# Thermofluids question, urgent



## qsz (Jun 20, 2007)

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/t...tf_pt2nov05.htm

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

My exam is tomorrow, so i'd really appreciate it if someone could explain to me how to do this question asap

Thanks alot


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## newton (Jun 20, 2007)

qsz,

You will have heat conduction through the walls /roof. You're given the total surface ares, so I don't guess that it matters.

Calculate the thermal resistance and use Q-dot = DELTA_T/R_thermal for the effective area.

The heat load rate due to the frozen food: (mass rate)(Cp)(Tfood - Troom)

Rate of heat removal: Load from conduction through the walls, load from lights and fan, plus the load due to the food.

For minimum power, use the Carnot efficiency for a refrigeration cycle: beta = Tc/(Th-Tc)

where Tc is the cold room temp in Kelvin and Th is the surrounding temp in Kelvin

I know there's not much detail, but I hope this helps.

Take care


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