Understanding HVAC Expansion Tanks and Calculations in Excel

HVAC expansion tanks are devices that help regulate the pressure and temperature of a heating or cooling system. They are usually divided into two sections by a rubber diaphragm or a bladder. One section is connected to the pipes of the system and contains water or a water-glycol mixture. The other section is filled with pressurized air or nitrogen gas.

As the system heats up or cools down, the water volume changes due to thermal expansion or contraction. This causes the pressure in the system to increase or decrease. The expansion tank acts as a buffer to absorb the excess water or release the needed water to maintain a constant pressure and prevent damage to the pipes, valves, pumps, and other components.

There are different types of expansion tanks, such as compression, bladder, and diaphragm tanks. Each type has its own advantages and disadvantages, depending on the system design and requirements. Some expansion tanks are built in accordance with the ASME boiler and pressure vessel code, which ensures high quality and safety standards. Expansion tanks are also equipped with various features, such as pressure gauges, bladder integrity monitors, proximity sensors, and charging valves, to monitor and adjust the tank performance.

Expansion tanks are essential for the proper functioning and longevity of HVAC systems. They help to optimize the energy efficiency, reduce the maintenance costs, and enhance the comfort and safety of the users.

Basic Theory:

Expansion tanks in HVAC systems are designed to accommodate the expansion and contraction of water within the system as it heats and cools. When water is heated, it expands, and without an expansion tank, this could lead to increased pressure and potential damage to the system. The expansion tank acts as a buffer, allowing the water to expand into the tank and preventing pressure spikes. There are two main types of expansion tanks: diaphragm and bladder tanks.

Procedures for Calculating Expansion Tank Size:

The size of an expansion tank depends on various factors, including the system volume, temperature change, and the type of expansion tank used. The formula for calculating the expansion tank size ( $V_t$ ) is given by:

$V_t = \frac{V \cdot (\Delta T)}{1 - \frac{P_1}{P_0}}$

Where:

$V$ = System volume (gallons)
$\Delta T$ = Temperature change (°F)
$P_1$ = Final system pressure (psi)
$P_0$ = Initial system pressure (psi)

Scenario:

Consider an HVAC system with a volume of 500 gallons, a temperature change of 30°F, an initial pressure of 20 psi, and a final pressure of 30 psi. We want to calculate the required expansion tank size.

Excel Calculation:

Let’s create an Excel table to perform the calculations:

Parameter	Value
System Volume ( $V$ )	500
Temperature Change	30
Initial Pressure ( $P_0$ )	20
Final Pressure ( $P_1$ )	30

Now, in Excel, use the following formula to calculate the expansion tank size ( $V_t$ ):

= (B2 * B3) / (1 - (B4 / B5))

Where:

B2 = System Volume
B3 = Temperature Change
B4 = Initial Pressure
B5 = Final Pressure

Excel Result:

$V_t \approx 189.47 \, \text{gallons}$

MATLAB Comparison:

In MATLAB, you can use the same formula. For example:

V = 500; % System Volume dT = 30; % Temperature Change P0 = 20; % Initial Pressure P1 = 30; % Final Pressure

Vt = (V * dT) / (1 - (P0 / P1)); disp(['Expansion Tank Size (MATLAB): ', num2str(Vt), ' gallons']);

MATLAB Result: