Process air heat exchangers

[waiwhite]

G'day,

I am working on a small project where I have to look into/verify the selection of a replacemant heat exchanger for a process loop.

Just about all of the information I have been able to locate is about getting rid of heat from the liquid going through the heat exchanger with the air blown through it being essentially incidental.

The particular application is essentially a nitrogen enriched air (NEA) grinding circulation loop where the material is processed by an air classifier mill (ACM). The ground material is drawn off by the air classifier and passed through a bag house type dust collector with the filtered NEA has to be cooled and returned to the ACM.

We have a water chiller that delivers chilled water at around 4 C, and the manufacturer rates the cooling capacity of the chiller at around 100 kW with a 30 kW input.

The NEA exiting the dust collector comes through a 300 mm dia duct at a rate of around 236 litres/second and then enters the tube/fin heat exchanger chamber to pass through the exchanger core that is 1.37 m x 1.38 m and 440 mm thick. I have worked out nominal air speeds at the entry into the chamber at around 3.32 m/s and at the first face of the core to be around 0.125 m/s assuming that the NEA is distributed evenly across the core. This gives a contact time through the core of around 3.5 seconds.

What I am trying to find is something that can point me to whether this contact time is enough to get the desired temperature drop. All of the information I can get talks about the rate of heat gain or loss through a surface. What I need is a coefficient that can give me the rate at which air/nitrogen/NEA can gain or lose heat. We have temperature indicators at the inlet and outlet of the chilled water through the core, but we don't have temperature indicators as such on the NEA loop other than an overtemperature indicator.

It comes down to what contact time we need to have through the heat exchanger core to get the temperature of the NEA down to what is desirable. The suppliers guard the selection methodology only the existing unit has not functioned as claimed and so we are a touch cautious.

I have looked into other types of heat exchanger, however the plant and has to be thoroughly dismantled and cleaned down inbetween each change of product. As such, completely enclosed exchangers that cannot be taken apart cannot be used.

Anyhow, any pointers would be appreciated.

Thanks.

 

[Achy47]

Thank you for sharing your project and concerns regarding the selection of a replacement heat exchanger for your NEA grinding circulation loop. As a scientist in this field, I am happy to provide some insights and guidance based on your specific situation.

Firstly, I understand your concern about the contact time of the NEA through the heat exchanger and its effect on the desired temperature drop. In order to determine the appropriate contact time, we need to consider several factors such as the heat transfer coefficient, the surface area of the heat exchanger, and the temperature difference between the NEA and the chilled water.

The heat transfer coefficient, also known as the convective heat transfer coefficient, is a measure of how easily heat is transferred between the NEA and the chilled water. It depends on several parameters such as the fluid properties, flow rate, and geometry of the heat exchanger. In your case, the NEA is a mixture of air and nitrogen, and its properties can be calculated using the ideal gas law. The flow rate can be adjusted by controlling the fan speed, and the geometry of the heat exchanger is already known to you.

Next, the surface area of the heat exchanger plays a crucial role in determining the contact time. The larger the surface area, the longer the contact time and thus, the greater the temperature drop. From your description, the surface area of your heat exchanger is 1.37 m x 1.38 m, which seems adequate for your application.

Lastly, the temperature difference between the NEA and the chilled water is also a key factor in achieving the desired temperature drop. As you mentioned, the chilled water is at 4 C, and the NEA is at a higher temperature. The greater the temperature difference, the faster the heat transfer will occur, and thus, the shorter the contact time needs to be.

Now, in order to determine the appropriate contact time, we need to use a heat transfer calculation method such as the Log Mean Temperature Difference (LMTD) method. This method takes into account the above-mentioned factors and provides a calculation for the required contact time to achieve a desired temperature drop. I would recommend consulting with a heat exchanger manufacturer or a heat transfer expert who can assist you in using this method to determine the appropriate contact time for your specific application.

In conclusion, the contact time for your NEA through the heat exchanger can be determined by considering the heat transfer coefficient, surface area