Performance & Analysis of Counter flow & Parallel flow Heat Exchangers , LMTD & Effectiveness

IntroductionNeed for heat transfer-

Heat Exchangers

Types of Heat ExchangersAlthough heat exchangers come in every

size and shape imaginable, there are two basic types.

They are- Tube & Shell Heat Exchanger

Plate Heat Exchanger

Tube & Shell Heat ExchangerThe most common type of heat exchanger

is a shell and tube heat exchanger. The system consists of a set of tubes enclosed in a shell.

Cont..• The fluid flowing inside the tubes is called the tube side fluid and

that flowing on the outside of the tubes is called the shell side fluid

• The tube side fluid is separated from the shell side fluid by a tube sheet(s)

• The tubes are rolled and press fitted or welded into the tube sheet to make it leak proof

• The higher pressure fluid is directed through the tubes and lower pressure fluid is circulated through the shell side.This is based on economy considerations as the tubes can be made to withstand higher pressure than the shell of the heat exchanger at a much lower cost

Plate Heat Exchanger•Plate heat exchangers employs plates

instead of tubes to separate the hot and the cold fluid

•Because of large surface area, plates provide an extremely large heat transfer area

Cont..•Plates are smaller than tubular heat

exchangers of the same capacity because of the large heat transfer area of the plates

•Plate heat exchangers are used for comparatively low pressure processes

•The reliability of plates is less in context to leakage as large gaskets used between plates

•

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Classification of Heat Exchangers

The heat exchangers are categorized on the basis of-

1. Flow configuration

2. Regeneration

Flow Configurations•Three basic flow arrangements are:- 1. Parallel Flow 2. Counter Flow 3. Cross Flow

Parallel Flow Heat Exchangers• In parallel flow heat exchangers both hot and cold streams

enter the heat exchanger at the same end and travel to the opposite end in parallel streams

• Energy is transferred along the length from the hot to the cold fluid so the outlet temperature asymptotically approach each other

• Parallel flow results in rapid initial rates of heat exchange near the entrance, but the transfer rates rapidly decrease as the temperature of two streams approach one another

• Hottest cold fluid temperature is less than the coldest hot fluid temperature

Fig- parallel flow heat exchanger

Counter Flow Heat Exchangers• In a counter flow heat exchanger, two streams enter

at opposite ends of a heat exchanger and flow in parallel but opposite directions

• Temperatures within the two streams tend to approach in a nearly linearly fashion, resulting in a much more uniform heating pattern

• In contrast to the co-current flow, the counter flow heat exchangers can have the hottest cold fluid temperature greater than the coldest hot fluid temperature

fig-counter flow heat exchanger

Cross flow heat exchangers•Cross flow exists when one fluid flows

perpendicular to the second fluid• In this arrangement one fluid flows

through the tubes and other fluid flows around the tube at 90 angle

•They find application when one of the fluid changes state e.g. boilers, condensers

˚

Fig – cross flow heat exchanger

fig-Cross flow heat exchangersEfficiency: 61%

Log Mean Temperature Difference•Heat flows between the hot and the cold

streams due to temperature difference across the tube acting as a driving force

•The temperature difference will vary along the length of the heat exchanger

•Fig:Fig-Temperature difference between hot and

cold process streams in parallel and counter flow

Cont..•The integrate average temperature

difference for either parallel or counter flow can be written as:

•The effective temperature calculated from this equation is known as log mean temperature difference

θ1=ΔT2

θ2=ΔT1and Δθ=ΔTlmand

Comparison •Each of the three types of heat

exchangers have their advantages and disadvantages

•Of the three the counter flow heat exchanger is the most efficient when comparing heat transfer rate per unit area

•The LMTD Δθ=ΔTlm is greater, than a similar parallel or cross flow heat exchanger

Mathematical explanation•The following elaboration shows that LMTD

counter flow heat exchanger is greater than LMTD parallel flow or cross flow heat exchanger

