Design of Heat Exchanger

Group Assignment in Industrial Engineering

Submitted To: Dr. K.V.S.S. Narayana Rao, NTIE

Submitted By: Sachin Jaynt, Roll No.: 80, PGDIE-42, NITIE

                           Vaishali Gurjer, Roll No.: 100, PGDIE-42, NITIE

Engineering Principles applied in Designing of  Heat Exchanger

Ma nufacture

The basic princip les of plate fin heat exchanger manufacture are the same for all sizes and  all materials. The corrugations, side-bars, parting sheets and cap sheets are held together in a jig under a predefined load, placed in a furnace and brazed to form the plate fin heat exchanger b lock. The header tanks and nozzles are then welded to the block, taking care that the brazed joints remain intact during the welding  process. Differences arise in the manner in which the brazing process is carried out. The methods in common use are salt bath brazing and vacuum brazing. In the salt bath process, the stacked assembly is preheated in a furnace to about 550 C, and then d ipped into a bath of fused salt composed mainly of fluorides or chlorides of alkali metals. The mo lten salt works as both flux and heating agent,  maintaining the furnace at a uniform temperature. In case of heat  exchangers made of aluminium, the molten salt removes  grease  and  the  tenacious  layer  of  aluminium  oxide,  which  would otherwise  weaken  the  joints.  Brazing  takes  place  in  the  bath  when  the temperature  is raised above the melting point of the brazing alloy. The brazed block is cleansed of the residual solid ified salt by dissolving in water, and then thoroughly dried.

In the vacuum brazing process, no flux or separate pre-heating furnace

is required. The assembled block is heated to brazing temperature by rad iation from electric  heaters  and  by  conduction from the exposed surfaces into  the interior of the block. The absence of oxygen in the brazing  environment ensured by application of high vacuum (Pressure ≈ 10mbar). The composition of the residual gas  is  further  improved  (lower oxygen content) by alternate evacuation and filling with an inert gas as many times as experience dictates. No washing or drying of the brazed block is required. Many metals, such as aluminium, stainless steel, copper and nickel alloys can be brazed satisfactorily in a vacuum furnace.

Full control over the entire production process from material roll to finished

packages creates the best conditions for efficiently producing high-quality heat

exchanger plates. Here is the AP&T way of making your manufacturing process

more efficient.

Feed-in

The material is fed in on a coil or as cut blanks. We have the necessary

experience to adapt the equipment to create the best functionality – from

stainless steel coil and copper foil coil or with blank feeding.

Cutting

On brazed heat exchanger plates, some of the channel holes and contour are

cut prior to embossing. In some cases, it is better to cut after embossing the

plate pattern. AP&T has experience and processes for both variants.

Embossing

Embossing with the right surface within narrow tolerances across the entire

heat exchanger plate is a vital property of the end product. The tolerances are

critical for brazability in brazed heat exchangers. In addition, the tolerances

affect the power exchange in heat exchangers with gaskets between the plates.

AP&T knows how to work with both of these types and has presses with the

right stability for the job – something that is verified with FEM analyses.

Automation

An effective and reliable automated process is a necessity for enduring

high production capacity and high end product quality. Over the years, we

have developed and adapted our range of feeders and conveyor systems

to standard products for handling small heat exchanger plates through

hole punching and embossing operations. Large plates are handled by

AP&T press robots and conventional conveyor systems.

Stacking – handling after pressing

In order to ensure product quality while retaining flexibility, AP&T offers a

number of methods after the press operation. For small plates, construction

of the heat exchanger is integrated by the desired number of plates being

stacked at the end of the press line. In conjunction with stacking, components

such as connecting plates are mounted to make the exchanger ready for

subsequent brazing. Large plates are picked with transfer or feeder. They are

then stacked or placed on a conveyor or fixture for subsequent assembly.

Service

AP&T’s service organization and experienced service technicians are

at your service around the world – both for installation and for support

during the service life of the machines. Within the framework of our One

Responsible Partner® concept, we take full responsibility for ensuring that

all component equipment works well together as a single unit. We are

happy to share our production  experience and can train both operators

and maintenance personnel.

plate construction

Depending upon type, some plates employ diagonal flow while others are designed for vertical flow (Figure 8). Plates are pressed in thicknesses between 0.020 and 0.036 inches (0.5 to 0.9 mm), and the degree of mechanical loading is important. The most severe case occurs when one process liquid is operating at the highest working

tube

Tube No. of passes No. of passes Side side No. of tubes/pass No. of passages/pass one Shell No. of cross passes No. of passes Side side (No. of baffles +1) two

Shell diameter No. of passages/pass B pressure, and the other is at zero pressure.

The maximum pressure differential is appliedacross the plate and results in a considerableunbalanced load that tends to close the typical 0.1 to 0.2 inch gap.

It’s essential, therefore, that some form of interplate support is provided to maintain the gap and two different plate forms do this.

One method is to press pipes into a plate with deep washboard corrugations to

provide contact points for about every 1 to 3 square inch of heat transfer surface   Another is the chevron plate of relatively shallow corrugations with support maintained by peak/peak contact (Figure 10). Alternate plates are arranged so that corrugations cross to provide a contact point for every 0.2 to 1 square inch of area. The plate then can handle a large differential pressure and the cross pattern forms a tortuous path that promotes substantial liquid turbulence, and thus, a very high heat transfer coefficient.

