Heat exchangers

Introduction

A heat exchanger is a component that allows the transfer of heat from one fluid (liquid or gas) to another fluid.   Reasons for heat transfer include the following:

1. To heat a cooler fluid by means of a hotter fluid
2. To reduce the temperature of a hot fluid by means of a cooler fluid
3. To boil a liquid by means of a hotter fluid
4. To condense a gaseous fluid by means of a cooler fluid
5. To boil a liquid while condensing a hotter gaseous fluid

Regardless of the function the heat exchanger fulfills, in order to transfer heat the fluids involved must be at different temperatures and they must come into thermal contact.  Heat can flow only from the hotter to the cooler fluid.

Types  of  Heat  Exchanger  Construction

Although heat exchangers come in every shape and size imaginable, the construction of most heat exchangers fall into one of two categories: tube and shell, or plate.  As in all mechanical devices, each type has its advantages and disadvantages.

Tube  and  Shell

The most basic and the most common type of heat exchanger construction is the tube and shell, as shown in Figure 1.   This type of heat exchanger consists of a set of tubes in a container called a shell.   The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid.   At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal.  In systems where the two fluids are at vastly different pressures, the higher pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side.   This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates shown on Figure 1 also act as baffles to direct the flow of fluid within the shell back and forth across the tubes.

Figure 1   Tube and Shell Heat Exchanger

Plate

A plate type heat exchanger, as illustrated in Figure 2, consists of plates instead of tubes to separate the hot and cold fluids.  The hot and cold fluids alternate between each of the plates.   Baffles direct the flow of fluid between plates.   Because each of the plates has a very large surface area, the plates  provide each of the fluids with an extremely large heat  transfer  area.    

Therefore  a  plate  type  heat  exchanger,  as  compared  to  a  similarly sized tube and shell heat exchanger, is capable of transferring much more heat.   

This is due  to  the  larger  area  the  plates  provide  over  tubes.    

Due  to  the  high  heat  transfer efficiency of the plates, plate type heat exchangers are usually very small when compared to a tube and shell type heat exchanger with the same heat transfer capacity.   

Plate type heat  exchangers  are  not  widely  used  because  of  the  inability  to  reliably  seal  the  large gaskets between each of the plates.  Because of this problem, plate type heat exchangers have  only  been  used  in  small,  low  pressure  applications  such  as  on  oil  coolers  for engines.    

However,  new  improvements  in  gasket  design  and  overall  heat  exchanger design have allowed some large scale applications of the plate type heat exchanger.   

As older facilities are upgraded or newly designed facilities are built, large plate type heat exchangers are replacing tube and shell heat exchangers and becoming more common.

Figure 2   Plate Heat Exchanger

Types  of  Heat  Exchangers

Because heat exchangers come in so many shapes, sizes, makes, and models, they are categorized according to common characteristics.  One common characteristic that can be used to categorize them is the direction of flow the two fluids have relative to each other.  The three categories are parallel flow, counter flow and cross flow.

Parallel flow, as illustrated in Figure 3, exists when both the tube side fluid and the shell side  fluid  flow  in  the  same  direction.     In  this  case,  the  two  fluids  enter  the  heat exchanger from the same end with a large temperature difference.  As the fluids transfer heat, hotter to cooler, the temperatures of the two fluids approach each other.   Note that the hottest cold-fluid temperature is always less than the coldest hot-fluid temperature.

Figure 3   Parallel Flow Heat Exchanger

Counter  flow,  as  illustrated  in  Figure  4,  exists  when  the  two  fluids  flow  in  opposite directions. Each of the fluids  enters  the heat exchanger at opposite ends.   Because the cooler fluid exits the counter flow heat exchanger at the end where the hot fluid enters the heat exchanger, the cooler fluid will approach the inlet temperature of the hot fluid. Counter flow heat exchangers are the most efficient of the three types.  In contrast to the parallel flow heat exchanger, the counter flow heat exchanger can have the hottest cold- fluid temperature greater than the coldest hot-fluid temperatue.

 

Figure 4   Counter Flow Heat Exchange

Cross  flow, as  illustrated in Figure 5, exists when one fluid flows perpendicular to the second fluid; that is, one fluid flows through tubes and the second fluid passes around the tubes at 90° angle.   Cross flow heat exchangers are usually found in applications where one  of  the  fluids  changes  state  (2-phase  flow).     An  example  is  a  steam  system's condenser, in which the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the tubes absorbs the heat from the steam, condensing it into water. Large volumes of vapor may be condensed using this type of heat exchanger flow

Figure 5   Cross Flow Heat Exchanger

Comparison  of  the  Types  of  Heat  Exchangers

Each of the three types of heat exchangers has advantages and disadvantages.   But of the three, the counter flow heat exchanger design is the most efficient when comparing heat transfer rate per unit surface area.  The efficiency of a counter flow heat exchanger is due to the fact that the average  T    (difference  in  temperature)  between  the  two  fluids  over  the  length  of  the  heat exchanger  is  maximized,  as  shown  in  Figure  4.    Therefore  the  log  mean  temperature  for  a counter  flow  heat  exchanger  is  larger  than  the  log  mean  temperature  for a  similar  parallel  or cross   flow   heat   exchanger.   (See   the   Thermodynamics,   Heat   Transfer,   and   Fluid   Flow Fundamentals Handbook for a review of log mean temperature).  This can be seen by comparing the graphs in Figure 3, Figure 4, and Figure 5.   The following exercise demonstrates how the higher log mean temperature of the counter flow heat exchanger results in a larger heat transfer rate.  The log mean temperature for a heat exchanger is calculated using the following equation.

