel opertaing at low pressure, genrally water tanks, Boilers etc. They are also used in high pressure application where the diameter is small.
2.      Hemispherical Head : Generally, the required thickness of the hemispherical head due to a given Pressure & temperature is half of cylindrical shell with equivalent diameter & Material.
3.      Elliptical and Torispherical (ASME Flangged & Dished) heads : they are very popular in pressure vessel, their thickness is ususally same as the cylinder to which they are attached.

This is a quick heads up for variety of heads used in industry. The other type are
1.      Conical or Toriconical head: they mostly form bottom end closure for the vessels, and act as hopper, or to give easy drainability. The important thing to consider while designing this heads are, a) they increase the overall height of the vessel. b) ASME need to do discontinuity analysis for half angle >30 deg, or else one need to go ahead with toriconical head to avoid the unbalanced force at the junction.
2.      Miscellaneous head: Many chemical process requires unusual vessel configuration, the heads of such vessel can have unlimited configurations. The design of such head is very complicated, and there is no straight forward formula for that.

Keep reading!