# Design of Expansion Tank in Chilled Water System

## Cleaned Transcript

Good morning, friends. In today's video, we are going to see the design of an expansion tank. It is one of the important items in a chilled water system located in the chilled water pump room or energy transfer station. The expansion tank is used for multiple reasons. One of the important points is that in a chilled water system, we have temperature fluctuations and thermal expansion. To overcome these issues in a closed-loop system, we use an expansion tank.

When we design the expansion tank, we need to add a few values carefully, or it will result in over-design or undersizing of the expansion tank. Let's discuss each requirement one by one.

Here is the formula for the expansion tank design, which I have taken from the Azure application handbook. For example, if you see here in the Azure application handbook, you can find the same formula. This formula is what I am going to use for the design of the expansion tank.

The first thing is the volume of water in the system. To find out the volume of water in the system, consider you have a closed-loop system like this. You have to consider the complete pipelines from the chiller to the last FCU, the complete supply and return pipelines. You also need to consider all the equipment, like chillers, AHUs, and FCUs. You have to consider the number of chillers and the number of FCUs you are using throughout the project.

For this project, I have used three pipe sizes: 50 mm, 75 mm, and 110 mm. Each pipe size has a different length: 8,500, 4,800, and 1,500 feet, respectively. We need to find out the gallon per foot. For example, for different pipe sizes, the gallon per foot volume is given. For instance, for a two-inch pipeline, it is 0.163 gallons per foot. I will add this value here. For the 75 mm and 110 mm pipe sizes, I will apply the values 0.367 and 0.653, respectively.

This is for one foot of pipeline. In our project, we have a total of 8,500 feet of two-inch pipeline. I will multiply to get the total gallons of the pipelines. This is not for the fittings or the equipment, just the pipelines. We have to consider a minimum of five percent additional as a safety factor because site conditions may vary. I always consider five percent. With this additional factor, we get 4,710 gallons.

In addition to that, we have to consider the equipment's water level. For example, in our project, we have two chillers, ten AHUs, and 700 FCUs. For the chiller, the fluid volume is given as 269 liters. Similarly, for the AHU and FCU, you can get the value from the technical data sheet. For the chiller, I added the same value: 269 liters. There are two chillers, so it is 538 liters. For the AHU, based on the technical data sheet, I consider 50 liters per unit, totaling 500 liters. For the FCU, I consider 5 liters per unit, totaling 3,500 liters. Converted to gallons, it is 1,198 gallons. Adding a five percent safety factor, the total is 5,969 gallons.

Next, we consider the lower and higher temperatures. For example, in a chilled water system, the temperature is 5.5 degrees Celsius, and the temperature from the building load is 11.5 degrees Celsius. The minimum temperature is 5.5 degrees Celsius, which we consider in this column. Converted to Fahrenheit, it is 41.9 degrees. The higher temperature is when the pump is off. We cannot consider the air temperature as the water temperature. Practically, the temperature inside will be about five degrees lower than the air temperature. If the outside temperature is 46 degrees, we can consider 41 degrees Celsius.

For the pressure values, we consider the atmospheric pressure, which is 1.01325 bar or 14.7 psi. The minimum pressure at the tank is when the system is not running. If the expansion tank is located on the ground floor, we consider the vertical static head. For example, if the height is 40 meters, we consider 4 bar. Similarly, if the expansion tank is located on the roof, we consider the same static head.

In our case, we have a 44-meter head building with the expansion tank on the ground floor. We consider 4.4 bar, converted to 64 psi. The absolute pressure at a lower temperature is the pressure value plus atmospheric pressure, totaling 79 psi. For the maximum pressure, we consider the pump head calculation, which is 6.9 bar, converted to 100.05 psi. Adding a correction factor of 10 psi, the total is 110.05 psi.

The specific volume of water at lower and higher temperatures is also considered. At 5.5 degrees Celsius, the specific volume is 0.01602 cubic feet per pound. At 109.4 degrees Fahrenheit, the specific volume is 0.016.

The linear coefficient of thermal expansion for carbon steel is 6.5 x 10^-6. This value is applied in the formula for the expansion tank design.

The formula is: Volume of Expansion = Volume of Water in the System (Vs) x (V2/V1 - 1) x (α x ΔT) / (1 - P1/P2). Applying all the values, we get the correct answer in gallons. Converting to liters, we multiply by 3.785.

Finally, we select the nearest expansion tank size. For example, if our answer is near 500 liters, we select a 500-liter expansion tank. Two important points to consider are the flange connection and working pressure. The flange connection can be ASME B16.5 (American standard) or BS EN 1092 (British standard). The working pressure of the system must also be satisfied.

These are the key considerations for selecting an expansion tank. Thank you for watching the video.