Heat Exchangers - Science topic

Several types of heat exchangers are commonly used in industrial settings, each with its own advantages and best use cases. Here are some of the most common types:

Plate Heat Exchangers: These are becoming preferred due to better heat transfer, easier maintenance and cleaning, modularity, and compactness. They are more efficient than shell and tube heat exchangers in many industrial and most HVAC applications.

The best type of heat exchanger for industrial use depends heavily on the particular process in which the heat exchanger is installed. Factors such as the type of fluids being used, the desired temperature change, the pressure, and the flow rate will all impact which type of heat exchanger is most suitable.

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Here's a general approach to estimating the fluid velocity without knowing the surface area:

1. Determine the flow rate: If you know the volumetric flow rate (Q) or mass flow rate (m) of the fluid passing through the exchanger, you can use this information in the calculation.

2. Estimate the cross-sectional area: If you can approximate the shape of the flow channel in the exchanger, you can estimate the cross-sectional area (A) based on that assumption. For example, if the flow channel is rectangular, you can estimate the area using the width and height of the channel. If the channel is circular, you can estimate the area using the radius or diameter of the channel.

3. Calculate the fluid velocity: Once you have an estimated cross-sectional area, you can calculate the fluid velocity (V) using the equation V = Q / A, where Q is the flow rate and A is the cross-sectional area.

It's important to note that this method provides an estimation and may not be as accurate as directly measuring the surface area of the exchanger. Additionally, the actual flow path and configuration of the exchanger may impact the fluid velocity distribution, so it's recommended to consult specific design guidelines or seek expert advice if possible.

Using magnetic waves instead of steam in a heat exchanger has several advantages and disadvantages:

Advantages:

Disadvantages:

It’s important to note that these are general points, and the specific advantages and disadvantages can vary depending on the application and system design.

I think this definition has some problem! the onlt parameter which can be considerd is entropy generation which should be positive, other things are only a difinition! not law!

air operated heat exchangers have air as the heat transfer fluid. For high performance of the hex material of high or reasonable thermal conductivity is preferred, e.g copper, aluminum, steel, stainless steel. An example is the domestic filament heater.

for water operated hex, water is the htf and the material similarly should be ideal for maximum heat transfer i.e., high thermal conductivity. In addition should be reasonably corrosion resistant. materials can be copper, steel, stainless steel. an example can be an economizer.

a steam boiler is a good example of a combination of both. The boiler tubes have hot gases as the htf and the shell side has water as the htf. here relatively affordable material with reasonable thermal conductivity and with an ability to withstand high pressure is required so schedule 40 steel can be used.

The mechanical energy produced by a Stirling engine using hot water at around 350K depends on factors like engine efficiency, temperature difference, and design. Generally, small engines might produce a few watts, while larger ones can generate several kilowatts or more. Specific values vary based on these factors.

The Rayleigh number (Ra) is a dimensionless number used to predict the flow regime (conduction, convection, or mixed) in a fluid when it is heated from below. In the context of a phase change material (PCM)-heatsink system with fins, the characteristic length is an important parameter for calculating the Rayleigh number.

The characteristic length (L) used in the Rayleigh number calculation can vary depending on the geometry of the system. In the case of a PCM-heatsink with fins, the characteristic length can be defined based on the specific geometry you are dealing with. Here are a few possibilities:

Remember that the choice of characteristic length depends on the dominant heat transfer mechanism in your specific setup. The key is to select a length scale that is relevant to the phenomenon you are trying to analyze.

Additionally, when dealing with PCM systems, keep in mind that the melting and solidification of the PCM can introduce additional complexity to the heat transfer process. You might need to consider the effects of latent heat and phase change in your analysis.

Before performing calculations, ensure that the physical properties of the fluid, PCM, and the geometry are accurately determined. It's recommended to consult relevant literature, research articles, or textbooks in the field of heat transfer to find appropriate values and guidance for your specific configuration.

Surendra Singh Rathore Surendra Singh Rathore sir thank you for reply, I tried with all algorithms.it is working only in coupled. In remaining all algorithms, getting the same result only difference is the number of iterations it is stuck(No response why ? ). I tried relaxation factor in all quantities as u said. Now am looking for other alternatives in coupled algorithm. because in this algorithm alteast iteration process is running.

