Heat exchanger calculation example

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# Heat exchanger calculation example

The conduction calculator deals with the type of heat transfer between substances that are in direct contact with each other. Heat exchange by conduction can be utilized to show heat loss through a barrier.

For a wall of steady thickness, the rate of heat loss is given by:. Thermtest is furnishing this item "as is". Thermtest does not provide any warranty of the item whatsoever, whether express, implied, or statutory, including, but not limited to, any warranty of merchantability or fitness for a particular purpose or any warranty that the contents of the item will be error-free.

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Do you like this page? How can we make this page better for you? Wikipedia — Heat Transfer. The heat transfer conduction calculator below is simple to use.

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Further Reading Wikipedia — Heat Transfer. Pin It on Pinterest. Select from database.Calculations of Heat Transfer. Conservation of energy theorem is also applied to heat transfer. In an isolated system, given heat is always equal to taken heat or heat change in the system is equal to zero. If two objects having different temperatures are in contact, heat transfer starts between them.

The amount of heat given is equal to the amount of heat taken. Object one has mass m 1temperature t 1 and specific heat capacity c 1object two has mass m 2temperature t 2 and specific heat capacity c 2.

If the mass of the block is 1,2kg, calculate the heat lost by the block. Example: The graph given below shows the relation between given heat and change in the temperatures of the three matters having same masses. Compare the specific heat capacities of these matters. Since the masses of these matters are equal, B has the greatest specific heat capacity because, with the same amount of heat, change in the temperature of the B is lower than the other two matters. Moreover, A has the minimum specific heat capacity, because the change in its temperature with the same amount of heat is larger than the others.

Finally, specific heat capacity of the C is between A and B. Tags: calculations of heat transfer examples of heat transfer heat transfer samples. Additional Information.The convective heat transfer coefficient hdefines, in part, the heat transfer due to convection. The convective heat transfer coefficient is sometimes referred to as a film coefficient and represents the thermal resistance of a relatively stagnant layer of fluid between a heat transfer surface and the fluid medium.

Common units used to measure the convective heat transfer coefficient are:. Convection involves the transfer of heat by the motion and mixing of "macroscopic" portions of a fluid that is, the flow of a fluid past a solid boundary. The term natural convection is used if this motion and mixing is caused by density variations resulting from temperature differences within the fluid. The term forced convection is used if this motion and mixing is caused by an outside force, such as a pump.

The transfer of heat from a hot water radiator to a room is an example of heat transfer by natural convection. The transfer of heat from the surface of a heat exchanger to the bulk of a fluid being pumped through the heat exchanger is an example of forced convection. Heat transfer by convection varies from situation to situation upon the fluid flow conditionsand it is frequently coupled with the mode of fluid flow.

In practice, analysis of heat transfer by convection is treated empirically by direct observation. Convection heat transfer is treated empirically because of the factors that affect the stagnant film thickness:. Convection involves the transfer of heat between a surface at a given temperature T s and fluid at a bulk temperature T b. The exact definition of the bulk temperature T b varies depending on the details of the situation.

For flow adjacent to a hot or cold surface, T b is the temperature of the fluid "far" from the surface. For boiling or condensation, T b is the saturation temperature of the fluid. For flow in a pipe, T b is the average temperature measured at a particular crosssection of the pipe. The basic relationship for heat transfer by convection has the same form as that for heat transfer by conduction:.

The convective heat transfer coefficient h is dependent upon the physical properties of the fluid and the physical situation. Typically, the convective heat transfer coefficient for laminar flow is relatively low compared to the convective heat transfer coefficient for turbulent flow. This is due to turbulent flow having a thinner stagnant fluid film layer on the heat transfer surface.

Values of h have been measured and tabulated for the commonly encountered fluids and flow situations occurring during heat transfer by convection. A 22 foot uninsulated steam line crosses a room. The outer diameter of the steam line is 18 in.

Calculate the heat transfer rate from the pipe into the room if the room temperature is 72 o F. Many applications involving convective heat transfer take place within pipes, tubes, or some similar cylindrical device.

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In such circumstances, the surface area of heat transfer normally given in the convection equation varies as heat passes through the cylinder. In addition, the temperature difference existing between the inside and the outside of the pipe, as well as the temperature differences along the pipe, necessitates the use of some average temperature value in order to analyze the problem.

This average temperature difference is called the log mean temperature difference LMTDdescribed earlier. It is the temperature difference at one end of the heat exchanger minus the temperature difference at the other end of the heat exchanger, divided by the natural logarithm of the ratio of these two temperature differences.

The above definition for LMTD involves two important assumptions: 1 the fluid specific heats do not vary significantly with temperature, and 2 the convection heat transfer coefficients are relatively constant throughout the heat exchanger.Heat exchangers are commonly used in industry, and proper design of a heat exchanger depends on many variables.

Moreover, engineers also use the logarithmic mean temperature difference LMTD to determine the temperature driving force for heat transfer in heat exchangers. A heat exchanger typically involves two flowing fluids separated by a solid wall. Many of the heat transfer processes encountered in industry involve composite systems and even involve a combination of both conduction and convection. Heat is first transferred from the hot fluid to the wall by convection, through the wall by conduction, and from the wall to the cold fluid again by convection.

With these composite systems, it is often convenient to work with an overall heat transfer coefficientknown as a U-factor. The overall heat transfer coefficient, U, is related to the total thermal resistance and depends on the geometry of the problem.

For example, heat transfer in a steam generator involves convection from the bulk of the reactor coolant to the steam generator inner tube surface, conduction through the tube wall, and convection boiling from the outer tube surface to the secondary side fluid. In cases of combined heat transfer for a heat exchanger, there are two values for h.

