Parallel Flow Heat Exchanger || Its 3 important FAQ’S ||

CONTENT

What is parallel flow heat exchanger?

A direct transfer type of heat exchanger in which both hot fluid and cold fluid flow in the same direction to exchange heat energy between them without transfer of any energy from the ambient. 

Parallel flow heat exchanger theory

Heat exchanger is defined as a steady flow adiabatic open system. Flow of both fluids (hot fluid and cold fluid) are in the same direction to exchange heat between. It is a categorized as direct transfer type heat exchanger in which fluids do not have any physical contact between them.  The pressure of both hot and cold fluid remains constant.
The Enthalpy loss of hot fluid is equal to the Enthalpy gain by cold liquid. The variation of temperature between hot fluid and cold fluid in the direction of the flow always decreases.
Fig: 1 Flow in parallel flow Heat Exchanger (Image credit: wikimedia)

Where,

Th,in: Temperature of inlet hot fluid

Th,out: Temperature of outlet cooled fluid 

Tc,in: Temperature of inlet cold fluid

Tc,out: Temperature of outlet warm fluid

Advantages of parallel flow heat exchanger

Loss of pressure is very low
It is simple in construction and cheap to build.

Parallel flow plate heat exchanger

A cluster of plates are placed in a systematic manner one above the other for the formation of a series of channels for fluid flow to exchange heat energy between them. The increase in surface area by the plates allows more heat transfer between the two fluids.
File:เครื่องแลกเปลี่ยนความร้อนแบบแผ่น.png
Fig: 2 Plate type heat exchanger (Image credit: wikimedia)

Parallel flow heat exchanger vs counter flow heat exchanger

The variation of temperature between hot fluid and cold fluid with respect to the flow direction is more pronounced in parallel flow heat exchanger. The entropy of parallel flow type heat exchanger is higher as compared to counter type heat exchanger. Counter flow heat exchanger is more efficient than parallel flow heat exchanger. Hence for the same heat transfer rate required in both cases, counter flow heat exchanger occupies lesser heat transfer area or more compact in size than parallel flow heat exchanger.

What is effectiveness of parallel flow heat exchanger?

‘The effectiveness (ϵ) of a heat exchanger is defined as the ratio of the actual heat transfer to the maximum possible heat transfer.’
Actual heat transfer (Q) = mh*Cph*(Th1 – Th2
= mc*Cpc*( Tc2 – Tc1)
Maximum possible heat transfer (Qmax) = Ch(Th1 – Tc1)

Parallel flow and counter flow heat exchanger experiment

Aim: To determine the effectiveness of the heat exchanger in parallel flow and counterflow.
The experiment setup consists of the following component,
  • Heater
  • Pump
  • Hot water inlet and outlet
  • Cold water inlet and outlet
  • Temperature sensor
  • Flow regulator

Procedure:

First, we have to switch ON the testing apparatus, Then switch ON the heater and set the temperature of the water heater. We have to Wait for the temperature of the water to raised upto the set point. Switch ON the pump for both hot and cold water. Set the mass flow rate of both hot and cold water using a flow regulator knob. All the temperature at inlet and exit are recorded. First, set the heat exchanger in a parallel configuration and note the readings.

Specific capacity of hot fluid: _________

Specific capacity of cold fluid: _________

  1. Adjusted mass flow rate of hot fluid (mh) are recorded
  2. Adjusted mass flow rate of cold fluid (mc) are recorded
  3. Set Inlet temp. of hot fluid are recorded (Th1)
  4. The Outlet temp. of hot fluid  are recorded (Th2)
  5. Inlet temp. Of cold fluid are recorded (Tc1)
  6. Outlet temp. of cold fluid are recorded (Tc2)

Application of Parallel flow heat exchanger

Used for furnace air preheat, which exchange heat between fresh cold air and furnace effluent flue gases.
Shell and tube type of heat exchanger on the ship used parallel flow heat exchanger.

