• Engn_sam
In summary, the heat exchanger will be totally dependent on free convection, so the required mass flow rate will be calculated by estimating the overall heat transfer coefficient.
Engn_sam
I am trying to design a heat exchanger for a stratified hot water tank which means that the heat transfer on that side of the exchanger will be totally dependent on free convection. I plan on using a shell and tube set-up but I have no idea on how to calculate the required mass flow rate from the hot side since the mass flow from the cold side is supposed to be governed by free convection. If I could find a way to calculate the overall heat transfer coefficient in that situation, I would be able to calculate the required mass flow rate.
Normally, I would just set the mass flow rate and boundary temperatures (Tin and Tout) and use that to calculate the required heat transfer rate (Q) using
Q = mass flow * specific heat capacity * ΔT​
then use the Q and the boundary temperatures of the other side to calculate the mass flow rate of the other side.
I got lost after realising that the only quantities I can set are the inlet and out let temperatures from the side and the inlet temperature from the hot water source, Since the mass flow of the tank side is solely determined by free convection, I need a way to estimate the product of overall heat transfer coefficient that the temperature difference in the tank can sustain; the temperature difference determines the density differences so I assume it would also determine the buoyancy which determines, in conjunction with the heat capacity, the max heat rate that can be sustained.

thank you

Engn_sam said:
If I could find a way to calculate the overall heat transfer coefficient [U0] in that situation, I would be able to calculate the required mass flow rate.
This is very difficult. With water both inside and outside the tubes, a reasonable first guess for U0 is 1000 W/(m2K).

insightful said:
This is very difficult. With water both inside and outside the tubes, a reasonable first guess for U0 is 1000 W/(m2K).
Yh it is, there are already solutions out there but they are all terribly confusing and I was just hoping someone on here would know of some sort of relationship between the UA (U being overall heat transfer coefficient and A being heat transfer area) product to inlet and outlet temperature. That way I can assume the U and solve for the A of the heat exchanger while also using it to get the mass flow of the hot side.

## 1. What is the overall heat transfer coefficient for free convection exchanger?

The overall heat transfer coefficient for free convection exchanger is a measure of the rate of heat transfer between a fluid and a solid surface in the absence of forced fluid motion. It takes into account the thermal conductivity of the materials involved, as well as the properties of the fluid such as its density, viscosity, and temperature.

## 2. How is the overall heat transfer coefficient for free convection exchanger calculated?

The overall heat transfer coefficient for free convection exchanger can be calculated using the following equation:
U = hA/k, where U is the overall heat transfer coefficient, h is the convective heat transfer coefficient, A is the surface area, and k is the thermal conductivity of the materials.

## 3. What factors affect the overall heat transfer coefficient for free convection exchanger?

The overall heat transfer coefficient for free convection exchanger is affected by various factors such as the properties of the fluid, the surface area, the temperature difference between the fluid and surface, as well as any obstructions or obstacles that may affect the flow of the fluid.

## 4. How does the overall heat transfer coefficient for free convection exchanger differ from forced convection?

The overall heat transfer coefficient for free convection exchanger is calculated based on the natural convection of the fluid, while forced convection involves the use of external forces such as pumps or fans to increase the rate of heat transfer. Additionally, the overall heat transfer coefficient for free convection exchanger is typically lower than that of forced convection.

## 5. Why is the overall heat transfer coefficient for free convection exchanger important in engineering and industrial applications?

The overall heat transfer coefficient for free convection exchanger is important as it helps engineers and industrial professionals determine the efficiency of heat transfer in various systems. It is crucial in designing and optimizing heat exchangers, as well as in understanding the heat transfer processes in natural convection systems.

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