AMCA International inmotion Magazine - Summer 2012 - (Page 10)

Dr. Michael BrenDel VP engIneerIng lau InduStrIeS dayton, ohIo in Achieving Energy Reduction Goals energy consumption of commercial buildings continues to be a major focus of attention as non-renewable energy sources are depleted and overall energy costs increase. The current ASHRAE Standard 90.1 released in 2010 provides a 30% reduction in building energy consumption as compared to the same standard issued back in 2004. A large portion of a building’s energy budget is consumed by the HVAC system. This article explores the role that fan energy efficiency plays in ASHRAE Standard 90.1 and how a proposed revision to that standard will help promote further energy reductions. Building HVAC systems are designed to provide occupant comfort through a combination of heating, air conditioning, and ventilation. Thermal loads and ventilation requirements determine the amount of airflow necessary to achieve HVAC performance targets. A fan provides continuous airflow and overcomes system resistance by increasing the total energy of the airstream. The rate at which energy is added to the airstream is called the air power and it is simply the product of the airflow rate and the fan total pressure rise. In turn, the fan total efficiency is the ratio of the delivered air power to the shaft power supplied to the fan – output power over input power. Clearly, a high efficiency fan will consume less energy than a low efficiency fan, but fan efficiency alone is not a sufficient metric to ensure reductions in fan energy consumption. For example, consider an application using a single AHU (airhandling unit) with a design capacity of 20,000 cfm and an internal total pressure drop of 2 in-wg. Two alternative systems are designed. System A has a total pressure drop of 3.5 in-wg while System B has a total pressure drop of 5 in-wg. Now suppose that the fan selected for System A has a total efficiency of 65% while the fan selected for System B has a total efficiency of 70%. At first glance we might expect that System B would consume less energy than System A because of the higher efficiency fan. However, a simple calculation would reveal that 10 Summer 2012 The role of Fan efficiency this is not the case. The fan input power, or shaft power, required by System A is 26.6 hp while the fan in System B requires 31.4 hp, an 18% difference. This example is summarized in Table 1 in accordance with Figure 1. It is the higher external total pressure drop imposed by System B that is driving the difference, not the fan efficiency. The key point is that the system resistance must be minimized and a high-efficiency fan must be selected to ensure the lowest energy consumption. Existing Requirements Several existing energy standards recognize both system pressure drop and fan efficiency as key factors in reducing energy consumption, although not in an obvious way. For many years, ASHRAE 90.1 has imposed a fan power limitation that has implications for both system pressure drop and fan efficiency. One form of the fan power limitation, Option 2 in the 2010 version of ASHRAE 90.1, states that the combined net shaft power for all applicable fans within a system must be less than a specified value. Written in a generic form, the requirement states: H ≤ αQ + A where H is the fan power limit (bhp), Q is the supply air flowrate (cfm), A is a pressure drop adjustment factor that provides Table 1: Energy consumption of fans. ahU System a Required Flowrate Internal Resistance External Resistance Total System Resistance Fan Total Efficiency Shaft Power inmotion 20,000 cfm 2 in-wg 3.5 in-wg 5.5 in-wg 65% 26.6 hp ahU System B 20,000 cfm 2 in-wg 5.0 in-wg 7.0 in-wg 70% 31.4 hp w w w. a m c a . o r g a m c a I n t e r nat I o na l http://WWW.AMCA.ORG

Table of Contents for the Digital Edition of AMCA International inmotion Magazine - Summer 2012

AMCA International inmotion Magazine - Summer 2012
President’s Message
Contents
From the Executive Director
Code Watch
News
The Role of Fan Efficiency in Achieving Energy Reduction Goals
Fan Motor Efficiency Grades in the European Market
Performance and Reliability Assurance of Custom Engineered and Manufactured Fans for Industrial and Utility Applications
Smoke Damper Testing and Maintenance for Service Life and Performance Assurance
Calendar
Advertisers Index

AMCA International inmotion Magazine - Summer 2012

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