Thanks

T.Saravanakumar

Muhammad Imran , Khaled Hossin , Shazia Farman Ali , I agree with you about the existing research gap in having the correlations for Zeotropic mixtures in the case of Plate type HX. I was also facing a similar issue. In my case, the intended fluid is CO2; and I didn't find any heat transfer coefficient as well as pressure drop correlation in the case of CO2 evaporation inside a plate type HX. However, there are numerous existing correlations for the in-tube flow boiling as well as condensation. I am hereby providing one reference for a unified correlation in the case of in-tube flow boiling inside a mini/micro/conventional channel. This correlation generally accounts the "hydraulic diameter" concept in a tube flow. As there is no existing correlation for CO2 and zeotropic mixtures in the case of flow boiling inside a plate HX, could this "hydraulic diameter" concept for mini/micro/conventional channel be a starting point for estimating the HTC and PD for flow inside a plate HX?

[1] Shah, M. M. (2017). Unified correlation for heat transfer during boiling in plain mini/micro and conventional channels. international journal of refrigeration, 74, 606-626.

Hi,

I guess you could chose one of the two fluid inter solvers. They are based on Volume of Fluid and easy to apply. But you should really take care about cell resolution for good results of course. To take account for natural convection just apply boussinesq or polynomial thermophysicalProps. For polynomials you need to fit function coefficients according to your const. Pressure and temperature range. Unfortunately it can happen, that property calculation is not that precise.

On the other hand it is possible to chose euler solvers instead of inter solvers of course. But in my opinion you need really deep knowledge about all those empirical coefficients.

Hope that helps.

Regards David

To formulate of this model you need to consider the amount of information included in it:

1.

2.

Thank you very much for your valuable feedback. I agree with you on the points stated above.

The Reynolds number UD/nu=2300 is based on a smooth circular pipe of diameter D, with STEADY mean flow velocity U and fluid of kinematic viscosity nu. Below this critical Reynolds number any perturbation due to an obstacle will not cause persistent turbulence to occur far downstream of the obstacle. Of course locally the wake of a blunt body placed in the pipe can be turbulent, but soon the flow will relaminarize if we travel further downstream. Above Re=2300 the flow does not need to be turbulent. It can be turbulent if there is a sufficiently large initial upstream perturbation. In principle the flow can remain laminar if the inlet is very smooth and care is taken to avoid vibrations. Experimentally fully developed laminar pipe flows have been achieved for Re=500. 000. It is important to realize that this critical Reynolds number does depend on the geometry of the cross-section of the channel. For a rectangular channel of height h and width w >>h, usually one considers a Reynolds number Re=Uh/nu based on the channel heigth. The critical Reynolds number for allowing turbulence is around Re=hU/nu=1100. There is however much less literature on flows through slit shaped channels than circular pipes. If you consider an open channel flow, clearly the critical Reynolds number will be quite different from Re=2300 and of course it does depend on the length scale used in the definition of this Reynolds number!

explain your idea

A counter current heat exchanger is a type of heat exchanger used to transfer heat between two fluids that flow in opposite directions, creating a counter current flow. The two fluids flow through separate channels separated by a thin, thermally conductive wall, which allows heat to transfer between the two fluids without mixing them. In this type of heat exchanger, the hot fluid enters at one end and flows in the opposite direction of the cooler fluid, which enters at the other end. As the fluids flow in opposite directions, heat is transferred from the hot fluid to the cooler fluid, resulting in a more efficient heat exchange process compared to other types of heat exchangers. This design ensures that the temperature difference between the two fluids remains high throughout the exchanger, allowing for maximum heat transfer. Counter current heat exchangers are commonly used in a variety of applications, including HVAC systems, chemical processing, and power generation.

I appreciate your enthusiasm Abdul Wahab. To converge the tear streams, we have to give correct appropriate initialization. This initialization you can find by breaking the tear open and doing trial and error analysis until the tear values are almost equal. If attaining this equality is difficult, then we can export the steady state with the tear open and run it in the dynamics by joining the tear. I hope it is understandable! For more information, you can have a look at the paper I attached.

Another reference text is the excellent

Fouling of Heat Exchangers - T.R. Bott

Web13 Apr 1995 · This unique and comprehensive text considers all aspects of heat exchanger fouling from the basic science of how surfaces become fouled to very practical ways of …

I would note that fouling is a massive cost factor in many industries.

yes by using ASPEN EDR

K= Q.Δx/A.ΔT

The transient hot wire method (THW) is a very popular, accurate, and precise technique to measure the thermal conductivity of gases, liquids, solids, nanofluids (hybrid Fluids), and refrigerants in a wide temperature and pressure range

Measure the heat transfer. Vary the inputs. Switch to water on the outside and air on the inside. Use Reynolds Analogy, St=f/2. Kays & London is an excellent reference. So is TEMA and HEI and various ASME documents.

