Engineered Systems - February 2009 - (Page 24) Maintaining minimum requirements may involve more than you think. However, while there are some areas — such as certain sensors — where you won’t want to skimp, owners do have some flexibility in reaching acceptable performance with lowest life-cycle cost. BY BRIAN SIKORSKI, P. ENG., CEM ometimes a requirement for humidification in a building is poorly defined, over-designed, and either over or underutilized. Humidity levels are notoriously difficult to control and maintain in buildings of any type. In particular, older buildings with leaky envelopes can lose so much moisture so quickly that it is almost impossible to maintain acceptable relative humidity levels. Fortunately, the human body has a great tolerance for comfort, and we are used to dry skin and static discharge in winter. So perhaps that is why building designers, owners, and operators are happy to leave humidification to specialized buildings like data centers, pharmacies, hospitals, galleries, and museums — especially in the Northwest, where extremely dry conditions only last a few days a year. For the purpose of our discussion, we will focus in humidification in health care applications — hospitals, laboratories, clinics, and extended care homes. COMFORT GUIDELINES ASHRAE recently published “Standard 170-2008 Ventilation of Health Care Facilities,” which requires many areas within hospitals, like operating rooms, to maintain humidity levels between 30% and 60% rh at temperatures between 20°C and 24°C (70° to 75°F). The standard quite closely follows the winter thermal comfort guidelines also developed by ASHRAE many years ago. The lower boundary of 30% prevents discomfort from cold and dry conditions that exacerbate many health conditions. The 60% rh upper boundary is meant to reduce the opportunity for mold growth. Stagnant warm and humid air above this control region is a breeding ground for mold growth and mildew, the spores of which when airborne can cause respiratory problems. The psychrometric chart in Figure 1 illustrates the accepted winter comfort zone. The psychrometric analysis is used to calculate the relative energy S in moist air, among other valuable physical properties, as we will see in the next example. FIGURE 1. Winter comfort zone for indoor air. 24 En gi neer ed S y stem s February 2009
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