7 replies to this topic

ok so ive found the fluid i want to use for cooling.

i have choosen a synthetic organic heat tranfer fluid which is made by dow chemicals it consists of a mixture of isomer of an alkylated aromatic.

anyways im designing a heat exchnager to cool styrene from ambient temp (21 degrees celcius) to 5 degrees celcius. the flow rate of the styrene is 5081.88 kg/hr or 1.412 Kg/s and the density is 906 Kg/m3.

i dont have a flow rate or temperature for the cooling liquid.

i do have physical porperties data of the coolant. density:878.5 kg/m3, specific heat: 1.714 kj/kgK, thermal conductivity:0.1368 W/mk, 1.80 mPa.sm, how would i go about calculating the temperatures and the flow rates of the cooling liquid.

dont get me wrong, i dont want it doing for me, just need some pointers on how i would go about assuming/calculating the unkown temperatures and flowrates thanks.

[djack77494]

just need some pointers on how i would go about assuming/calculating the unkown temperatures and flowrates thanks.

azizi1,
Solving this problem is based on performing an energy balance. The energy lost by the styrene is the same as the energy gained by the heat transfer fluid. Think in terms of mass flowrate (kg/hr) * specific heat (kJ/kg/K) * change in temperature (K).

[Art Montemayor]

Aziz:

Those of us who are "old-timers" on this Student Forum are here for a variety of reasons:

• to help students with tips on how to be better students;
• give advice on how the real engineering is going to be like for graduating students;
• share real-life engineering experiences with students and future engineers;
• assist in understanding specific engineering problems and helping students understand how to attack a specific problem;
• correct erroneous ideas about what is involved in engineering;
• lend a hand in checking out proposed calculations and methodologies on problem solving;
• explain difficult or complex operations and theories in engineering and operations involving actual processing plants, equipment, and processes.
• many other areas where a helping hand is needed.

Your query seems to ask for help in calculating the flowrate and terminal temperature of a fluid used in a heat exchanger. I would first ask you for some personal background in order to give you the best possible, meaningful help that I can muster. It is important to know:

1. Your present level of Chemical Engineering study; is it 1st, 2nd, or later year?
2. What ChE courses have you successfully completed?
3. Your university and location.

With this information I can respond to your query much more efficiently since I will have knowledge of what is probably expected of you and how to address you such that I don't repeat a lot of basic information and equations that you already know (or should know).

Can you give us some help by responding with answers to the above requests?

I await your reply.

Art Montemayor

[gvdlans]

Art,

Could you please increase your font size since this is very hard to read...

Guido

[Art Montemayor]

Thank you Guido.

In my haste to make my way home in traffic, I failed to increase my font size to #3 - which is my favorite since it is the "geriatric" size for us old timers with increasingly poorer vision. I had trouble reading it myself when I saw it this morning.

hi, i am studying chemical engineering, i am in my third year of a four year MEng course, i have done heat transfer modules in my first and second year and i am at aston university in birmingham uk.

thank you

[Art Montemayor]

Aziz:

Thanks for your response. Immediately after I posted my first reply, I noted that Doug beat me to the punch and gave you a very good, albeit succinct answer. His response is exactly what I would repeat now that I definitely know your preparation and experience in heat transfer.

I can add a little more detail to what Doug states and perhaps that will assist you in further understanding how to attack a future problem of this nature. We Chemical Engineers often rely on a basic “golden rule”: when stuck or befuddled on how to attack a problem, make a heat and material balance of the system. This often works and, in this case, is just the right remedy. There is nothing to memorize and just simple logic that needs to be applied, with some very simple equations that you’ve already studied and hopefully retained. The steps are:

Make a heat balance, just as Doug states:

Energy (heat) lost by the styrene = the energy (heat) gained by the heat transfer fluid (HTF)

Therefore, since you know that:
Q (sensible heat transferred) = W Cp ΔT

Then,
Heat lost by the styrene = (Weight of Styrene) (Cp of Styrene) (21 – 5) ^oC
Heat gained by the HTF = (Weight of HTF) (Cp of HTF) (T2 – T1) ^oC

Now you have the heat balance equation identified:

(Weight of Styrene) (Cp of Styrene) (21 – 5) ^oC = (Weight of HTF) (Cp of HTF) (T2 – T1) ^oC

You know the Styrene flow rate and you should know (or be able to establish) the inlet and outlet temperature of the HTF and its Cp, so that all you have left is the HTF flow rate to calculate. Depending on the HTF, you should be able to set both temperatures. If you can only deduce one HTF temperature, you then have a situation where you can either set the HTF flow rate or the remaining temperature. The important thing to digest and retain is that the heat picked up by the HTF is dependent on the flow rate you circulate and on (usually the outlet) the HTF temperatures. Remember to make sure all your units are consistent in your calculations. Note that the higher the HTF flow rate, the higher the pumping cost, the bigger the piping, the bigger the pressure drop, and the smaller the exchanger. This is a classical setup for a future process optimization.

Remember that you want an HTF temperature that will work with the size and type of heat exchanger that you select. You don’t want to create a “temperature cross” or establish a temperature you physically cannot achieve because of heat exchanger efficiency or construction. This is common sense and plays a large role in determining how you finally set the design parameters of the ultimate heat exchanger. Since you still have another year of study left, you probably haven’t gone too deep into the various types of heat transfer equipment and their construction characteristics, limitations, and advantages. Therefore, you are probably going to have to assume conservative limitations on the “approach temperature” (how close to the exit HTF temperature the Styrene can achieve). I believe that, for where you are at in your studies, you don’t have to come up with an “optimized” design; you merely have to achieve the heat transfer in a credible and logical manner. The above method allows you to do that. Bear in mind that no losses to the ambient or to the surrounding metal and insulation have been considered, so the design is conservative when you assume the basic heat balance. Take note that the thermal conductivity of either fluid does not enter into play yet. All you need the rate of the sensible heat transfer for now.

I hope this explanation helps you out in your understanding and setting up an algorithm leading to a logical resolution.

thank you for the information, what i have done is specified the HTF inlet temperature and massflow rate. the inlet temperature of -10 degrees celcius and a massflow of 4500kg/hr (1.25kg/s). if the styrene is beeing cooled from 21 to 5 degrees celcius woth a mass flow of 5081.88kg/hr (1.41kg/s) the HTF goes in at -10 (specified) and comes out at 2.66 degrees celcius.

does this seem ok?