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Description
INTRODUCTION
Dedicated outdoor air systems (DOAS) (Mumma 2001), when used, are generally required to employ total energy recovery, or an enthalpy wheel (EW), by ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings (ASHRAE 2004b). The enthalpy wheel transfers excess moisture and sensible heat contained in the outdoor airstream to the relatively dry and cool exhaust airstream during the summer (i.e., precooling and dehumidification). Similarly, the entering outdoor air (OA) is heated and humidified during the winter by recovering sensible heat and moisture from the relatively warm and humid exhaust air. This free heating and humidification is the advantage that should not be overlooked in the EW application. However, observed systems with EWs have often not utilized the EW to maximum benefit, particularly during periods of low ambient dew-point temperatures (DPTs). When the OA is dry, humidification of the supply air (SA) is required to maintain the space relative humidity (RH)--a key health and thermal comfort factor in buildings. ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy (ASHRAE 2004a), recommends that the RH be maintained in the 30%-60% range.
Most technical literature on EWs is focused on their operation as an aid during cooling to reduce both the design load and energy use. Consequently, no simple and universally accepted EW humidification performance control sequence currently exists for low DPT OA conditions, hereafter referred to as dry conditions. The objective of this paper is first to justify the use of the EW for humidification under dry conditions, then introduce a simple binary EW control. The EW binary control was evaluated in a university campus facility (Mumma and Jeong 2005).
Enthalpy Wheel Overview
In the mid 1970s, two EW products were introduced to the HVAC industry. One was the oxidized aluminum wheel made of corrugated aluminum foil. The corrugated aluminum foil assembly is dipped into a bromide solution to cause the aluminum to oxidize and form a layer of alumina, a known desiccant. The second EW product introduced then used silica gel as the desiccant. The silica gel desiccant was bonded to an aluminum foil assembly, or matrix.
In the 1980s, molecular sieves, also known as synthetic zeolite desiccants, that could be designed at the molecular level emerged. Fabrication of silica and other compounds were derived from the semiconductor industry. Manufacturing processing advances that allowed a breathable layer of desiccant to be bonded to the corrugated aluminum foil surface made molecular sieve EWs possible.
The type of desiccant used determines the moisture transfer performance of the EW, while the sensible heat transfer of the wheel depends on the thermal properties of the matrix-desiccant material combinations. Currently the vast majority of EWs employ either silica gel or molecular sieve desiccants.
Silica Gel. Silica gel can absorb up to 40% of... |
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