Pump Head Calculation In Hvac

[Janne Evers]

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Head is the height at which a pump can raise water. Basically, designers need to calculate this value in order to select a suitable pump. Pumps play a very important role, especially in water-side systems (i.e., hot and cold water systems), in lifting water up to the desired height range. Height could be easily calculated when there are no losses along the flow path. Since the piping system includes elbows and tee joints, frictional losses should be considered for effective selection.

The farthest routing will be calculated from the CAD/Revit floor plan. Input the values for pipe size, GPM, units, equivalent length of foot, and head loss (ft/100 ft). Calculations will be carried out on the supply, return, and equipment sides. GPM will be known as per the flow distribution and respective pipe sizes. If 4" pipe has a head loss of 3.89 ft/100ft, 60 feet linear feet of strainer will have 2.3 feet of head loss. Calculation is to be continued on similar lines.

Following this, the total pump head will be added, and the obtained head will be used while selecting the pump. The image below shows the pump selection software, where GPM and water head will be used for selection.

Pump head calculations are fundamental in the world of fluid mechanics and engineering. They play a crucial role in determining the performance and efficiency of pumps in various applications, ranging from industrial processes to municipal water supply systems. In this article, we will delve into the essential concepts and equations involved in pump head calculations, shedding light on the significance of these calculations in engineering design.

Pump head, often referred to as total head or total dynamic head (TDH), represents the total energy imparted to a fluid by a pump. It quantifies the combination of pressure energy and kinetic energy that a pump imparts to the fluid as it moves through the system. Understanding pump head is essential because it helps engineers assess the pump's performance, select the right pump for a given application, and design efficient fluid transport systems.

• Static Head (Hs): Static head is the vertical distance between the pump's suction and discharge points. It accounts for the potential energy change due to elevation. If the discharge point is higher than the suction point, static head is positive, and if it's lower, static head is negative.
• Velocity Head (Hv): Velocity head is the kinetic energy imparted to the fluid as it moves through the pipes. It depends on the fluid's velocity and is calculated using the equation:

• Pressure Head (Hp): Pressure head represents the energy added to the fluid by the pump to overcome pressure losses in the system. It can be calculated using Bernoulli's equation:

Understanding this equation allows engineers to design efficient pump systems by considering factors such as the required flow rate, pipe dimensions, elevation differences, and pressure requirements.

1. Pump Selection: Engineers use pump head calculations to select the appropriate pump for a specific application. By determining the required total head, they can choose a pump that can meet these requirements efficiently.
2. System Design: Pump head calculations are crucial in designing fluid transport systems. Engineers can size pipes and select appropriate fittings to minimize friction losses and maximize system efficiency.
3. Energy Efficiency: Understanding pump head helps in optimizing pump operation for energy efficiency. By minimizing unnecessary head, engineers can reduce energy consumption and operating costs.
4. Maintenance and Troubleshooting: Monitoring pump head over time can help detect changes in system performance, indicating the need for maintenance or troubleshooting issues such as blockages or leaks.

To illustrate the concept of pump head calculations, let's consider a simplified scenario involving a water pump used for irrigation. In this scenario, we want to determine the total pump head required for efficient water distribution from a reservoir to a field.

In this example, the total pump head required for the irrigation system is 30 meters. This means the pump must be able to provide enough energy to lift the water 20 meters vertically, overcome frictional losses, maintain a certain velocity, and provide additional pressure as needed.

Pump head is an essential parameter for chilled water pumps as well as condenser water pumps. It determines if the pumps are able to deliver the required water flow rate. So, how do you calculate the required pump head?

To calculate chilled water pump head, first determine the total water flow required. Then, identify the farthest equipment on the piping system. Next, find the total head loss of the longest piping loop. Finally, calculate the required pump head using the Hazen-Williams Equation.

I, unfortunately, did not have the chance to work on the detailed design of chilled water pumps and chilled water piping systems. Hence, I seek help from one of my friends who had done pump head calculations before.

There are a few ways to calculate pump head but the one that I find quite easy to understand and implement is by using the equivalent pipe length method with the Hazen-Williams Equation.

Imagine when you go to the gym. A guy says he can do 10 reps. But, there is no context to it. What is 10 reps? He must also let you know 10 reps at what weight. For instance, 100 lbs. So, he can do 10 reps at 100 lbs.

The same applies to chilled water pumps and condenser water pumps and basically any pumps. A pump delivers a certain amount of water flow after it overcame all of the friction or technically known as head loss.

When the head loss of a valve or pipe fitting is expressed in meter, it is equivalent to the head loss of straight pipe in meter. For instance, if a valve has a head loss of 0.35 meter, it has the same resistance (head loss) as a 0.35 meter long straight pipe.

The Hazen-Williams Equation is also suggested by ASHRAE as one of the methods to calculate pump head. However, it is applicable to water only which is not a problem for HVAC chilled water pumps and condenser water pumps.

Basically, you need to finish like 99% of the chilled water piping system design. The last thing you need to do is to calculate the total head loss of the system and then, select the suitable pump to deliver the required water flow rate.

Then, you also need to find out the water pressure drop of the associated equipment along the way. Usually, for equipment like AHUs and FCUs, you can find the water pressure drop in the datasheet provided by the manufacturer.

Once again, you can get my Chilled Water System (eBook) to quickly learn more about chilled water system. But, if you want to learn how to design a chilled water system from start to end, I encourage you check out my Chilled Water System Design Course.

Yu is an HVAC professional with over 10 years of experience. He design, install, commission and service various kind of HVAC systems. Ever since he created this website, Yu has helped hundreds of homeowners and engineers.

In close loop systems there is no need to include the vertical height because the return side (which is also vertical, downward) cancel out the supply side. You only need to account for the friction loss and equipment pressure drop.

If a coil has a given pressure drop, X PSIG, at some Y GPM flow rate, then we would expect to measure Y GPM when X PSIG is measured as the pressure drop. Example: A coil has a design flow rate required of 650 GPM. At this flow rate, the coil has a pressure drop of 10 feet, the pipe and valves at the coil have a pressure drop of 3 feet, and its two-way control valve has a pressure drop of 12 feet. At 650 GPM, the total pressure drop is 25 feet. If a pump is delivering 25 feet to this zone, the flow should measure 650 GPM if the two-way control valve goes wide open. If, when the valve opens, there are only 15 feet of head available, there will not be 650 GPM flow rate. 25 feet is the control head of this coil and valve.

When we add a variable speed drive (VFD) to this pump and put a differential sensor (DP) across the pump, what will happen? If the pump is off and there is a call for cooling, the two-way valve will be open, but at what speed will the pump operate?

The constant speed pump using the PLEV profile would cost $6,800 per year based on 24/7 operation. The variable speed pump with a control head of 60 with the DP sensor across the pump would cost $5,300 per year. The savings would be in the hatched area below.

What happens if the sensor is moved out in the system so it includes the coil, two-way control valve, and interconnecting piping? Now we can take advantage of the variable pressure drop in the supply and return piping as well as the mechanical equipment room. The control head is 25 feet instead of 60 feet. The curve below shows the improved area of savings. The area under the curve is greater resulting in greater savings. The cost of operation is $3,477 per year. An additional savings of over 15% from the sensor located at the pump.

The variable speed hydronic system control head is the minimum pressure differential that the variable speed pump should product to assure the pump can provide the required flow rate to the index circuit. In hydronic systems the value of control head is the differential pressure drop in feet of head downstream from the sensor location to achieve design flow rate.

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