### What is Example – Calculation of Heat Exchanger – Definition

There is the convective heat transfer coefficient h for the fluid film inside the tubes and a convective heat transfer coefficient for the fluid film outside the tubes.

In order to solve certain heat exchanger problems, engineers often use a log mean temperature difference LMTDwhich is used to determine the temperature driving force for heat transfer in heat exchangers.

LMTD is introduced due to the fact, the temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. The heat transfer through the wall of heat exchanger at a given location is given by the following equation:. Here the value of overall heat transfer coefficient can be assumed as a constant. On the other hand the temperature difference continuously varies with location especially in counter-flow arrangement.

In order to determine the total heat flow, either the heat flow should be summed up using elemental areas and the temperature difference at the location or more conveniently engineers can average the value of temperature difference.

It can be seen from the figure that the temperature difference varies along the flow and the arithmetic average may not be the real average, therefore engineers use the logarithmic mean temperature difference. The larger the LMTD, the more heat is transferred. It can be seen from the figure that the temperature difference varies along the flow and the arithmetic average may not be the real average.

This holds both for parallel-flow arrangement, where the streams enter from the same end, and for counter-flow arrangement, where they enter from different ends. In a cross-flow, in which one system, usually the heat sink, has the same nominal temperature at all points on the heat transfer surface, a similar relation between exchanged heat and LMTD holds, but with a correction factor.

A correction factor is also required for other more complex geometries, such as a shell and tube exchanger with baffles. Steam generators and condensers are also examples of components found in nuclear facilities where the concept of LMTD is needed to address certain problems. When the subcooled water enters the steam generator, it must be heated up to its boiling point and then it must be evaporated.

Because evaporation is taking place at constant temperature, it cannot be used a single LMTD. In this case the heat exchanger has to be treated as a combination of two or three when superheat occurs heat exchangers. Pinch point is the location in heat exchanger where the temperature difference between hot and cold fluid is minimum at that location.

The log mean temperature difference LMTD method discussed in previous section is easy to use in heat exchanger analysis when the inlet and the outlet temperatures of the hot and cold fluids are known or can be determined from an energy balance. Therefore, the LMTD method is very suitable for determining the size and performance of a heat exchanger. This method is based on a dimensionless parameter called the heat transfer effectiveness, defined as:. As can be seen, the effectiveness is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate. To define the effectiveness of a heat exchanger, we must first determine the maximum possible heat transfer rate, q maxfor the heat exchanger. The specific heat of the oil is 2.

Calculate the logarithmic mean temperature difference.The specific heat of the oil is 2. Calculate the logarithmic mean temperature difference. Determine the area of this heat exchanger required for this performance.

## What is Heat Exchanger Analysis – Performance Calculation – Definition

To calculated the area of this heat exchanger, we have to calculate the heat flow rate using mass flow rate of oil and LMTD. The required area of this heat exchanger can be then directly calculated using general heat transfer equation:. Heat Exchangers. We hope, this article, Example — Calculation of Heat Exchangerhelps you.

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Design Heat Exchanger

Main purpose of this website is to help the public to learn some interesting and important information about thermal engineering. Example — Problem with solution. Calculation of Heat Exchanger. Consider a parallel-flow heat exchanger. Calculate the logarithmic mean temperature difference and determine the area. Thermal Engineering. Theodore L. Bergman, Adrienne S. Lavine, Frank P. ISBN: Heat and Mass Transfer. Yunus A. McGraw-Hill Education, May Nuclear and Reactor Physics: J.Heat exchangers are typically classified according to flow arrangement and type of construction.

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The simplest heat exchanger is one for which the hot and cold fluids move in the same or opposite directions in a concentric tube or double-pipe construction.

Figure The two configurations differ according to whether the fluid moving over the tubes is unmixed or mixed. In this case the fluid temperature varies with and.

Since the tube flow is unmixed, both fluids are unmixed in the finned exchanger, while one fluid is mixed and the other unmixed in the unfinned exchanger.

### Example – Calculation of Heat Exchanger

To develop the methodology for heat exchanger analysis and design, we look at the problem of heat transfer from a fluid inside a tube to another fluid outside.

It is useful to define an overall heat transfer coefficient per unit length as. We wish to know the temperature distribution along the tube and the amount of heat transferred.

For heatingthe heat flow from the pipe wall in a length is. From Next: Generalized Conduction and Previous: Thermodynamics and Propulsion.

The basic component of a heat exchanger can be viewed as a tube with one fluid running through it and another fluid flowing by on the outside. There are thus three heat transfer operations that need to be described: Convective heat transfer from fluid to the inner wall of the tube, Conductive heat transfer through the tube wall, and Convective heat transfer from the outer tube wall to the outside fluid.To approximate the results of a heat transfer system; enter the fluid data and enter 5 of the 6 available inputs under Flow Rates and Temperatures.

A box will be highlighted yellow if it needs input. The currently calculating value will always be highlighted in green. Your inlet temperature on the cold side needs to be less than your inlet temperature on the hot side. Your cold outlet temperature can't be hotter than the hot inlet temperature. Your outlet temperature must be hotter than your inlet temperature.

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Since fluid properties can vary by manufacturer and chemical composition, the properties stated may not be exactly representative of the actual conditions. This Heat Transfer Calculator is intended to be used to approximate a heat transfer system and is not intended to provide engineering recommendations. Every effort has been made to ensure the accuracy of the Heat Transfer Calculator on this site, however, since fluid properties vary by manufacturer, chemistry, composition and site conditions and we cannot make a guarantee or be held responsible for any errors that have been made.

If you spot an error on this site, we would be grateful if you could report it to us by using the contact link at the top of this page and we will review the data and correct it as soon as possible. Home Email. Technical Resources. Photo Gallery. About Us.  