Homework Help: Heat Transfer and mass flow rate

1. Nov 10, 2017

mrcleanhair

Hi, wondered if anyone can give me a pointer on this problem I am working on.

6 Diesel engines each with power 55kW running, given output cooling water temperature of 80°C, and required return temp of 60°C. The coolant is to be fed through a heat exchanger, with the heat exchanger outlet to be cooled via evaporative cooling from small cooling tower.

Since I have Tout and Tin for the Heat EX/Engine loop, I can use steady flow energy eq to find mass flow, once I have made assumptions of thermal efficiency of engines, (30% of heat energy transferred to coolant),

55*6=330 @30% = 99kW
Cp water @ 80°C = 4.196kJ/kg

Q=mcpDT, so rearranging gives 99/(4.196*(80-65) = 1.57kg/s (heat energy transferred to coolant)

Then, I am assuming a mains water feed to heat ex, with average assumed values;
Cold side inlet temp: 10°C
Cp water @10°C: 4.192kJ/kg
Mains feed at 3l per min (0.051kg/s)

Using SFEE and rearranging mh*Cph*(80-65)=mh*Cpc*(T4-10)

10+(1.57*4.196*15)/(0.051*4.192)=472.2

I am clearly missing something fundamental, as even I know that is totally incorrect. Any helpis hugely appreciated.

2. Nov 10, 2017

DoItForYourself

If I understood well, the lost heat (30%) is transferred in the coolant (water) and the coolant passes through a heat ex, cooled down to 60°C and then returns again to the engines [cold side: cooling water (from cooling tower) and hot side: the coolant (water)].
Is this right?

I also have some questions:
Why did you write (80-65) and not (80-60)?
Why did you assume so low feed for cooling water? As long as they contain water, both sides of the heat ex have almost the same Cp. I think you must consider T4 to be the biggest accepted value for the returned cooling water temperature (depends on the design of cooling tower and the weather conditions). Then you can find minimum mc.

3. Nov 10, 2017

mrcleanhair

Hi,

Appreciate the reply. Apologies, typo on my part. Delta T is 15 degrees.

The cooling water is circulating from the engines, through the heat ex and returning to the engines water jacket at 65 degrees.

The cooling tower system is used to cool the output of the heat exchanger. I assume this is to keep fluids separate in the system.

I will look again at my assumptions of flow rate of mains water feed.

Many thanks.

4. Nov 10, 2017

DoItForYourself

Hi too,

Ok.

The reason why the cooling water does not enter directly the jacket of the engines is because it may contain small amounts of substances that can cause or promote corrosion/fouling/growth of microorganisms (although additives have already been added in the cooling tower to avoid this situation). If one of these cases occurs, then the damage (functional and economical) will be great because the engines are expensive.

You can also use a typical cooling tower's ΔT, in order to get an idea of the mains water feed's order of magnitude.