J-B weld original would be the best option since you're talking about the plastic part of your radiator: https://www.amazon.com/J-B-Weld-8265S-Cold-Weld-Reinforced/dp/B0006O1ICE

It can glue everything. I used it to glue acrylic to aluminium and acrylic to polyethylene to create pipe-flange leak-free joints. My acrylic-aluminium joint holds 0.35 bar gauge pressure of water and the acrylic-polyethylene holds about 0.15 bar gauge of water.

Before going with J-B weld original, I tried plastic specific J-B weld epoxies. None of the worked. Only J-B weld original saved me.

Hello Nekin Joshua ,

You can find the answer to your question on the Internet by simply typing the phrase "do electric vehicles have radiators" into the Google search field. One of the most important EV components that needs to have its temperature regulated is the battery. If the battery overheats, it can catch fire or explode. Most EV batteries are liquid cooled with the heat dissipated by a radiator.

Regards,

Tom Cuff

May I recommend you to use ULTEM or PEI filament (Polyetherimide); It is similar to PEEK but better in thermal resistance.

Are you looking for a laboratory that would perform tests on a heat exchanger to determine fouling? That would be extremely expensive and entirely impractical unless you had some critical motivation and funding. Are you looking for experimental studies on heat exchanger fouling that have already been performed? There are many in the literature. Are you looking for codes, standards, and methods? There also many of these (see HEI, TEMA, EPRI, ASME, ASHRAE, AIChE, and many more).

N = L1.L2/P1.P2, N(number of tubes), L1 (total length of flow normal to tube back), L2 (length of the no-flow side), P1 (distance between the center of one tube to the next in each column of tubes i.e., vertical separation distances), P2 (horizontal separation distance between two adjacent tubes)

see text: Heat Exchanger Design Handbook by T. Kuppan Chapter 4 section 3.2, pp 178-179

Prof. Udit Singh

It will depend on the physics that the exchanger has, and it comes from a difficult subject, physical kinetics.

Best Regards.

Maybe this video can be helpful for you since your info is somewhat limited :)

I hope you manage the modelling!

With the given shell and tube type heat exchanger, determine the maximum pressure tube can sustain with the hoop stress criteria. Then corresponding to that temperature, choose any fluid whose saturation pressure is above maximum pressure, so that no phase change cannot occur. Regarding shell side, generally ambient pressure fluid is supplied.

What are the temperatures and shapes of the surfaces to be insulated?

I think the temperature is not greater than 250 degree Celcius. You can use glass wool for high temperatures. For temperature up to 60 degree Celcius, you may use Thermocole but Thermocole can be put easily only on flat surface or outside the pipes or cylindrical surface if pipe section of Thermcole is available.

Plate heat exchangers are often used when the cooling fluid isn't clean because a plate heat exchanger can be dismantled and cleaned fairly easily, much more so than one that requires cutting and welding. Several things have been tried, including: rinsing, periodically reversing the flow, pulsing the flow, and introducing abrasive particles. What about acoustic stimulation to discourage accumulation of stuff on the surface; that is, keep the crud suspended and perhaps filter it out. You could also consider rather than a filter, running part of the stream through a centrifugal device to remove sludge without interrupting service. Go out and see some of these in action. One place you will find plate heat exchangers is a stationary combustion gas turbine. I've seen a variety of these at power plants. Get someone to take you on a *real* tour of a plant--not the "visitors" tour but the "maintenance" tour.

Chemical exergy fraction = 1.0401 + 0.1728 H/C + 0.0432 O/C + 0.2169 (S/C) (1 − 2.0628 H/C )

Thanks Simon. I have tried the solution in the link but didn't work in my case. WIll try to ave it as 3mf and see if it works on Ansys.

Thanks anyway.

2. Fins are never used on the surface in contact with a fluid undergoing phase change.

The fitting can be selected based on its conductivity

Recursive vs. one-step calculations are of little concern with current computer resources and readily available software. The bigger issue arises from the fact that these two methods solve the same separable differential equation, but using different assumptions when separating the variables before integrating. This means that the two different methods handle some things differently, like implicitly/explicitly averaging properties, heat transfer coefficients, etc. You should consider EPRI TR-107397 (Service Water Heat Exchanger Testing Guidelines). The principal author of 1997 version is Jeffrey Rabensteine of Power Generation Technologies, here in Knoxville. The 2015 version was updated by Lindon C. Thomas, who is a friend and also lives in Knoxville. Dr. Thomas is a renown authority on the P-NTU method and wrote a textbook covering the subject https://www.amazon.com/Lindon-C-Thomas/e/B001HOVPI4

Are you talking about pinch points and heat release diagrams? You want to avoid pinch points. Here's a typical HRSG heat release diagram.

