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What is head? definition

Head refers to how high (meters or feet) a pump can move water or another fluid up through a building. Head is a crucial part of calculating what types of pumps you will need in a system, and how they should be arranged.

For Example
Let's say you have a single pump in the basement of a 6-story hospital building, but that pump's head is not capable of pumping the water up to that 6th floor.

You might think you can just increase the waterflow rate (pump water through faster), but this would actually decrease the head because the increased flow causes more friction in the system.

One way to increase the head of the system is to add another pump in series in that system. These two pumps could maintain the same flow rate while combining their separate head measurements to move the water up through the building.

What is an air-side economizer? definition HVAC

An economizer is not always a single component but rather a control method that relies on several components in a system. Primarily, an economizer is used to reduce energy consumption.

In HVAC Applications
An air-side economizer includes programming the system to bring in more outside air when its temperature is lower than the inside air. The system opens up an outside air damper to bring in more outside air. To balance the airflow volume in the system a return air damper in the return air duct closes by the same amount.

Since the outside air is already chilled, there isn't a need to push chilled water all the way from a chiller in a central plant to the air handling unit's chilled water coil to absorb heat from the air to cool it down.

This reduced need for mechanical cooling allows the system to save on that energy consumption.

Series and parallel pumps: what's the difference?

Series Pumps
Pumps in a series are installed one after another in the piping. This is usually to achieve the necessary head to pump water from a lower level to a higher level while maintaining pressure standards and waterflow. This is possible because two pumps in series allow their respective heads to be added together to achieve a higher overall head. However, these pumps will pump at a constant (not variable) flow rate.

One disadvantage of series pumps is if one pump goes out, the waterflow is interrupted at that point in the system.

Parallel Pumps
Pumps in parallel are arranged parallel to one another, rather than one after another. This allows each pump to function independently of the others. If one fails, the waterflow will not be interrupted, or at least minimally. Parallel pumping is often used to achieve high waterflow rates. The waterflow rate of each pump can be added to the others to increase this value and match the load. Head values of parallel pumps are not additive, however, as you see in series pumps.

What is head? Click here to learn more.

What is static pressure? definition HVAC

Static pressure is the pressure created by everything else in the ductwork that resists the airflow from the fan. This can include anything from the ductwork itself to the various dampers, coils, and other components installed in the system.

Static pressure is measured in "inches of water column" or simply "in. wc".

When static pressure increases, the HVAC system must work harder to overcome it. This means the system must increase the volume of air it is pushing and which also increases the energy being used.

How much more energy is needed? The 3 Fan Laws can help which you can read about here. 

Why is static pressure measured in "in. wc"? Why not PSI?

Why is static pressure measure in "inches of water column"? Why not PSI?

Water column (wc) is related to PSI, or pounds per square inch.

1 psi = 28 in. wc
1 in. wc = 1/28 psi

1/28 psi is a small value. Using "in. wc" helps us represent these smaller pressure values in larger units

The History and Reason for Inches of Water Column
U-tube manometers used in older systems to measure static pressure were installed with water inside a u-shaped portion of the piping. As gas or some other medium filled the piping, it pushed the water through the u-tube and created a column of water. As the pressure of the gas or medium increased, it pushed the water even higher up on that column. This column, as you can imagine, was measured in "inches."

Here's a YouTube video you can check out for more information.

What are the 3 Fan Laws?

The 3 Fan Laws help us understand how different variables in an air-side system affect one another. While the math behind them may seem complex, you don't need to know them to practically apply these 3 laws.

The basic takeaway
Fan Law #1 shows us that if the fan speed changes, the airflow volume will change at the same rate. Fan Law #2 shows us that if the fan speed changes, the static pressure will change at the square of the change of the fan speed. Fan Law #3 shows us that if the fan speed changes, the power will increase at the cube (to the third power) of the change in fan speed.

Minor changes to variables in your HVAC system can cause bigger changes to other variables in your system, which is important for maintaining the system with normal operating parameters based on its design.

Want to go even deeper? We can break everything down for you so it makes more sense.

CFM2 = CFM1 x (RPM2 / RPM1)

Okay, but what does that mean?

CFM2 - The new airflow volume in "cubic feet per minute."
CFM1 - What the airflow volume was before in "cubic feet per minute."
RPM2 - The new speed of the fan motor in "revolutions per minute."
RPM1 - What the speed of the fan was before in "revolutions per minute."

What this shows us is that the change in airflow volume in a system is exactly proportional to a change in fan speed.

For example, if you need to increase your airflow volume by a certain percentage, you'll have to increase your fan speed by that same percentage to achieve the desired airflow volume.

SP2 = SP1 x ( (RPM2 / RPM1) ^ 2 )

Here we see a relationship between the static pressure changes in a system and the fan speed. But what do all of these variables mean?

SP2 - The new static pressure in the system.
SP1 - The old static pressure in the system.
RPM2 - The new speed of the fan in "revolutions per minute."
RPM1 - What the speed of the fan was before in "revolutions per minute."

This shows us that the static pressure in a system will change in proportion to the square of the change in fan speed. In other words, if your fan speed increases by 5% (1.05), the static pressure will need to increase by the square of that change: (1.05 x 1.05 = 1.10) or a 10% increase.

For example, a static pressure of 2.0" wc would increase to 2.20" wc with just a 5% increase in the fan speed.

HP2 = HP1 x ( (RPM2 / RPM1) ^ 3 )

Here we see a relationship between the power used by the fan motor and the fan speed. In other words, the energy being used is related to the speed of the fan in this formula. But what do all of these variables mean?

HP2 - The new amount of power being drawn to the fan motor in horsepower, but could be represented in other units.
HP1 - What the power being drawn to the fan motor was before.
RPM2 - The new speed of the fan in "revolutions per minute."
RPM1 - What the speed of the fan was before in "revolutions per minute."

This shows us that the power needed to meet the demand will change in proportion to the cube of the change in the fan speed. In other words, if your fan speed increases by 5% (1.05), the power will need to increase at the cube of that 5% change: (1.05 x 1.05 x 1.05 = ~1.16) or 16%.

For example, if a variable frequency drive is currently allowing the fan motor to operate at 10 horsepower, a 5% increase in fan speed would require 15% more horsepower to keep up with the load: 11.5 horsepower.

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