Engineered Systems - February 2009 - (Page 27) ASHRAE Standard 170P restricts the use of reservoir and pan-type humidifiers t because these systems contain standing water that g is open to air and can induce mold and bacteria growth. Other types of closed steam-injection type humidifiers are acceptable. answer to the first part of the question in winter — we don’t need to unless we are getting too many comfort complaints or have special medical conditions that require it. For energy cost considerations, the acceptable low region should be the target at all times during occupied periods. Table 1 illustrates the acceptable scenarios for a cold day for a 40,000 cfm system humidifying outdoor air at realistic dry winter conditions. As can be seen, the difference between the low end of 20°C and 30% rh and 24°C/60% rh is a substantial energy cost increase from $290 to $819 or $529/day. The latent energy requirement nearly triples between low and high conditions. HUMIDITY REQUIREMENTS Unfortunately, we can’t just keep the humidity and temperature in the optimum energy state for various reasons — not the least being the inability to measure and control accurately with standard HVACgrade sensors and controllers. So what is required to humidify a space like an operating room or a delivery room? Well, there are several systems that can be used, but the simplest method for general humidification — though perhaps not the purest method — is the use of plant steam piped into a humidifier dispersion grid located in the supply duct work of a major air-handling system. The addition of low pressure (10 to 15 psi) steam from a humidification coil enables water to be injected and evaporated into the airstream within the minimum length of duct run. If the coil is large enough and properly designed, and the air velocity is kept low enough (1,000 ft/ min or less), then this method will evaporate steam in distance of less than 6 ft (1.8 m). Good dispersion will avoid the saturation of any duct insulation and filter media. STEAM CONCERNS Boiler water chemical additives are often cited as a concern, but according to ASHRAE, plant steam can be introduced into health care spaces — as long as it meets the USDH FDA requirements of 173.310 in the Code of Federal Regulations CFR21. This code, of course, cites numerous possible chemical additives and their limitations in the steam, especially if the steam comes in contact with milk or milk products such as in food preparation areas. Fortunately, these requirements are reasonably easy to meet, but a chemical analysis of your plant steam is recommended. The requirements can be found at http://edocket.access.gpo.gov/cfr_2004/ aprqtr/pdf/21cfr173.310.pdf. Health Canada discusses safety “Guidelines for Incidental Additive Submissions” under its Food and Nutrition division (see http://www.hc-sc. gc.ca/fn-an/legislation/guide-ld/guide_incidental_addit_indirects-eng.php) and mentions boiler water treatment compounds, but unlike the FDA document, no direct reference to humidification is made. ASHRAE Standard 170P restricts the use of reservoir and pan-type humidifiers because these systems contain standing water that is open to air and can induce mold and bacteria growth. Other types of closed steam-injection type humidifiers are acceptable. Steam-to-steam humidifiers that use a heat exchanger supplied with plant steam or electric heating elements to boil steam from a clean water source like a water purification system — can also be used, but this can be very expensive. Use of domestic (potable) water for this type of application can lead quickly to maintenance problems if mineral levels in the water are too high and cause buildup on control components like fill valves. STREAMLINING In some cases, humidification may be required to be delivered at the source, like the example given above with a large general supply fan, or at the load like added to air supply at the branch that serves the space — like an individual operating room. The problem with this method is that the owner is required to maintain several small humid- ROOF VENTILATORS BRILLIANTLY SIMPLE! 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