Category Archives: Heat exchanger

Energy Required to Heat Air

Recently I came across this requirement of calculating heat required to heat air (for AHU), I came across two simplified formulas, as follows.

Please also note the learning from this workout at the bottom!

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Learning!

for delta T, Centigrade vs Fahrenheit are different!

I was thinking, as long as its Delta (Difference between two temperature), °F and °C doesn’t matter! I was WRONG… See following example

Raise temperature of air from 10 °C to 110°C, Temperature difference is 100°C

Where as in Fahrenheit, its 50°F (10°C) to 230°F (110°C) i,e difference is 180°F!

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Hibernation over!

Hello all!

Im not sure how many of you missed my online presence, but it was me who was missing it the most.

Lets start the journey again, I’ll be posting more on my learnings soon.

keep posted.

Sumit

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TEMA Shell & Tube Heatexchanger Checklist

Few of Post Follower has asked me for more help on TEMA design, Though I can’t give a ‘Ready made’ Solution as its against principle of teaching something 😉

A small checklist attached below will definitely help for mechanical design of
Shell and Tube heat exchanger as per TEMA

(PS: if some one able to break the password of the sheet, please let me know 🙂

TEMA-Checklist-Edition-9

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Heat exchanger : Good Ref. Site

To have more ‘Visual’ Clarity, visit following site!

http://www.hcheattransfer.com/shell_and_tube.html

Take Care!

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Tube Holes in Tubesheet : Point to ne noted while designing heat exchanger

Tube hole finish affects the mechanical strength and leak tightness of an expanded tube-to-tubesheet joint. In general:
(1) A rough tube hole provides more mechanical strength than a smooth tube hole. This is influenced by a complex relationship of modulus of elasticity, yield strength and hardness of the materials being used.
(2) A smooth tube hole does not provide the mechanical strength that a rough tube hole does, but it can provide a pressure tight joint at a lower level of wall reduction.

(3) Very light wall tubes require a smoother tube hole finish than heavier wall tubes.

(4) Significant longitudinal scratches can provide leak paths through an expanded tube-to-tubesheet joint and should therefore be removed.

Hence its important to show roughness during design, and ensure it durign fabrication.

Good day!

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Design of heat exchanger step :5 : Parameter Calculations

In the process of evaluation of U, we need to calculate various parameters. Below paragraph mentions few of them.

1. Reynolds number (Re) Reynolds number, which relates inertial forces to viscous forces and thereby characterizes the type of flow regime
2. Prandtl number (Pr), which relates the thermal properties of the fluid to the conductivity of the pipe.
3. Nusselt number (Nu) , a dimensionless group defining the relative significance of the film heat transfer coefficient to the conductivity of the pipe wall

All above three parameters are linked as shown below
Calculation of Reynold’s Number

where:

Dh is the hydraulic diameter of the pipe; its characteristic travelled length, , (m).

Q is the volumetric flow rate (m3/s).

A is the pipe cross-sectional area (m²).

v is the mean velocity of the object relative to the fluid (SI units: m/s).

mu is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s)).

nue is the kinematic viscosity ( (m²/s).

rho is the density of the fluid (kg/m³).

Calculation of Prandtle number :

Where

Based on these parameter, we can now calculated Heat transfer co-efficient on either side.

Where kw is thermal conductivity of the bulk fluid.

With this you are now equipped to calculate over all heat transfer.
 

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Design of heat exchanger step :5 : Parameter Calculations

In the process of evaluation of U, we need to calculate various parameters. Below paragraph mentions few of them.

1. Reynolds number (Re) Reynolds number, which relates inertial forces to viscous forces and thereby characterizes the type of flow regime
2. Prandtl number (Pr), which relates the thermal properties of the fluid to the conductivity of the pipe.
3. Nusselt number (Nu) , a dimensionless group defining the relative significance of the film heat transfer coefficient to the conductivity of the pipe wall

All above three parameters are linked as shown below
Calculation of Reynold’s Number

where:

Dh is the hydraulic diameter of the pipe; its characteristic travelled length, , (m).

Q is the volumetric flow rate (m3/s).

A is the pipe cross-sectional area (m²).

v is the mean velocity of the object relative to the fluid (SI units: m/s).

mu is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s)).

nue is the kinematic viscosity ( (m²/s).

rho is the density of the fluid (kg/m³).

Calculation of Prandtle number :

Where

Based on these parameter, we can now calculated Heat transfer co-efficient on either side.

Where kw is thermal conductivity of the bulk fluid.

With this you are now equipped to calculate over all heat transfer.
 

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