Hello guys........Hope all are enjoying the start of these weekend !!

Today i wanna add a full stop for many peoples fantasies of 'U' value calculation, the most common line that i heard from many engineers was Consider U as 150 for Reactors and 450 for Condensers, in return if you ask them what are the units for those 150 and 450, many of them will take some strategic time out as in IPL.

An Engineer is the one who holds the ability to deal any of the situation with a smart work, but not the one who follow the Thumb blindly, i'm not criticizing the Thumb, as many cases i too prefer the Thumb rules in practical cases but my intention isAn Engineer should be able to derive the U value if needed.

And for that we need to know some basics, before going into topic.

**What are Heat Transfer Co-Efficient?**Heat transfer coefficient is a quantitative characteristic of convective heat transfer between a fluid medium (a fluid) and the surface (wall) flowed over by the fluid.

*Also Read:*

**What is Overall Heat Transfer Co-efficient?**Overall Heat Transfer Co-efficient is the combination of the possible Individual Heat Transfer Co-efficient, which are due to the following Heat Transfer modes,

1. Rate of convective heat transfer from Fluid to Medium Wall, and

2. Conductive Heat Transfer from Medium Walls one end to other end,

3. Convective Heat Transfer from Medium wall to inside Fluid.

Credits: TLV.com |

**What is Thermal Conductivity?**Thermal Conductivity of a material characterizes the Rate of Heat Transfer of a material, whether the heat is flowing in or out.

The thermal conductivity -

*k -*for some typical materials (varies with temperature)

- Polypropylene PP :
*0.1 - 0.22 W/mK* - Stainless steel :
*16 - 24 W/mK* - Aluminum :
*205 - 250 W/mK* - Glass lining
*: 1.2 W/mK*

**What are the units of Overall Heat Transfer Co-efficient and How are they derived?**British Units for U are Btu/(hr-ft

^{2}°F) , SI units for U are Watts/Sq.m .°K or KCal/Sq.m .sec.°K

They are basically derived from the formula Q = U x A x LMTD,

where Q holds the units Watts , A - Sq.m, LMTD - °K or °C.

So, that's how U units were derived.

Credits: engineersedge.com |

**What will be the Trend of Convective Heat Transfer Co- efficients?**The Convective Heat Transfer Trend will be as follows:

Fluid | Convective heat transfer coefficient (h) |

Water | about 1000 W/(m^{2}°C) |

Hot Water | 1000 – 6000 W/(m^{2})°C |

Steam | 6000 – 15000 W/(m^{2}°C) |

The convection heat transfer coefficient -

*h*- depends on

- The type of fluid - gas or liquid,
- The flow properties such as velocity,
- Other flow and temperature dependent properties.

- Air -
*10 to 100 W/m*^{2}K - Water -
*500 to 10 000 W/m*^{2}K

*Recommended Read:*

**U value Calculation:**
For calculation of U value we need some basic data regarding the Jacket fluid properties, Vessel Mass properties, Jacket properties, Equipment Physical data etc.

General Data includes: Vessel Dia, Vessel MOC, Vessel wall thickness.

For Calculating Overall Heat Transfer Coefficient we need Individual Heat Transfer Co-efficients used in the following co-relation.

The1 / U = ( 1 / hj ) + ( Fj ) + ( X/K ) + Fv + ( 1/hi )

**hj**,

**hi**need to be calculated primarily,

**Jacket Side Heat Transfer Co-efficient Calculation:**For Calculation of

**hj**, we need to know Pipe coil ID, Flowrate in pipe, Fouling factor Fj, Fluid properties[Density, Viscosity, Specific heat, Conductivity],

With an example i'll illustrate here,

Let pipe ID is 2", Flowrate(Q) is 10 Cu.m/hr, Fouling factor(Fj) = 0.0002 h.m

^{2}.°C/Kcal,

Density (rho) 1000 Kg/Cu.m, Viscosity(Î¼) 0.01 cP, Specific heat (Cp) 0.9 KCal/Kg.°C, Conductivity(k) 0.2 KCal/m.h.°C,

Viscosity @ wall is (Î¼

_{w}) = 0.01 cP

Velocity, V = Q/A = 2.7410 m/sec

Reynold's number, Nre = D x V x rho / Î¼ =17638677,

Prandtl's number, Npr = Cp x Î¼

_{ /}k = 0.2093

using the below co-relation ,

**hj**x D / K = 0.023 x ( Nre ^ 0.8 ) x ( Npr ^ 0.33 ) x (( Î¼

_{/}Î¼

_{w})^0.14)

**hj**= 27202.81 Kcal/h.m

^{2}.°C

*Also Read:*

**Vessel Side Heat Transfer Co-efficient Calculation:**For this calculation we need to know the inside fluid properties, which includes Density, Viscosity, Conductivity, Specific Heat, Viscosity @ wall.

Within an example i'll illustrate here,

Agitator RPM 36, Agitator Dia 36", Density (rho) 900 Kg/Cu.m, Viscosity(Î¼) 2cP, Specific Heat(Cp) 1KCal/Kg.°C, Conductivity (k) 0.5 KCal/m.h.°C, Viscosity @wall (Î¼

_{w}) 1 cP, Fouling factor(Fv) 0.0002 h.m

^{2}.°C/Kcal,

Reynold's number, Nre = N x D^2 x rho / Î¼ = 313548,

Prandtl's number, Npr = Cp x Î¼

_{ /}k = 14.4

Now, Individual Heat Transfer Co-efficient

**hi**is Calculated by following co-relation,

**hi**x D / K = 0.53 x (Nre ^0.66) x ( Npr ^ 0.33) x (( Î¼

_{/}Î¼

_{w})^0.24)

**hi**= 1441.16 Kcal/h.m

^{2}.°C

So, Now our task came to an end,

U value Calculation,

1 / U = 1/

**hj**+ Fj + X/K + Fv + 1/

**hi**

1 / U =

`1/27202.8 + 0.0002 + 0.0007 + 0.0002 + 1/1441.2`

U = 537.019 Kcal/h.m

^{2}.°C

That's it............Task Completed Cheers....!!

Actually one more thing i should have been included in the basic "What are the factor do the U value depend?", anyway by the end of this tutorial you should have learned this,

Getting mails asking me to demonstrate with some example data, and that's why today i've illustrated the formulae with data,

If you understood clearly then say cheers, or any queries, feel free to contact me, M happy to help,

Comments are most appreciated.....!!!

*You May Also Like:*

**A**bout The Author

Cheerz AJ ðŸ’œðŸ’œðŸ’œ

ReplyDeleteIf possible share this with your friends and say cheers...!!!!

DeleteTable 1. Typical overall coefficients for jacketed glass lined steel vessels

ReplyDeleteDuty U (W m−2K−1)

Distillation/Evaporation 350

Heating 310

Cooling 200

Cooling (chilled service) 100

Table 2. Typical overall coefficients for jacketed carbon and stainless steel vessels

Duty U (W m−2K−1)

Heating 400

Cooling 350

Cooling (chilled service) 150

A typical overall coefficient for a well designed coil would be 400 to 600 Wm2K−1.

These are Thumb and we can't strongly depend on these all,

DeleteThis comment has been removed by the author.

ReplyDeleteAj.. In batch reactor heating/ cooling,

ReplyDeleteIsothermal& Nonisothermal medium relates to.. ????

Sorry, can't get what you are asking...!! Query need to be some more clear

DeleteWhat is HVAC system?,explain it with diagram.

ReplyDelete