Circuiting of fluid coils is important to the performance and life of the coil, yet for even seasoned professionals it can be confusing. However, it really isn't as difficult as it seems.
What is circuiting?
In the simplest terms, circuiting means the number of tubes on any coil being fed by each header. Every coil has a specific number of rows, and a specific number of tubes within each row. For example, in the illustration below we show a coil that is 8 tubes high and 4 rows deep for a total of 32 tubes (in another case, you could have a coil that is 24 tubes high and 8 rows deep for a total of 192 tubes). In this initial example, the "supply" enters from the right and the "return" exits on the left. (Note: Most fluid coils have the supply and return connections on the same end of the coil.)
Hot or cold fluid enters through the supply connection, fills up the manifold, and then simultaneously feeds the tubes in each circuit. The number of tubes that are fed by each header (or number of circuits) ultimately depends on the performance you need from that coil. Circuiting is really a balancing act of tube velocity and pressure drop.
Determining the right number of circuits
To ensure proper heat transfer, fluid must travel through coils at the right speed. Some coils only have a small number of circuits while others have quite a few. If fluid travels too quickly through the coil, the heat transfer will be inefficient, the coil could have a high pressure drop, and this situation could cause tube erosion. If it travels too slowly, only a little heat transfer will occur. By selecting the appropriate number of circuits, you control the fluid speed and thus the heat transfer efficiency of the coil.
Fewer circuits serves to speed up the fluid in the coil, while more circuits slow it down. For example, feeding a larger number of tubes simultaneously spreads the flow of fluid to multiple circuits, which slows down the speed at which it flows through the coil. The number of circuits and the inside diameter of the tubes allows you to calculate total flow rate (in GPM) of the coil.
If a coil has 8 tubes per row and is 4 rows deep, as shown in our example, then the circuiting is as follows:
- Half Circuit – 4 feeds
- Quarter Circuit – 2 feeds
- Full Circuit – 8 feeds
- Double Circuit – 16 feeds
Finally, here are 3 rules of thumb to follow regarding coil circuiting:
- The number of tubes you feed must divide evenly into the number of tubes in the coil or you will have dropped tubes (tubes that aren't fed fluid).
- The coil must have an even number of passes if you want connections to end up on the same end of the coil. With an odd number of passes, you will have fluid connections on opposite ends.
- The number of circuits will determine the resulting pressure drop by determining the number of circuits and thus the fluid velocity and effective tube length for each circuit (acceptable fluid tube velocity is between 2 and 7 feet per second).
If you want to ensure that the most efficient number of circuits are being used for your replacement coils, simply contact the team at Campbell-Sevey.