# Thermodynamics, Calculating the required mass flow

• Engineering
• bardia sepehrnia
In summary, the conversation is about a question that involves solving for the mass flow of air and cooling a power plant. The question is unclear and the specific heat capacity of air and power plant's output are given. There is confusion about the calculations and it is pointed out that the mass flow is unrealistically large. It is also mentioned that it is unrealistic to cool a power plant with air only. The conversation ends with gratitude for the help.
bardia sepehrnia
Homework Statement
1-A power plant generates 150 MW of electrical power. It uses a supply of 1000 MW from a geothermal source and rejects energy to the atmosphere through a cooling tower.
Determine:
a) The rejected power to the atmosphere in MW
b) How much air should be flowed to the cooling tower (kg/s) if its temperature cannot be increased more than 10C.
Relevant Equations
Qin-Qout=W
Q=m*cp*(T2-T1)
I have solved this question and it seemed pretty easy, but I got an extremely large number for the mass flow, I had to post the question here to make sure I did it correctly. Any help will be appreciated.

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Two Three Four things.

I see a calculation that is hard to decipher:

The 1 in cp= 1.00##\zeta## looks like a 7.​
Your k and K look exactly the same​
And how can 8.5 105 kJ/s divided by 10 kJ/kg yield 17 106 kg/s ? Did you check your math ?​

The last one is more about the exercise: cooling towers evaporate water, they are not air cooled heat exchangers.

##\ ##

BvU said:
Two Three Four things.

I see a calculation that is hard to decipher:

The 1 in cp= 1.00##\zeta## looks like a 7.​
Your k and K look exactly the same​
And how can 8.5 105 kJ/s divided by 10 kJ/kg yield 17 106 kg/s ? Did you check your math ?​

The last one is more about the exercise: cooling towers evaporate water, they are not air cooled heat exchangers.

##\ ##
The question is kind of vague so I had to assume lots of things but it clearly says in question: "how much air has to flow... "and when it says :'its temperature", the question is referring to air's temperature. So I'm pretty sure it's being cooled by air.

The cp (specific heat capacity at constant pressure) of air is 1.005kJ/kg*K
The 850MW is 850*10^3 kW.

I can see I messed up the math. So (850*10^3 [kJ/s])/(1.005[kJ/kg*K]*10[K])=84577kg/s
But the mass flow is still unrealistically large.

I agree it's unrealistic, but the math is as it is. Also, your answer suggests an unrealistic precision.

But it is unrealistic to cool a power plant with air only, and that is the responsibility of the problem composer, not yours.

##\ ##

bardia sepehrnia
BvU said:
I agree it's unrealistic, but the math is as it is. Also, your answer suggests an unrealistic precision.

But it is unrealistic to cool a power plant with air only, and that is the responsibility of the problem composer, not yours.

##\ ##
Ok. Thank you for the help

BvU

## 1. What is thermodynamics?

Thermodynamics is the branch of physics that deals with the relationship between heat, work, and energy. It studies how energy is transferred and transformed in physical systems, and how these processes affect the properties of matter.

## 2. What is mass flow?

Mass flow is the rate at which mass is transferred through a given cross-sectional area in a given amount of time. It is commonly measured in units of mass per unit time, such as kilograms per second.

## 3. How do you calculate the required mass flow?

The required mass flow can be calculated using the equation: mass flow rate = density x velocity x cross-sectional area. The density and velocity can be determined experimentally or through mathematical models, while the cross-sectional area can be measured or estimated.

## 4. What factors affect the required mass flow?

The required mass flow is affected by several factors, including the properties of the fluid (such as density and viscosity), the geometry of the system, the temperature and pressure of the fluid, and the desired flow rate or velocity.

## 5. How is thermodynamics used to calculate the required mass flow?

Thermodynamics principles, such as the conservation of energy and the laws of thermodynamics, are used to understand and analyze the processes involved in calculating the required mass flow. These principles help to determine the relationships between various factors and how they affect the mass flow rate.

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