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

Actually what is done…• Most large heat exchangers are not purely parallel or

counter or cross flow, rather a mixture of two or three heat exchangers is used

• The reason for this mixture is to maximize the efficiency of heat exchangers within the restrictions placed on design such as size, cost, operating pressure, required efficiency, type of fluid processed and temperature

• Two fluids are made to pass each other several times within a single heat exchanger to increase the performance

Cont…• When the fluids pass each other more than one time

it is called multi-pass heat exchanger

• If the fluids pass each other single time it is called single-pass heat exchanger

• Commonly the multi pass heat exchangers reverses the flow several times by the use of “U” tubes

• A second method to achieve multi passes is by the use of baffles on the shell side of the heat exchangers

Regeneration Heat exchangers are also classified by

their function in a particular system as 1. Regenerative heat exchanger 2. Non-Regenerative heat exchanger

Regenerative Heat Exchanger•A regenerative heat exchanger is the one

in which the same fluid is both the cooling fluid and the cooled fluid

• i.e. hot fluid gives heat to incoming cool fluid heating it and itself being cooled

•Usually found in high temperature systems

Fig – Regenerative heating

Non-Regeneration • In a non regenerative system, the hot fluid

is cooled by fluid from a separate system•The energy removed is not returned to the

system

Effectiveness-NTU Method•NTU or number of transfer units is used to

calculate the rate of heat transfer in heat exchangers

•NTU method used when there is insufficient information to calculate the LMTD

•So when the inlet and outlet temperatures are not specified properly, the NTU method is use

The Method• To define the effectiveness of a heat exchanger, we need

to find the maximum possible heat transfer that can be hypothetically achieved in a counter flow heat exchanger of infinite length

• Therefore one fluid will experience the maximum possible temperature difference , which is (temperature difference between the inlet temperature of hot stream and cold stream)

• The method proceeds by calculating the heat capacity rates( mass flow rate multiplied by specific heat) and Сһ Сс

Cont..for the hot and cold fluid, and denoting the smaller

one as A quantity =

Is found out where Ɋmax is the maximum heat that could be transferred between the fluids in unit time

must be used as it is the fluid with the lowest heat capacity that undergoes max possible temperature change

Сmin

Ɋmax

Cont…•The other fluid changes temperature more

slowly along the heat exchanger length•We are concerned only with the fluid

undergoing the maximum temperature change•The effectiveness(E), is the ratio of heat

transfer rate and maximum possible heat transfer rate

E = Ɋ/ ɊmaxWhere Ɋ= =

Cont…•Effectiveness is a dimensionless quantity

between 0 and 1• If we know E for a particular heat

exchanger and we know the inlet conditions of the two flow streams, we can calculate the amount of heat being transferred between the fluids by

Ɋ=E For any heat exchanger it can be shown

that E=

Cont…• For a given geometry, E can be calculated using

correlations in terms of “ heat capacity ratio” Heat capacity ratio=and the NTU

Where U = overall heat transfer rate A = overall heat transfer area

Cont…•The effectiveness of a parallel flow heat

exchanger is calculated with E

Or the effectiveness of a counter flow heat exchanger is calculated with

E

Cont… For heat capacity ratio =1 E=

Similarly effectiveness relationships can be derived for other heat exchangers also and are differentiated from one another depending on the flow regime, number of passes and whether a flow stream is mixed or unmixed.

1+

Cont…The heat capacity ratio is =0, is a special

case of evaporation or condensation where phase change takes place in a heat exchanger. Hence in this special case the heat exchanger behaviour is independent of flow arrangements. Therefore the effectiveness is given by

E=

Conclusion •Heat exchangers serve as the backbone of

the industry in which it is used•Their effectiveness directly relates to the

economy of the organization and the tasks performed with them

•So the arrangements of heat exchangers are very important

•The proper functioning of the units are necessary as they save energy, cost & product damage

Thank you