Energy vs. Airflow: The Battle Is Really Cost vs. Comfort

BELOIT, WI, August 31, 2017 — Regal Beloit Corporation is a global leader in the manufacture and distribution of electric motors, mechanical and electrical motion controls, and power generation products, with operations in 27 countries. Regal's product brands meet customer requirements in demanding applications used around the globe in industries such as: heating, ventilation, air conditioning, commercial refrigeration, food processing, pharmaceutical, chemical processing, material handling, medical, oil and gas, construction, manufacturing, power generation, agriculture and mining. To ensure the success of our customers throughout the lifecycle of our products, Regal employs a global service team that focuses on design, installation, troubleshooting, condition monitoring, product repair/rebuild, and application training.

Abstract

There's a popular misconception that increasing the efficiency of a belt drive results in a decrease in energy consumption. This is not always the case; for a true analysis, one must also look at the resultant airflow. The impacts on airflow extend beyond the HVAC box; dampers, building management systems and system setup all play roles. This article will look at the systematic effect of all mechanical and electrical components as they relate to the overall efficiency of a system and their consequential effects on airflow.  

Ensuring Peak Performance

Ensuring peak performance of a commercial heating, ventilating and air conditioning (HVAC) system can be a difficult proposition. Commercial and industrial air handlers are used to exchange, condition and circulate air to achieve occupancy comfort in a given space. These systems typically contain a fan, heating and cooling coils, dampers, actuators and controls, filters and a belt drive. The air handlers are connected to a system of ductwork that carries the conditioned air throughout a building. You then must understand the different types of systems: constant air volume (CV); variable air volume (VAV); and multi-zone. The efficiency of these systems depends on the operational conditions of each component in the series. If any fails to perform, the whole system suffers. 

According to the U.S. Department of Energy (2009), the commercial sector spent $192 billion on energy; of that, $80 billion (41 percent) was spent on HVAC equipment. Costs are going to rise in conjunction with electricity prices. For that reason, building engineers, owners and managers are looking at the performances of their equipment and deciding if they can improve them, either through retrofitting or recommissioning. Couple this with the fact that half of commercial buildings in the U.S. are more than 36 years old, and you'll see many people sharing a similar challenge. 

The efficiency of fans must always be measured in terms of total system efficiency, where each of the components' individual efficiencies are added together. Losses from all the components in a system, including the electric motor, belt drive, frequency drive, aerodynamic design and efficiency of the fan wheel all go into reducing a system's total efficiency. 

The total efficiency is measured by comparing the electrical power consumption of the electrical motor to the mechanical power output of the fan. 

Htotal = Hmotor + Hbelt drive + Hfrequency drive + Hdynamic loss + Hfan wheel

Motors (Hmotor) are a critical component to a system's efficiency. Brushless DC motors, which are electronically commutated (ECM), are highly regarded within the industry. These motors are becoming more prevalent in the residential sector and are starting to be seen in higher-end furnaces. Government regulations are driving this sector to adopt this technology quickly. 

The commercial sector is dramatically different. NEMA premium motor is still considered by many to be the top-of-the-line integral horsepower motor (IHP), as it currently boasts the highest efficiency rating. However, with permanent magnet motors starting to be released in an IHP NEMA footprint, encompassing a similar architecture to their residential "little brother," the standards are about to increase. 

Motors are an easy upgrade justification; it simply is a matter of how efficiently they can translate energy into motion. With every percentage increase, there is a subsequent decrease in energy consumption. 

A belt drive (Hbelt drive) relies upon multiple variables to function properly. A belt drive's ability to efficiently transmit power from a motor to a fan depends upon several factors:

  • Belt tension
  • Sheave wear
  • Alignment
  • Belt selection
  • Quality of components used

If any of these variables aren't properly maintained, the system won't perform at its intended design level. The belt will slip, the fan won't deliver the required airflow, and the power consumption will decrease as the motor becomes unloaded. 

Many building engineers don't view the air exchanger from the perspective of maintaining peak efficiency and rarely monitor the cubic feet per minute (cfm) output of the fan. As such, best practices for installation and maintenance of belt-driven components often aren't prioritized, and there is a general lack of knowledge in the industry as to the effects of improper procedure as a function of airflow. The result is the belt drive almost always operates below its peak efficiency. 

