Engineered Systems - March 2008 - (Page 61)

but also due to the fact that in most areas of the country, the requirements of ANSI/ASHRAE/IESNA Standard 90.1 also require the use of an airside or waterside economizer. The airside economizer is most commonly employed and varies the amount of outdoor air introduced for energy conservation purposes. When an airside economizer process is employed in conjunction with volumetric flow reduction in the space, to maintain a given rate of outdoor air introduction the primary outdoor air fraction of the air to the space (Zp) must increase as primary airflow to the space decreases. This means that as ambient conditions get colder, and the airside economizer reduces the primary outdoor air fraction at the system, more air must be processed, conditioned, and delivered to the occupied space to provide a given level of ventilation. As a result, using the mixed air path to introduce outdoor air is a very poor design choice. Another factor that makes this challenge especially difficult for VAV reheat systems is that they provide primary supply air to the space based on the need for sensible cooling. In addition to people, this includes heat transmission through the exterior envelope and interior partitions, solar gains, and heat from lights and any kind of power consuming equipment. External thermal load characteristics vary with time of year as well as time of day. Designed equipment loads may be off, on standby, may have never even been installed, or are different than planned. Many HVAC engineers overestimate these loads in their computations “just to be safe.” Because “design” cooling loads rarely occur, the actual amount of primary air delivered to a space is usually less, and frequently much less, than design flows. As a matter of practice, overdesign overdrives minimum airflows and artificially increases the need for terminal reheat. ASHRAE Standards 62.1-2004 and 90.1-2004 provide clear direction in this situation. Section 6.5.2.1 of Standard 90.1 prohibits the use of all simultaneous heating and cooling, even when the cooling is accomplished as part of an airside economizer. Reheat is permitted only under limited exception clauses. As a general rule, it can be easily demonstrated that to deliver a given amount of ventilation air, more energy is consumed in the form of fan and reheat energy by reducing the outdoor air fraction and delivering more air than by increasing the outdoor air fraction and reducing the total amount of air delivered. The exception to this is when a system’s outdoor air fraction is driven by an anomalous space, such as a conference room. There is also a point where the use of recirculation in a classical VAV reheat design becomes cost-ineffective and energy-inefficient. Section 5.4 of Standard 62.1-2004 tells us the following: “Ventilation System Controls. Mechanical ventilation systems shall include controls, manual or automatic, that enable the fan system to operate whenever the spaces served are occupied. The system shall be designed to maintain the minimum outdoor airflow as required by Section 6 under any load condition.” It also tells us: “VAV systems with fixed outdoor air damper positions must comply with this requirement at minimum supply airflow.” This is why since the 1998 edition, the IMC has required “special controls” for VAV systems other than those delivering 100% outdoor air. Those familiar with Standard 62-1989 should recognize 1 the above equation as the denominator from the l p q t multiple spaces equation. What this essentially means is that instead of making this computation once at the system level, Standard 62.12004 now requires it to be computed for each zone. This essentially gets us to the same end-point, although via a more roundabout and torturous route. SPECIAL CONTROLS The need for “special controls” has been touched on superficially in the ASHRAE Handbook – Systems and Equipment4 since 1992, but fails to tell designers how to actually accomplish this. This can, however, be derived by understanding the requirements of Standards 62.1 and 90.1 and the subject of air-handling system control. Most HVAC system designers erroneously believe that minimizing the outdoor air fraction on a VAV system saves energy. However, when introducing outdoor air for ventilation through the mixed air path, precisely the opposite is true. Reducing total system mass flow and increasing the outdoor air fraction on the system reduces the amount of outdoor air which must be introduced, preheat energy, reheat energy, and fan energy requirements. It also reduces indoor air contaminant levels by breaking the cycle of concentration recirculation causes. The best economies actually occur when minimum airflows are delivered, and flow potential is most minimized when 100% outdoor air is used. The energy implications of this can be easily verified with a few relatively simple computations. To effectively deliver adequate ventilation with a VAV reheat system, the system controls need to accomplish the following: • The system must be able to measure and monitor, in real time, the delivered outdoor air fraction at the system. Theoretically, this could be accomplished several ways: • Monitoring outdoor air and supply air totals. • Monitoring outdoor air and return air totals. • Monitoring outdoor air, return air, and mixed air temperatures. • In all cases, flow measurement is required at the VAV box. • Designers should also pay particular attention to the capabilities and limitations of the flow measurement technologies employed, and entrance conditions can make, or break, a design. w w w. esmag a zine . c o m 61 http://www.esmagazine.com

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Engineered Systems - March 2008 Contents Editor’s Note Back2Basics HVAC Challenge Case In Point Commissioning Building Automation Energy Wiz HVACR Designer Tips Application Checklist Exemplary Design = Elementary Success The Modern Unit Ventilator On A Mission VAV Systems And Green Design – Part II Issues & Events Products Glossary Classifieds Advertiser Index Tomorrow’s Engineer

Engineered Systems - March 2008

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