Check https://www.sciencedirect.com/topics/engineering/cooling-coil

Hi Hannah

The heater and cooler block are meant as shortcuts when we know what a heat exchanger must do but we don't want to model in detail. If you want to size the exchanger you will have to convert it into a shell and tube or air called etc, and then use either the Hysys calculations or EDR to calculate the size.

Regards

Kevin

thank you all for your suggestions.

Luigi Candido yes the model is in 3D and converges in steady state..

I will try to do what you have suggested and see the result.

Try to solve using different solver and you can also change meshing type

Dear Khalid! yes you may able to use such radiators, however, consider thermal efficiency (as it might ought to loss). Perhaps, Varying the parameters to optimal state could help!

Simulation software is very helpful in a lot of ways. Firstly, it allows you to develop a better idea about the big picture of your project without having to run too many tests. Sometimes the simulation results can be more accurate compared to the experimental results, since there are more factors that can affect the uncertainty of the trials, operating errors, systematic errors and random errors, etc. I believe a lot of consulting engineers use the simulation software, such as fluent, starccm+, comsol in their research. The simulation result can serve as a strong indicator for the researchers when they scale up their reactors. In terms of the demanding situation for this type of skills, yes, as far as I know, people who are skillful in this area is highly competitive.

Hi Dr Tarikayehu Amanuel . I agree with Dr Paul Gateau .

I presume you are cooling moist air using a coolant (water) having a lower temperature than the moist air dew point temperature. Depending on the level of detail required, you may either

a) assume thorough mixing of the condensate formed and the air flow, i.e. assuming equilibrium condition, or

b) take into account the fact that moisture might be condensed 'independently' from the air cooling, i.e having non-equilibrium flow.

Case a) is the simplest, and the gas side heat transfer coefficient might be established using the method of Silver or Bell/Ghaly. Of course you will need to integrate along the heat exchanger, as the gas side heat transfer coefficient will tend to vary in the flow direction.

Case b) is more complex, however, any 'film model' (e.g. the model of Colburn/Hougen) can be applied. Note also that the heat transfer coefficient along the height of the fin might vary significantly (due to the vapor condensation rate), i.e. the traditional calculation of fin efficiency becomes void.

Case a) will most likely provide erratic air exit temperature and condensation rates, but for some mysterious reason the heat duty might not be that far off. Case b) will be more correct, and will most likely result in more condensate formed but less air cooling than case a). Which method ends up being correct will depend on the degree of mixing between the condensate and air.

As far as design methodology is concerned, it will be a trial-and error procedure, as all heat exchanger designs are.

Dear Dr. Thitiwut Srichalongrat ,

perhaps it would be possible to make a resistance measurement with a simple digital tester, keeping one tip on a conductive part and moving the other by placing it on the parts to be investigated ....

Simple but it could work.

My best regards, Pierluigi Traverso.

It would really depend on the cooling targets but heat pipe based thermal management provide an answer to the reliability requirements needed for aircraft.

Hi, usually the point of the phase change has be introduced by yourself as an input parameter. You can't estimate it through the simulation.

Hi,

It can be a numerical problem. You can try to increase the number of cycles for the convergence from the different setups for the convergence; after you have to reinitialize your simulation and so you have to run.

bye

Looks like a simple electric resistance heater in a tube would do this low duty. An 8kW element looks about right but needs to be suitable for high temperature and the gases used, so probably stainless steel casing and tube needed.

I recommend that you get this excellent book. It will be very helpful. Well, this book develops several exercises similar to the work that you have to do. This book was very useful to me.

Bergman, T. L., Incropera, F. P., DeWitt, D. P., & Lavine, A. S. (2011). Fundamentals of heat and mass transfer. John Wiley & Sons.

There are a number of methods for measuring the heat of an exothermic reaction. But in any case, not so simple equipment is needed. One option is to use a bomb calorimeter (see https://en.wikipedia.org/wiki/Calorimeter). An example can be found here https://arc.aiaa.org/doi/abs/10.2514/3.25795?journalCode=jsr.

Another option is to find a laboratory that performs thermogravimetric analysis (TGA, see, for example, https://link.springer.com/chapter/10.1007/978-3-030-11599-9_7) or differential scanning calorimetry (DSC). From the processing of these data, one can obtain the heat of reaction. An example can be found here Kong Y., Hay J. N. The measurement of the crystallinity of polymers by DSC //Polymer. 2002. V. 43. Iss. 14. Pp. 3873-3878 or here Faleeva J. M. et al. Exothermic effect during torrefaction //Journal of Physics: Conference Series. IOP Publishing, 2018. V. 946. Iss. 1. Art. # 012033; Zaichenko V. M. et al. Thermal effects during biomass torrefaction //Solid Fuel Chemistry. 2020. V. 54. Iss. 4. pp. 228-231.

