https://www.sciencedirect.com/search?qs=design%20an%20helical%20heat%20exchanger&show=25&sortBy=relevance

Md Washim Akram thank you for your quick reply. I am giving a look at the file. For what concerns the file "picnnez2016", I think this is not completely on what is my design based on. The geometry that I aiming for is shown in the picture called "Helix". Anyway, I will finish to read the other two files and let you know.

Moreover, do you know some law/relation that I could apply for the fluid outside the pipe of the helix in case of natural convection? Something related to the Nusselt number and the Rayleigh number.

I assume both fluids are single phase. If so, the friction and heat transfer coefficent for the inner tube fluid is well established, both from numerical analysis, and by experiments. Good references on this are e.g. Kakac/Shah/Aung: Handbook of Single-Phase Convective Heat transfer, or Hewitt et al: Heat Exchanger Design Handbook, vol. 2. Note that for laminar flow, the heat transfer and friction coefficient are significantly enhanced due to the secondary motion caused by the centrifugal effects. Also, the transition to turbulence is delayed.

Concerning the annular flow, there is almost no information available. There are some scattered experimental data (S. Garimella, E. Næss) published, but not sufficient to support generalized correlations. The secondary motion effects are significant in the laminar region; however the transition to turbulence and the enhancement in the turbulent region is questionable. My recommendation in the turbulent region is to assume a straight annular pipe and calculate accordingly.

Assuming you are able to establish heat transfer and pressure drop values, the design process is straight forward using e.g. LMTD- or NTU-methods, which are explained in most textbooks on heat transfer (Holman, Incropera et al, Cengel,....)

Good luck!

Erling Næss thank you for your reply. Yes, both fluids are in single phase and both fluids are water. The fluid behavior inside the tube should be turbulent and especially fully developed turbulent. For what concerns, the value of the heat transfer I am working on it. For the pressure drop, I still do not know. Moreover, if I will be able to compute this two unknowns, I am not sure that then the design will be straightforward. In the book, I did not find the LMTD method or the NTU-methods for the helical heat exchanger. I found these methods for shell and tube or other heat exchangers. I am not sure that I could apply this even for the helical heat exchanger. Do you think that I could apply these methods even for this type of heat exchanger?

I am in doubt about this, because in this problem I have that the pump moves only the hot fluid. The cold fluid is at "rest" in the tank. Well, technically it moves according to the buoyancy force if there is a difference in temperature from the top to the bottom of the tank. Even because, if there is no difference, the delta T logarithmic will be 0.

To give you a better idea of the problem, I attached a photo. The fluid is single phase and it is liquid water for both sides, the tank and the heat exchanger. The input parameters are: T1, d tube, D coil, inlet velocity, Pitch and T bath. I do not know the temperature of the wall of the tube (inside and outside) and I want to know the outlet temperature of the coil (T2), the overall heat transfer (U) coefficient and the amount of heat transfer rate (Q). Do you know I could I design such type of heat exchanger and calculate all the unknowns?

Yes, this type of heat exchanger can be calculated, and the parameters you seek (T2 and Q) can be solved.

Firstly, the LMTD Method is not restricted to the type of heat exchanger at hand; it relates to the flow configuration. In your case, assuming for a moment that the Cold fluid temperature is constant in the tank, we have LMTD=(T1-T2)/ln[(T1-Tbath)/(T2-Tbath)]. The heat transfer coefficient for the hot fluid inside the coil can be determined from suitable correlations for helically coiled tubes, as I indicated in my earlier reply. The cold shellside fluid is calculated assuming free-convection flow (hot fluid in the boundary layer rising due to buoyancy). Correlations may be found in many basic heat transfer textbooks. (If a correlation for several tubes stacked above each other is not found; assume something between a single tube and a vertical wall stretching the entire height of the coil.)

In off-design mode, this becomes an iterative solution since T2 is not known in LMTD. Therefore, for such applications I recommend using the NTU-effectiveness method instead of LMTD; it can solve explicitly for T2 (also found in most textbooks on heat transfer).

Erling Næss thank you again for your very quick reply. Even the NTU-effectiveness method is not restricted to the type of heat exchanger, right?

In this way, I could use the relation for the counter flow or parallel flow (then the choice is up to the most efficient way and I guess is the counterflow).

However, I still have a doubt. The relation that I will apply, is for a general counter (or parallel) flow. Probably the formula that correlates the effectiveness with the NTU for this type of heat exchanger, is a bit different. Do you think that the committed error using this formula is small or negligible?

Thank you again for all your reply.

If you make the assumption that the cold storage temperature is uniform, there is no difference between countercurrent and cocurrent flow. The efficiency becomes E=1-exp(-NTU)

Look at VDI- Wärmeatlas or Heat Atlas

Erling Næss if I apply the NTU-effectiveness method I do not even need to calculate hi with all the relations found in the literature. I can just take an assumption on T2 and then iterate until the convergence is reached. What do you think about this?

Wilfried Roetzel thank you for your answer. I am looking at it. In the method that is described, I need even the temperature of the wall inside and outside the tube. Now I have to see if through an iterative process I am able to compute those.

If hi>>ho, you may ignore hi. Otherwise, you must include its impact on the overall heat transfer coefficient (U). Remember, NTU=U.A/(m.cp), and it is the external fluid temperature you assume constant (not the wall temperature).

In off-design (rating) mode, the NTU Method is non-iterative; in design mode the LMTD Method is non-iterative.

The pitch has an important influence if it is too small then parts of the tube do not have a good condition for fluid flow and thus a lower heat transfer.

Since the convective layer temperature increases toward the top you should have the internal flow from top to bottom for maximal efficiency (counter current).

Erling Næss yes, for the calculation that I made hi>>ho.

Anyway, why is the NTU Method non-iterative in off-design (rating) mode? If I made some hypothesis to calculate hi and I want to refine the value founded, should not I iterate and see when its values stabilize itself?

Concerning counterflow and parallel flow with non-uniform overall heat transfer coefficients: One should arrange the flow direction in such a way that the product of the local overall heat transfer coefficient and the local temperature difference becomes as uniform as possible. For more accurate calculations the two- or three-point method is recommendet. See Heat Atlas or VDI-Wärmeatlas.

Wilfried Roetzel Sorry you for the late reply. Thank you for your advice. I will do it.

The signalled paper concerns to a specific application of the helical-coil heat exchanger ― coil pipe inserted between concentric cylinders: R.K. Patil, B.W. Shende, P.K. Ghosh, "Designing a helical-coil heat exchanger", Chemical Engineering, December 13, 1982, 85-88.

Carlos Araújo Queiroz Thank you for your reply. I solved it and I don't need it anymore.

Hi Marcello Nitti, I am encountered with a similar problem of yours and the bath in your case is replaced by phase change material (PCM). I was wondering if you could share me the procedure you used to design the heat exchanger?

Thanks in advance!

Hello, Marcello, I am working on a similar problem only I am cooling gas with water and I know T1 and T2 and I have to calculate TBath. I am searching everywhere for this type of heat exchanger without success. Could you please share with me how did you solve this problem.

Thank you very much.

Hi Marcello Nitti , I am Veena and I am currently developing a mini powerplant as a part of my project. As a part of it, I am involved in designing a helical coil heat exchanger. I am encountered with a similar problem of yours. I have aware of the pitch, diameter of the tube, the diameter of the coil, the velocity of fluid inside the coil, mass flow rate, T inlet of the coil and the T of the secondary fluid. I want to calculate Q, T out and the overall heat transfer coefficient. I was wondering if you could share with me the procedure you used to design the heat exchanger?  Thanks in advance!