7 replies to this topic

[pali]

can any one give me the relation between heat dissipated, temperature difference and flow rate of fluid?.
PLZ, tq.

[djack77494]

tq,
The energy balance and heat transfer equations provide you the relationships you need. These are very basic to the field of chemical engineering. Essentially, you have:

Q = m * Cp * (T2 - T1) [defines change in energy]

Q = U * A * (T2 - T1)lm [defines heatt ransfer]

Please refer to any of the many standard textbooks for definitions of the above as well as more detailed information.
Doug

[pali]

dear Doug,
can u verify my calculations:

Q=mCpT
where m = flow rate (l/min)
Cp = specific heat capacity (water 4.2 J/gK)
T = temperature difference

therefore,

given m=130l/min and T=5K

Q= 45.5 kW (J/s)

from my understanding, to dissipate 45.5 kW of heat, the water flow rate should be 130 l/min. And to dissipate 100 kW we need to have water flow rate of 285.7 l/min

thanks

[pleckner]

@pali:

'm' needs to be in mass flow units NOT volume flow. Check your units!

[elpepe]

Hi,
I don't understand the whole problem yet. If you have a plate at a fixed T1, and a flow of air at a different T2, suppose a laminar flow. Which equation has to be used to know the heat transfer rate?. Suppose I know the heat transfer coefficient for the geometry and type of flux (either natural or forced convection) I am using, why I would use the equation involving the mass flow?. Or the opposite?. What are the essential differences between both equations and in which cases have to be used? I know there's a lot of books out there but still I could not get the key of the physics involved. Can anybody answer this?

[Allen]

This post typifies what can be wrong about this sort of forum. I have no idea what level Pali is as a student but the question relates to one of the most basic elements of heat transfer addressed in the first year of any Chemical Engineering course.

Art Montemayer eloquently states in the previous thread:

There is nothing quite as lazy as a lazy student.

I think this applies equally here. Instead of asking other members to provide an answer, my advice to Pali is dig out your lecture notes and text books and do some serious reading. You'll benefit in the long term.

[JEBradley]

Just to re-iterate - 130 l/min is a volumetric flow and you have to change to mass flow. Also standardise you units here (you work in J/s (W) later so its best to change minutes to seconds now by dividing by 60).

m=V x density
m=(130 l/min x 1 kg/l) / 60 s/min = 2.167 kg/s

so Q=mcPT = 2.167 kg/s x 4.2 kJ/kgK x 5 K = 45.5 kJ/s (kW)

So that answers the question of heat dissipation, and yes 100 kW would require 100/45.5 x 130 =
285.7 l/min.

Q=UA(LMTD) (the heat balance) normally works hand in hand with this equation. If your just starting studying heat transfer then this will probably crop up next lesson!!! Plugging the 45.5 kW you just calculated into this equation will enable you to find the heat transfer area, A required and therefore select an adequate heat exchanger for the job.

Elpepe - this is for conductive heat transfer. For convection it would be a different problem )

[elpepe]

JE Bradley,
Thanks. When you say conductive transfer you mean about using the equation that takes in account Cp and mass flow rate, don't you? It can be rapidly seen the second equation (using the heat transfer coefficient) applies to problems related with convection.
As a student (and I hope Allen don't think Art Montemayor's quotation does apply to me!), I understand tha maybe studying so many equations leads to a lack of the feeling about the actual physics of the problems themselves.
I mean: in the problem of a flow passing or impinging a flat plate, why they are thinking the heat from the plate can be totally used (and removed) to warm the fluid?, and using the mass flow rate and heat capacity of the fluid one can get the heat transferred?. I understand one can warm a fluid through a hot plate in an almost "static" situation and also for some extent of "slow" motion of the fluid, but it seems unreliable for a forced convection for example.
Of course, what do "static", "slow" etc. mean is related to velocities, geometry, fluid properties etc, and there's a lot of dimensional analysis to take all these stuff in account. But generally all these analysis leads to the Newton's Law of Cooling equation and the "heat transfer coefficient" defined in terms of Reynolds, Prandtl, Nusselt numbers..., i.e. the second equation.
I don't know what kind of problem is Pali dealing with but at first, some basic analysis has to be done about the physics of the problem, before use any equation.