Thermodynamics concepts and theories used in heat exchanger 8

[leowang]

Hi, Everyone
I have not touched the thermodynamics for almost twenty years.
I almost forgot what I learned in school. Suddenly, I have a chance to look at heat exchangers. Since time is quite limited, I really don't have time to go over the thermodynamics textbook in such short time. Is there someone could help me highlight that the most important and useful thermodynamics concepts and theories(used more often in heat exchanger field) that I have to review in order to better understand heat exchanger performance? Thanks!

leowang

 

[Montemayor]

leowang:

I've designed and fabricated heat exchangers without any elementary or advanced Thermodynamics concepts or relationships being applied. I've also gone into deep Thermo work in producing cryogens, producing power, Rankine cycle, combined cycles, Expanders, turbines, compressors, etc. so I'm fairly familiar with Thermodynamic applications. I don't think you'll have a prerequisite in having to bone up on Thermodynamics just to get involved in heat exchanger design or fabrication. Heat exchangers, of course, are employed in Thermodynamic cycles (conversion of energy) but they don't have any Thermodynamic theory or knowledge in their design - or at least I don't recall ever coming across an instance.

 

[25362]

Here is a list of some thermodynamic basic concepts:

Thermodynamic equilibrium. Two systems in contact are considered in thermodynamic equilibrium when no change occurs in any macroscopic property.

Temperature. It is a macroscopic property. Two systems have the same temperature if they are in thermodynamic equilibrium. Let's call it T, and measure it in K (kelvins).

Zeroth law of thermodynamics. If two systems A and B are both in thermodynamic equilibrium with system C, then A and B are in thermodynamic equilibrium with each other.

Heat. It is energy in transit and its transfer from one object to another is due to temperature differences alone. Thus one may call it [Δ]Q. Once heat has been transferred to an object, the internal energy of the object has increased, not that it contains more heat. This is to reflect the fact that processes other than heating -such as the transfer of mechanical or electrical energy- can also change the object's temperature.

Heat capacity and specific heat. The heat [Δ]Q transferred to an object and the resulting change in the object's temperature [Δ]T are directly proportional:

[Δ]Q = C [Δ]T​

C is the heat capacity of the object. Different substances are characterized in terms of specific heat c, or heat capacity per unit mass. Since heat is a measure of energy transfer, its unit is the joule, J, and those for C would be J/K. The SI units of heat capacity are J/(kg.K).
Thus, we can write

[Δ]Q = mc[Δ]T​

Equilibrium temperature. When two objects in thermal contact are thermally insulated from the surroundings heat flows from the hotter object (1) to the cooler one (2), and all the energy leaving the hotter object ends up in the cooler one.
One can write:

m₁c₁[Δ]T₁ + m₂c₂[Δ]T₂ = 0​

where [Δ]T₁ is negative.

Mechanisms of heat transfer. Three commonly occur: conduction, involving direct physical contact; convection, involving energy transfer by the bulk flow of a fluid; radiation, energy transfer by electromagnetic waves. The first two mechanisms are dominant for example, in shell-and-tube heat exchangers, the third one plays a role, for example, in fired heaters.

Heat energy flow by conduction. For a rectangular slab of material of surface A, m², the heat-flow rate in J/s, or watts, perpendicular to A is

H = - kA([Δ]T/[Δ]x)​

The minus sign indicates that heat transfer is opposite to the direction of increasing temperature, that is, from hotter to cooler, x is the material (slab) thickness, given in m, and k is thermal conductivity expressed in W/(m.K). When a series of materials are in contact the heat-flow rate H is equal through the slabs since energy doesn't accumulate or dissapear at the interface between them.

Heat transfer by convection. As with conduction is approximately proportional to the temperature difference. The calculation of heat transfer (or loss) involves understanding details associated with moving fluids.

Heat transfer by radiation. The rate of energy loss by radiation is given by the Stefan-Boltzmann law:

P = e[σ]AT⁴​

where P is in watts, A is the surface area of the emitting surface, m², T, the temperature in kelvins, and [σ] a constant called the S-B constant, approximately 5.67*10^-8 W/(m².K⁴).

The non-dimensional quantity e is called emissivity, it ranges from 0 to 1, and measures the material's effectiveness in emitting radiation. Materials not only emit but also absorb radiation, and it turns out that the same quantity e describes a material's effectiveness as an absorber. A material with e=1 absorbs all radiation incident on it, it would appear black at normal temperatures, and is therefore called a blackbody. However, it would glow brightly when sufficiently heated.

In vacuum where conduction and convection cannot occur, all energy transfer is by radiation.
That's why in Thermos bottles and Dewar flasks -whose insulation is the vacuum between the layers of glass- energy loss is by radiation.

Since the rate of energy transfer by radiation depends on the fourth power of the temperature, it dominates at high temperatures, and is generally less important at low temperatures.

I hope this synopsis serves the purpose.