Increasing the efficiency of a system always results in better performance, but a popular misconception is that increasing the efficiency of the belt drive will reduce energy consumption. This is not always the case; sometimes it can increase energy usage. 

If you were to ask, "What's the least amount of energy the air handler can use?", the simple answer is: don't turn it on. Obviously, this wouldn't make the fan spin and thus provide no ventilation to the space — which is the whole point of the system — so that won't work. A better response to "What is the least amount of energy the air handler can use?" needs to have the caveat of making sure it's providing the proper airflow. So, consider this: the optimal efficiency of an air handler is its ability to produce the optimal airflow while requiring the least amount of energy to do so. 

 

Per the affinity laws, cubic feet per minute of airflow is directly proportional to the rotational speed of the fan in revolutions per minute (rpm). Fan power is proportional to the third power of the ratio of shaft speed:

Q1 = Q2 x (N1/N2)

P1 = P2 x (N1/N2)3

Where Q is the volumetric flow rate, P is the power, and N is the rotational speed. This means that if the fan speed is increased by 10 percent, the resultant power consumption increases by 33 percent! 

If you increase the efficiency of the belt drive, you decrease the slippage at the fan sheave; subsequently, the fan rpm increases. But since the motor is now doing more work to drive the increased load, and as was just demonstrated, this would increase the power consumption if all factors remained equal.

One parameter that can affect the power consumption is the difference between wrapped and notched v-belts. Wrapped v-belts are only at best 95 percent efficient, whereas notched are up to 98 percent. This increase in efficiency is due to several factors, one of which is a lower bending stress. So, one way to increase efficiency and reduce the increased energy consumption is to use notched v-belts wherever possible.

So the big question becomes: "Is that extra airflow needed?" If so, the by-product very well may be an increase in energy expenditure. This is because the motor is doing more work to create that extra air. Of course, if extra airflow is needed, then the biggest benefit is increased occupancy comfort, which is the top priority of any well-designed HVAC system. If extra air is not needed, the ratio of the belt drive can be changed to keep the driven speed equal to what it was before installing the new components; the resultant will be a decrease in energy due to the lower fan speed and increased power transmission efficiency. 

Today, many HVAC systems utilize variable frequency drives. These can either speed up or slow down the fan to meet static pressure requirements. So, if a belt drive is operating inefficiently (i.e., slipping), the VFD will tell the motor to speed up; increasing the motor speed increases its power consumption. A belt drive operating at peak efficiency will allow the motor to hit static pressure easier, and thus the VFD will tell the motor to slow down, resulting in significant energy savings.

We highly recommend working with an air balancer to ensure that the space is receiving the proper airflow after you've optimized the system and installed the new belt-driven components to the manufacturer's specifications. This will ensure that you're maintaining the proper airflow in the system, maximizing occupancy comfort, and minimizing the electrical energy consumption to make it all happen. At the end of the day, the least amount of work you must do to get a job done correctly is your best approach. The same philosophy applies to your HVAC equipment. 

References:

Source: http://www.eia.gov/consumption/commercial/reports/2012/buildstock/index.cfm
U.S. Energy Information Administration
A Look at the U.S. Commercial Building Stock: Results from EIA's 2012 Commercial Buildings Energy Consumption Survey (CBECS) – March 2015    

Source: https://www1.eere.energy.gov/buildings/publications/pdfs/corporate/bt_stateindustry.pdf
Energy Efficiency Trends in Residential and Commercial Buildings

Source: http://www.engineeringtoolbox.com/fan-affinity-laws-d_196.html
Fan affinity laws

About Regal Beloit Corporation

Regal Beloit Corporation (NYSE: RBC) is a leading manufacturer of electric motors, electrical motion controls, power generation and power transmission products serving markets throughout the world. The company is comprised of three business segments: Commercial and Industrial Systems, Climate Solutions and Power Transmission Solutions. Regal is headquartered in Beloit, Wisconsin, and has manufacturing, sales and service facilities throughout the United States, Canada, Latin America, Europe and Asia.

Regal Beloit Corporation Application Considerations

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Media Contact
Jackie Catalano
Director, Branding and Marketing Communications, Regal
Tel:  859 727 5271


Technical Contact
Jesse Dupius
North American HVAC Industry Manager, Regal
Jesse.Dupius@regalbeloit.com
Tel:  647 222 8299

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