A thin walled double pipe parallel flow heat exchanger

The arrangement in which one fluid flows inside a pipe and the other fluid flows between the outer surface of the first pipe and the inner surface of another pipe that surrounds the first. These pipes are concentric in nature. 

Counter and parallel flow heat exchanger

Both counter and parallel flow heat exchanger are direct transfer type heat exchanger.
The flow direction of the hot and clod fluid in case of counter typer heat exchanger is opposite to each other whereas in case of parallel flow the direction of hot and cold fluids same.
The Log Mean Temperature Difference (LMTD) of  is higher in case of counter flow as compared to the parallel flow heat exchanger and so counter flow heat exchanger are smaller in size for same energy transfer.

Parallel flow heat exchanger calculations

When both hot and cold fluid enters the heat exchanger from the same side, flow in a parallel direction and exit from the same side is known as a parallel flow heat exchanger.
Fig 3: Graph for parallel flow heat exchanger
Aim is to calculate the total heat transfer rate (Q) between hot and cold fluids in the parallel flow heat exchanger.
Where,
Thi is Inlet temperature of hot fluid
The is Exit temperature of hot fluid
Tci is Inlet temperature of cold fluid
Tce is Exit temperature of cold fluid 
ΔTi = Inlet temperature difference
     = Thi – Tci
ΔTe = Exit temperature difference
     = The – Tce
Q = U x A x ΔTm
Where,
U = Overall heat transfer coefficient
A = Total heat transfer area of heat exchanger
ΔTm= Log mean temperature difference

Double pipe parallel flow heat exchanger

It has a simple construction in which one pipe is inserted concentrically to the other. Hot fluid and cold fluid enters heat exchanger from the same side and also flow in the same direction to exchange enthalpy between them.

In case of parallel flow heat exchanger what is the value of maximum effectiveness.

‘The effectiveness of a heat exchanger is defined as the ratio between the actual heat transfer rate taking place between hot and cold fluid and the maximum possible heat transfer rate between them.’
The value of maximum effectiveness in a parallel flow can be 50%.

Parallel flow heat exchanger derivation

To derive an equation for Mean Temperature Difference(MTD) and total heat transfer rate (Q)of the parallel flow heat exchanger.
Consider differential heat transfer area ΔA of the heat exchanger of length Δx through which the differential heat transfer rate between hot and cold fluids is dq.
Then, dq = U x ΔT x dA
Where dA = B * dx, and ΔT = Th – Tc = f(x)
Boundary conditions,
At x = 0 (i.e Inlet) ΔT = ΔTi = Thi – Tci
At x = L (i.e exit) ΔT = ΔTe = The – Tce
Also,
dq = -mh*cph*dt
   = +mc*cpc*dt
ΔT = Th – Tc
d(ΔT) = dTh – dTc
d(ΔT) = -dq[(1/mh*cph) + (1/mc*cpc)]
dq = U*(dA)*ΔT 
    = U*ΔT*(BdX)
dq = -U*(dA)*ΔT*[(1/mh*cph) + (1/mc*cpc)]
Integrating both side by separating variable

Parallel flow heat exchanger diagram

File:Straight-tube heat exchanger 2-pass.PNG
Fig 4: Parallel flow heat exchanger (image credit: wikimedia)

Parallel flow heat exchanger equations

The equation for total heat exchanged
Where,
U = Overall heat transfer coefficient
A = Total heat transfer area of heat exchanger
봗m = Log mean temperature difference
The equation for Log Mean Temp Difference.
Where,
Thi is Inlet temperature of hot fluid
The is Exit temperature of hot fluid
Tci is Inlet temperature of cold fluid
Tce is Exit temperature of cold fluid 
ΔTi = Inlet temperature difference
     = Thi – Tci
ΔTe = Exit temperature difference
     = The – Tce

Parallel flow heat exchanger example

Shell and tube
Double pipe
Plate type

Parallel flow heat exchanger graph

 

Fig 5: Temperature distribution graph

Advantages and disadvantages of parallel flow heat exchanger

Advantage:

It is simple in construction and cheap to build.
Quick fetches
Low pressure loss

Disadvantage:

Less effectiveness
Size is bigger for same heat transfer

Identify the characteristics of parallel flow heat exchangers.