 

[lilliput1]

Learn also the definition of log mean temperature & learn & use counterflow. Main type of HEX = shell & tube; plate.

 

[rmw]

IN addition to LMTD and counter flow (vs cross flow and co-current flow) I suggest you bone up on terms like 'approach temperatures' and 'terminal temperature difference', Reynolds number, Prandtl number, thermal conductivity, heat capacity, heat transfer co-efficients, film co-efficients, density, viscosity, to name some more.

Suggestion: Go this website

http://www.processassociates.com/process/tools.htm

and root around in the sections titled "Heat Transfer", "Fluid Flow and Hydraulics" and "Basics", and if you plow through it all, and come to understand what it all means, you should be pretty conversant about HX's.

rmw

 

[amorrison]

thread391-125007

 

[davefitz]

Thermodynamics has its most useful application in evaluating power cycles, while the science of heat transfer is more useful in designing heat exchangers.

Since a large amount of the expertise used in designing a successful ( ie competitive , long lasting , meet performance requirements) heat exchanger is obtained from many years of experience, most successul heat exchangers are based on proprietary design standards that are held close by brand name companies. The companion effects of corrosion, pressure vessel technology , fabrication details, metallurgy, dynamic effects, and fluid mechanics must all be addressed in a successful design , which can rarely be uniformly addressed the first time concieved by a rookie designer.

But specific to the question that was asked, thermodynamic rules include:
a)heat will be transferred from the hotter to the colder fluid or mass if indiviual processes are considered in isolation.
b) the maximum amount of heat that can be transferred from one fluid to the other fluid if an infinite amount of heat exchanger surface is provided
c) work must be done in order to push the fluids thru the heat exchanger, generally as a frictional pressure dropd) when mating heat transfer processes are considered together, the net change in entropy must be positive
e) there is conservation of energy, as stated by the first law of thermo, and that the various forms of energy such as work, potential energy,kinetic energy, chemical energy, and heat can be considerd as equivalent in terms of energy content.
f) some forms of energy have a higher potential to do work, sometimes called the "exergy" of a fluid.

 

[Tobalcane]

25362

Where did you learn all of these concepts?

Go Mechanical Engineering
Tobalcane

 

[pmover]

tobalcane,

theoretical concepts are taught in an academic institution of higher learning. practical or "real-world" applications are learned from seasoned, forthwright, knowledgable, and competent individuals in the business of designing and constructing heat exchangers to meet the design process conditions. of course, there is some research and development done to improve heat transfer, but the fundamental equations do not really change.

hope this helps!
-pmover

 

[unclesyd]

Here is one very important point that all of the above left out. I don't think it's taught in school but ios derived from the school of hard knocks.

Do all the theoretical design work both thermodynamically and mechanical required by following all the necessary codes and heat exchanger design practices and standards.

After this is complete add 10% extra tubes. This takes care of many present and future problems.

 

[rmw]

While I have used Unclesyd's safety factor method to some extent or another, you should know that how it can affect your design.

First, some elements of your design already have added extra surface, which is either extra tubes, or longer tubes, and that is the fouling factor that you use. Fouling factor is nothing more than additional surface area to account for anticipated fouling.

Second, increasing surface area by adding tubes alone can change the fluid flow velocities inside the tubes, and can change the heat exchange characteristics of your heat exchanger design. If it is not critical, it might not matter, however if yours is a velocity sensitive performance calculation, longer tubes instead of more tubes might be your preference for additional safety margin. Slower tubeside velocities can also promote settling out of contaminants if any. If your design is temperature sensitive, longer tubes could be going in the wrong direction.

Some Hx designers put a few (1-5) additional tubes just in case manufacturing problems produce an error that cannot be solved by any other means than by plugging tubes or tube holes. Some Hx's leave the manufacturer day one with plugged tubes without overall performance loss.

rmw

 

[gwolf]

Try "Heat Transfer" by Holman, I have found it very useful after a long period of absence from this field. It has a lot to say on heat exchangers and is quite good for bringing you back up to speed if you understood it once.

I'm sure that if you get this book you will feel a lot more confident.

Heat Transfer by J.P. Holman, McGraw-Hill, ISBN 0-07-Y66459-5

 

[SamOntario]

If you are considering Plate Type Heat Exchanger, you may like to take a look this thread:

http://www.eng-tips.com/viewthread.cfm?qid=118735&page=1

The thread did not give any specific answers but could contribute to your recall of memory:

Heat transferred Q = U x A x LMTD
where U = Overall Heat Transfer Coefficient(HTC)
A= surface area
LMTD= log mean temperature difference

If both hot side and cold side happened to be liquids, Q also equals Hot side heat transfer or cold side heat transfer = U x A x LMTD = Mc x CPc x dTc = Mh x CPh x dTc

 

[SamOntario]

The equation put into words: cold side Mass flow x Specific energy x temperature difference of cold side liquid = hot side mass flow x specific engy of hot side liquid x temperature difference of hot side liquid.