Heat exchanger effectiveness

[eddiej90]

I am designing a heat exchanger to be used in a house to circulate air from the outside. I have very basic knowledge on the subject and would appreciate any input. I have looked up various topics on the subject and am using "introduction to thermal and fluid engineering" by Kaminski. However after reading the procedure in calculations I am still unsure.

I have decided on a counterflow heat exchanger with air to air properties.
I know the volume of the house, and the air change rate is 3 times this per hour.
I know the temperatures of Hot air in and Cold air in.
I also know or have calculated Specific heat Capacity, Density and Mass Flow Rate.

I have set my calculations up on a spreadsheet, however i am now stuck as some equations require Hot temperature out and cold temperature out.

Thanks in advance

 

[mfb]

Those temperatures will depend on the efficiency of your heat exchanger (and maybe humidity).
In the limit of perfect efficiency, they are equal to the temperatures inside/outside.

and the air change rate is 3 times this per hour.

Are you sure this is necessary?

 

[bigfooted]

I don't know this specific book, but usually you will need to make an assumption on your heat exchanger like that it has a constant wall temperature at the contact surface, or a constant heat flux. You can then calculate the outlet temperatures. The book probably deals with these two cases for the heat transfer problem of a fluid flowing through a pipe or something.

 

[AlephZero]

You know one of the outlet air temperatures, because that is the air temperature you want inside the house. (You didn't say whether you are heating or cooling the house, so I don't know if that is your "hot" or "cold" outlet temperature).

If the heat exchanger is operating at a steady state, you know the heat going into the exchanger = the heat coming out. So you can calculate the outlet temperature of the other air stream, from the mass flow rates and specific heats.

Then you get to the hard part: actually designing a heat exchanger that exchanges the right amount of heat energy.

 

[alphy]

I would first like to commend you for taking the initiative to research and learn about heat exchanger design. It is a complex and important aspect of thermal and fluid engineering.

To provide some input on your specific situation, the effectiveness of a heat exchanger is a measure of how well it transfers heat from one fluid to another. It is typically expressed as a percentage, with 100% representing perfect heat transfer. The higher the effectiveness, the more efficient the heat exchanger is at transferring heat.

In your case, using a counterflow heat exchanger is a good choice as it allows for the highest possible effectiveness. This means that the hot and cold air streams are flowing in opposite directions, maximizing the temperature difference between the two fluids.

To calculate the effectiveness of your heat exchanger, you will need to know the inlet and outlet temperatures of both the hot and cold air streams. This is where you may be stuck, as you mentioned that some equations require the outlet temperatures. In order to determine these, you will need to consider the heat transfer rate and the specific heat capacity of the air.

I would recommend consulting with a more experienced engineer or seeking additional resources to help guide you through the calculations. It is also important to consider any other factors that may affect the effectiveness of your heat exchanger, such as pressure drop and heat loss.

Overall, designing a heat exchanger for a house is a complex task, but with proper research and calculations, you can ensure that it efficiently transfers heat and provides a comfortable living environment. Best of luck with your project!