Heat Transfer and mass flow rate

In summary: I used 15 degrees as the ambient temperature, as that is a common value. I assumed that the mains water has a temperature of 10 degrees.
  • #1
mrcleanhair
2
0
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.
 
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  • #2
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
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
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.
 

FAQ: Heat Transfer and mass flow rate

What is heat transfer and how does it occur?

Heat transfer is the movement of thermal energy from a hotter object to a colder object. This can occur through three different mechanisms: conduction, convection, and radiation. Conduction is the transfer of heat through a solid material, convection is the transfer of heat through a fluid, and radiation is the transfer of heat through electromagnetic waves.

How is heat transfer measured?

Heat transfer is measured in units of joules (J) or calories (cal). The rate of heat transfer, or the amount of heat that is transferred per unit time, is measured in watts (W) or calories per second (cal/s). The temperature difference between the two objects is also a factor in determining the rate of heat transfer.

What is the difference between heat transfer and mass flow rate?

Heat transfer and mass flow rate are two separate concepts. Heat transfer involves the movement of thermal energy, while mass flow rate refers to the amount of mass that is flowing through a particular area per unit time. Heat transfer can occur without mass flow, such as in the case of radiation, while mass flow rate can occur without heat transfer, such as in the case of a fluid flowing through a pipe.

How does heat transfer affect the mass flow rate?

The amount of heat transfer can affect the mass flow rate in a system. For example, in a heat exchanger, the rate of heat transfer can impact the flow rate of a fluid through the system. As heat is transferred from one fluid to another, it can change the temperature and density of the fluid, which can then affect its flow rate.

What factors can affect the rate of heat transfer and mass flow rate?

The rate of heat transfer and mass flow rate can be affected by a variety of factors, including the temperature difference between the two objects, the thermal conductivity of the materials involved, the area of contact between the objects, and the velocity of the fluid. Other factors such as external conditions and the properties of the materials can also play a role.

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