# Homework Help: Thermofluids question

1. Jun 20, 2007

### qsz

I posted this somewehre else, but perhaps this is a better location:

Hey guys i'm trying to prepare for an exam, and i've come across a question I have no idea how to do.

I can draw the diagram and all but i'm at loss as to how i'd go about solving this. The lecturers do not give us solutions, let alone worked solutions, so if someone could help me out here (a worked solution of some sort would be great, but that's probably asking too much)

My question is as follows:

A cold room is to be erected above ground inside a building and maintained at -20°C when the building is at 21°C. The cold room shell will have an effective surface area of 500m2, constructed with two layers: one 60mm thick with conductivity 0.42 W/m.K, the other 125mm thick with conductivity 0.05 W/m.K. When operating, 400 kg/hr of frozen food with a specific heat of 3.85 kJ/kg.K is to be brought into the room at -11°C and brought to room temperature. Fans and lights consume 9.5 kW of electrical energy within the room. Assuming heat transfer through the shell depends only upon its conductivity, draw a diagram showing the various energy flows, and calculate the:

a. rate of heat flow through the cold room shell;

b. heat load caused by the frozen food;

c. rate of heat removal required to maintain the cold room temperature;

d. minimum power required to pump the heat from the room to the building.

And just to show it's not an assignment question or anything, here is the link to the past paper.. It's question A4

http://www.mech.uwa.edu.au/unit/MECH2403/tf_exams/tf_pastexams/papers/tf_pt2nov05.htm [Broken]

Help would be greatly appreciated guys :) It's probably a walk in the park for you physicists

Last edited by a moderator: May 2, 2017
2. Jun 21, 2007

### Chi Meson

No this is a nasty little problem
formulas would include the rate of heat conduction Q/t = (kA deltaT)/L
Luckily the temperature difference stays constant.
note that this is "joules per second" equivalent to power, right?

c. Heat is flowing in through the walls. That heat must be taken out, by the second. Fans and light consume electric energy and turn it into, guess what? That heat must be removed too, every second.

d.one word: Carnot

b. The food must also have heat taken out of it, how much heat? But I don't think this heat is to be included in parts c & d, because the amount of heat removed from the food will diminish logarithmically over time, and I think parts c & d are concerned with the constant temperature difference.

3. Jun 24, 2007

### qsz

Thanks for that, I'll see if it helps