You can evaluate the Nu number as the integral of the local Nu numer of your pipes surfaces. The last one can be easily evaluated with COMSOL since it is proportional to dT/dn|A where n is the normal vector and A is the surface of the pipe. The local Nu number depends also on the bulk temperature which is can be evaluated by integrating u*T and dividing by the integral of u*A.

Good luck

Hi Md Wasi Uddin

I think this article might helpful for your research:

Wish you all the best luck!

KHOA

In compact heat exchanger, when the heat transfer coefficient is needed locally, and we have numerical results we can determine the temperature difference between the wall, and the bulk temperature above it. The bulk temperature is obtained considering the mixing cup of fluid in each section. The bulk temperature could be obtained as a function of length and a polinomial curve can be fitted for the heat exchanger. It's a additional work but the results are goods at least in my case.

Intrested

Seemingly of some possible interest for your query: https://www.researchgate.net/post/What_is_the_solution_to_dissolve_the_scale_of_CaSO4_and_MgSO4_from_RO_membrane_surface

I think it may be due to BCs. Can you please your problem in detail, so that I may help you in diverg. Issue

Hi Parag Patil

Have you noticed that my linearization is incorrect, because I took your C = 1, when C = –NTU (Number of Transfer Units) and NTU > 0? I'm not a Chemical Engineer, but I rechecked the info on MathWorks and Wiki; the effectiveness (ε) of a heat exchanger is bounded: 0 < ε < 1. The X, or the heat capacity ratio, Cr = Cmin/Cmax. Since both Cmin > 0 and Cmax > 0, then Cr is also bounded 0 < Cr < 1. Can you verify if the above figure is the surface plot of the E-NTU function? I tend to agree with Prof. José Arzola-Ruiz that linearization is not necessary.

Mauricio Aguilar Cardenas Were you finally able to model rarefied flow in Star-CCM+?

Thanks for your help @ Luigi Candido

Ashkan Ghafari Okay makes sense..

Yea i decided to run each case separately and import the design points to the DoE, may be inconvenient but I’m running out of time to come up with a more practical approach

If it is isothermal there will be no temperature change. I assume that the tubes are adding (or removing ) heat to control the shell side temperature at isothermal conditions.

You can do it by

1. Split the laminar and turbulent regions manually by using the BC panel.

2. Use a transition model

for more help, you can used this link where a similar question has been asked

I do not think you needed to remove the heater instead you keep the two but instead of eliminating one, you can introduce a controller (using logic blocks) to either adjust the heating role played by the heater when once the product stream is ready for the heat exchanger or to completely switch off the heater. Zeeshan Uddin

Hi Ahmed Usaama Ifthikhar,

What particular type of HX you have Designed?? There are many options for air-to-air HX with a variety of arrangements, fins, geometries, etc. You will have to specify first.

however, Compact HX (by A. Louis London, W. Kays) is a very comprehensive book for air-to-air HX.

regards,

Ahmad

A vertical shell and tube heat exchanger in which the tubes extend through oversized holes in a liquid distribution plate. Liquid flows through the holes and down each tube exterior surface as a falling film. More details here:

Hi Kruger Eren

Please refer to section 18.5.1

Try contacting authors, it may resolve your issue.

I would prefer to mesh the solid and use wall thickness and material conductivity.

In a heat exchanger, considering the conduction and convective heat transfer is enough to do a good calculation.

dear Kruger Eren

maybe you can adapt it out of some literature research... so, you have an additional parameter for your case study.

Imagine a heat exchanger made of spiral finned tube shown in the file. It consists of 4 rows of tubes diverted parallel to the air flow. In the experiments I measured the pressure drop of the air passing through the closed rectangular channel. I modeled this in COMSOL. When I take the difference in the mean pressures at the inlet and outlet in the Comsol Model, it comes out much lower than the experimental results. But when I take the difference of the maximum and minimum pressures in the Comsol model, the results are very close to the experimental results. Is the second method acceptable for finding the pressure drop in numerical models? I wanted to ask this. The problem here is that I accepted the fins as thin wall, because my computer was insufficient. There is no problem in heat transfer, but it is very low in pressure calculation because of this acceptance.

Q=m Cp delta T