Mixing and variable length

To obtain optimum thermal and pressure drop performance while using a minimum

number of heat exchanger plates, mixing and variable length plates are available for several APV paraflow plate heat exchanger models. These plates are manufactured to the standard widths specified for the particular heat exchanger involved but are offered in different corrugation patterns and

plate lengths. Since each type of plate has its own predictable performance characteristics, it is possible to calculate heat transfer surface,

which more precisely matches the required thermal duty without oversizing the exchanger. This results in the use of fewer plates and a smaller, less expensive, exchanger frame.

To achieve mixing, plates–which have been pressed with different corrugation angles–are combined within a single heat exchanger frame. This results in flow passages that differ significantly in their flow characteristics, and thus, heat transfer capability from passages created by using plates that have the same corrugation pattern.

SUMMARY ON

Design and Construction of a Concentric Tube Heat Exchanger

Folaranmi Joshua

Department of Mechanical Engineering, Federal University of Technology 

Minna, Niger State, Nigeria

E-mail: <folajo@yahoo.com>

SUMMARY submitted by VAISHALI GURJER, PGDIE 42, NITIE, Roll No. 100

Introduction

The concentric tube heat exchanger was designed in order to study the process of heat transfer between two fluids through a solid partition. It was designed for a counter-flow arrangement and the logarithmic mean temperature difference (LMTD) method of analysis was adopted. Water was used as fluid for the experiment. The temperatures of the hot and cold water supplied to the equipment were 87^o and 27^oC, respectively and the outlet temperature of the water after the experiment was 73^oC for hot and 37^oC for cold water. The results of the experiment were tabulated and a graph of the mean temperatures was drawn. The heat exchanger was 73.4% efficient and has an overall coefficient of heat transfer of 711W/m2 K and 48^oC Log Mean Temperature Difference. The research takes into account different types of heat exchangers. 

There are three main types of heat exchangers:

a. The Recuperative type

b. The Regenerative type  

c. The Evaporative type

This research paper is on recuperative type of heat exchanger, which can further be classified, based on the relative directions of the flow of the hot and  cold fluids, into three types:

a. Parallel flow, when both the fluids move in parallel in the same direction.

b. Counter flow, when the fluids move in parallel but in opposite directions.

c. Cross flow, when the directions of flow are mutually perpendicular. 

The objectives of the research work are:

(i) To design and construct a concentric- tube heat exchanger in which two tubes are concentrically arranged and either of the fluids(hot or cold) flows through the tube and the other through the annulus.

(ii)To carry out test on the concentric- tube heat exchanger and obtain values which will be compared to theoretically determined ones.

Theory of Design and Analysis

Design Considerations

In designing heat exchangers, a number of factors that need to be considered are: 

1. Resistance to heat transfer should be minimized

2. Contingencies should be anticipated via safety margins; for example, allowance for fouling during operation.

3. The equipment should be sturdy.

4. Cost and material requirements should be kept low.

5. Corrosion should be avoided.

6. Pumping cost should be kept low.

7. Space required should be kept low.

8. Required weight should be kept low. 

Design involves trade-off among factors not related to heat transfer. Meeting the objective of minimized thermal resistance implies thin wall separating fluids. Thin walls may not be compatible with sturdiness.

The Energy Balance

Since the flow in a tube is completely enclosed, an energy balance may be applied to determine how the mean temperature (T_m) x varies with position along the tube and how the total convective heat transfer Q_conv is related to the difference in temperatures at the tube inlet and outlet.

This gives us the following equation, which can be used to find T_m and Q_conv:

DQ_conv +  M(CvT_m +  pv) + [M(CvT_m +  pv) d(pv)/dx] = 0,

The Overall Heat Transfer Coefficient 

 A heat exchanger typically involves two flowing fluids separated by a solid wall. 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. Any radiation effects are usually included in the convection heat transfer coefficients.

Fouling Factor

 The performance of heat exchanger usually deteriorate with time as a result of scaling or deposits from over the interior surface. Scaling or deposits on the inside and outside of the tubes is really a gradual build-up of layers of dirt due to impurities in the fluid, chemical reaction between the fluid and the metal, rust etc. The deposits can severely affect the overall heat transfer  coefficient U. It is related to the overall heat transfer coefficient under clean conditions and under fouled conditions by the equation:

1/U_foul = R_f + 1/U_clean

Logarithmic Mean Temperature Difference 

 The method used in the analysis of the heat exchanger in this research work is the Logarithmic Mean Temperature Difference (LMTD), and it is defined as that temperature difference which, if constant, would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference.

In order to derive expression for LMTD, the following assumptions were made:  The overall heat transfer coefficient U is constant, the flow conditions are steady, the specific heats and mass flow rates of both fluids are constant, the is no loss of heat to the surroundings, there is no change phase either of the fluid during the heat transfer, the change in potential and kinetic energies are negligible, axial conduction along the tubes of the heat exchanger is negligible (Saunders 1981).

In this design, counter-flow LMTD was adopted because it is always greater than that for a parallel flow unit, hence counter-flow heat exchanger can transfer more heat than parallel-flow one; in other words a counter-flow heat exchanger needs a smaller heating surface for the same rate of heat transfer.

Conclusion

This study as a whole offers an overview of an analytical method applicable to the design of concentric tube heat exchanger (counter-flow type). Logarithmic mean temperature difference (LMTD) method was used in the design analysis. The overall heat coefficient and the efficiency were computed. Results obtained show that the heat exchanger was effective.