# How to Design a Heat Exchanger Without Knowing Tube and Shell Diameters

• pxp004
In summary, the conversation discusses the process of designing a heat exchanger with 100% efficiency. The overall heat transfer coefficient (U) is dependent on transfer coefficients on the shell and tube side, and the resistance of the pipe walls. To calculate these coefficients, the Reynolds number of the pipes and shell is needed, which is based on their diameters. Without knowing the specifications of the tubes and shell, the designer must make assumptions about the construction of the heat exchanger. This can be done by using flow rates, temperature data, and thermal properties of the fluids, along with some rules of thumb about maximum velocities inside a tube. The number of tubes required can then be calculated, and the design can be adjusted until it can handle the
pxp004
Lets assume an exchanger that is 100% efficient (no heat loss). Duty (Q) and LMTD is known. Your overall heat transfer coefficient (U) is dependent on your transfer coefficients on the shell and tube side, and the resistance of the pipe walls (we're going to ignore entrance effects and fouling) . In order to calculate the transfer coefficients for the tube and shell sides, you need your Reynolds number in order to calculate Nusselt Number. The Reynolds Number is based on the diameter of your pipes and shell. So from a design perspective, if

Q = U*A*LMTD​
How would you calculate the surface area needed without knowing the specs (diameters) of your tubes and shell?
And without any software such as ASPEN, HTRI, etc. The only information I have available are flowrates, temperature data, and thermal properties of the fluids.

As a general rule, the designer must make some assumptions about the construction of the HE in order to proceed. In other words, you can sometimes use the flow rate of the coolant along with some rules of thumb about max. velocities inside a tube to come up with an estimate of the tube ID. There are only so many tube IDs and thickness schedules to choose from.

From that information, you can calculate how many tubes are required to handle the total flow of the coolant, and then calculate the other parameters of the HE from that. Check that the HE design can produce the duty. If it can't, then you start over with different assumptions. Rinse and repeat until everything converges.

Engineers used to do this without software, but it took a while longer for things to converge into a reasonable design. To speed things along, design charts for some of this effort were prepared and used.

## 1. What is a heat exchanger?

A heat exchanger is a device that is used to transfer heat between two fluids. It is commonly used in industrial processes and heating and cooling systems.

## 2. Why is it important to properly size a heat exchanger?

The size of a heat exchanger directly affects its efficiency and performance. If it is too small, it may not be able to transfer enough heat, while if it is too large, it may be unnecessarily expensive and take up too much space.

## 3. How do you determine the appropriate size for a heat exchanger?

The size of a heat exchanger is determined by several factors, including the flow rate of the fluids, the temperature difference between the two fluids, and the desired heat transfer rate. This can be calculated using equations and software programs specifically designed for heat exchanger sizing.

## 4. What are the most common types of heat exchangers?

The most common types of heat exchangers include shell and tube, plate and frame, and finned tube heat exchangers. Each type has its own advantages and is suitable for different applications.

## 5. Can a heat exchanger be oversized?

Yes, a heat exchanger can be oversized, which means it is larger than necessary for the required heat transfer. This can lead to higher costs and reduced efficiency. It is important to carefully calculate and select the appropriate size for a heat exchanger to avoid oversizing.

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