The parallel flow heat exchanger is characterized by direct flow type heat exchanger in which direction of flow is same for both hot and cold fluid during energy transfer.

LMTD equation for parallel flow heat exchanger

It is the parameter that takes into account the variation of ΔT (Temperature difference of inlet side and exit side of heat exchanger) with respect to the direction of hot fluid flow by averaging it all along the length of the heat exchanger from inlet to exit.
Log Mean Temperature Difference (LMTD) is the ratio of difference of difference of inlet temperature and difference difference in exit temperature to Log of the ratio of difference of difference of inlet temperature and difference of difference in exit temperature.
Where,
Thi is Inlet temperature of hot fluid
The is Exit temperature of hot fluid
Tci is Inlet temperature of cold fluid
Tce is Exit temperature of cold fluid 
ΔTi = Inlet temperature difference
    = Thi – Tci
ΔTe = Exit temperature difference
    = The – Tce

Optimization of parallel flow heat exchanger

Shell and tube type parallel flow heat exchanger can be optimized by a new type of anti-vibration clamping baffle. The geometric parameter like baffle distance and baffle width also influence its performance. Type of flow is an important parameter to be considered for the optimization of the heat exchanger.

Define temperature gradient in case of Parallel flow heat exchange

The difference of temperature between temperature difference in inlet side and exit side of heat exchanger is known as temperature gradient. In the case of a parallel flow heat exchanger, it is not uniform and gradually decrease in the direction of flow.
Fig 6: Temperature gradient in parallel flow (image credit: wikimedia)

In Which Condition we should use parallel flow heat exchanger?

The limit of the exit temperature of cold fluid is exit temperature of hot fluid in case of parallel flow heat exchanger. So, it is mainly used where limiting transfer of heat is recommended.

Numerical question:

Que: Hot water at 46℃ enters the heat exchanger to increase the enthalpy of water that enters at 10℃ and comes out of the heat exchager at 38℃. The mass flow rate of hot fluid is 25 l/s, and the mass flow rate of cold fluid is 19 l/s. If no heat losses take place during heat transfer, What is the temperature of the hot fluid at the exit?

Sol: Given inlet temperature of hot fluid (T1) = 46℃

     Given inlet temperature of cold fluid (T3) = 10℃ 

     Given exit temperature of cold fluid (T4) = 38℃

     To find exit temperature of hot fluid (T2) = X

     Density of water () = 1000 kg/m3

     Mass flow rate of hot fluid (mh)= 25 l/s

     Mass flow rate of cold fluid (mc) = 19 l/s

     Heat capacity of water (c) = 4186 J/kg-K

Heat lost by hot water is the same as the heat gained by the cold fluid.

mh*c*(T1-T2) = mc*c*(T3 – T4)

25 (46 – T2) = 19 (38 – 10)

T2 = 24.72℃

The exit temperature of the hot water is 24.72℃

FAQ/Short Notes

Where does parallel flow heat exchanger used

The parallel flow heat exchanger is mainly used where limited transfer of heat is recommended.The limit of the exit temperature of cold fluid is exit temperature of hot fluid in case of parallel flow heat exchanger.

Crossflow vs parallel flow heat exchanger

For the same heat transfer rate required in both cases, counter flow heat exchanger occupies lesser heat transfer area or more compact in size than parallel flow heat exchanger.

 

When water is heated and oil is cooled in a heat exchanger.  will it follow a counterflow path or parallel flow path?

Both type of heat exchanger can be used, but counter flow type heat exchanger will occupy less space as compared to parallel flow type heat exchanger.

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About Krishna Singh

I am Krishna K Singh working as a project scientist. Holding 5 years of industrial experience in Cross Mechanical domain. Presently, I am working on Deep sea mining projects which are of international importance.I am glad to be a part of Lambdageeks family and want to help students with in-depth understanding of Mechanical and Thermal.
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