	∆T2 – ∆T1
			Equation  2-1

In ∆T1/∆T2

∆T_in =

.

Heat  transfer  in  a  heat  exchanger  is  by  conduction  and  convection.    The  rate  of  heat transfer, "Q", in a heat exchanger is calculated using the following equation.

Equation  2-2

 

 Q = U_OA_O∆ T_im

.

Where:

Q     =  Heat transfer rate (BTU/hr)

Uo   =   Overall heat transfer coefficient (BTU/hr-ft²-°F)

A_o    =   Cross sectional heat transfer area (ft²)

∆T_in= Log mean temperature difference (°F)

Consider  the  following  example  of  a  heat  exchanger  operated  under  identical  conditions  as  a counter flow and then a parallel flow heat exchanger.

 T₁   =  represents the hot fluid temperature

T_1in  = 200°F

T_1out = 145°F

U_o  =   70 BTU/hr – ft² – °F

A_o  =   75ft²

T₂ = represents the cold fluid temperature

T_2in = 80°F

T_2out = 120°F

(145 – 80oF)

–

(200 – 120oF)

Counter flow ∆T_in  =

(145 – 80oF)

(200 – 120oF)

                                     

In

                                       =   72^oF  #

          Parallel flow ∆T_in    =

	(200 – 80oF)

(145 – 120oF)

                                                In 

                                        =   61^oF #

.

Inserting  the  above  values  into  heat  transfer  Equation  (2-2)  for  the  counter  flow  heat exchanger yields the following result

hr –  ft2 – °F

         Q =     70                    (75ft²) ( 72^oF)

              =  3.8 x 10⁵ BTU/hr

Inserting  the  above  values  into  the  heat  transfer  Equation  (2-2)  for  parallel  flow  heat exchanger yields the following result.

	.

BTU

       

hr –  ft2 – °F

         Q  =     70                    (75ft²) ( 61^oF)

 

                 =  3.2 x 10⁵ BTU/hr

The results demonstrate that given the same operating conditions, operating the same heat exchanger  in  a  counter  flow  manner  will  result  in  a  greater  heat  transfer  rate  than operating in parallel flow.

In actuality, most large heat exchangers are not purely parallel flow, counter flow, or cross flow; they are usually a combination of the two or all three types of heat exchangers.   This is due to the fact that actual heat exchangers are more complex than the simple components shown in the idealized  figures  used  above  to  depict  each  type  of  heat  exchanger.     The  reason  for  the combination of the various types is to maximize the efficiency of the heat exchanger within the restrictions placed on the design.   That is, size, cost, weight, required efficiency, type of fluids, operating  pressures,  and  temperatures,  all  help  determine  the  complexity  of  a  specific  heat exchanger.

One method that combines the characteristics of two or more heat exchangers and improves the performance of a heat exchanger is to have the two fluids pass each other several times within a single heat exchanger.    When a heat exchanger's fluids pass each other more than once, a heat exchanger is called a multi-pass heat exchanger.  If the fluids pass each other only once, the heat exchanger is called a single-pass  heat  exchanger.   See Figure 6 for an example of both types. Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more sets of "U" bends in the tubes.  The "U" bends allow the fluid to flow back and forth across the length of the heat exchanger.   A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger.   These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect.

Figure 6   Single and Multi-Pass Heat Exchangers

Heat  exchangers  are  also  classified  by  their  function  in  a  particular  system.    One  common classification is regenerative or nonregenerative.  A regenerative heat exchanger is one in which the same fluid is both the cooling fluid and the cooled fluid, as illustrated in Figure 7.   That is, the hot fluid leaving a system gives up its heat to "regenerate" or heat up the fluid returning to the system.  Regenerative heat exchangers are usually found in high temperature systems where a portion of the system's fluid is removed from the main process, and then returned.   Because the fluid removed from the main process contains energy (heat), the heat from the fluid leaving the main system is used to reheat (regenerate) the returning fluid instead of being rejected to an external  cooling  medium  to  improve  efficiency.    It  is  important  to  remember  that  the  term regenerative/nonregenerative only refers to "how" a heat exchanger functions in a system, and does not indicate any single type (tube and shell, plate, parallel flow, counter flow, etc

In a nonregenerative heat exchanger, as illustrated in Figure 7, the hot fluid is cooled by fluid from a separate system and the energy (heat) removed is not returned to the system.

Figure 7   Regenerative and Non-Regenerative Heat Exchangers

Types  of  Heat Exchangers  Summary

·        There are two methods of constructing heat exchangers: plate type and tube type.

·        Parallel flow - the hot fluid and the coolant flow in the same direction.

·        Counter flow - The hot fluid and the coolant flow in opposite directions.

·        Cross flow - the hot fluid and the coolant flow at 90° angles (perpendicular) to each other.

·        The four heat exchanger parts identified were:

o   Tubes

o   Tube Sheet

o   Shell

o   Baffles

·        Single-pass heat exchangers have fluids that pass each other only once.

·        Multi-pass heat exchangers have fluids that pass each other more than once through the use of U tubes and baffles.

·        Regenerative heat exchangers use the same fluid for heating and cooling.

·        Non-regenerative heat exchangers use separate fluids for heating and cooling.