Consulting-Specifying Engineer - September 2008 - (Page 61)

Equipment Lifecycles Putting pressure on steam humidification Tips for saving energy, water, installation, and maintenance costs when installing steam humidification. BY LYNNE WASNER and JIM LUNDGREEN, DRI-STEEM Corp., Eden Prairie, Minn. C arefully choosing steam humidification equipment can help you reduce or eliminate water waste in the form of condensate. As a result, you can significantly reduce steam humidification system lifecycle costs through water and energy savings and, in pressurized steam applications, also reduce boiler chemical use, installation costs, and maintenance requirements. First, some background: A steam humidification system generates pressurized steam in a boiler or nonpressurized steam in a tank. This steam travels through piping to a dispersion assembly and into an airstream. This airstream, typically 50 to 55 F (or cooler in some specialty applications), blows across stainless steel dispersion tubes, causing some steam in the dispersion tubes to condense and drain from the dispersion assembly. Second, some physics: • Energy used to change water to steam is wasted whenever steam condenses before it gets used for humidification, wasting about 8,000 Btu for every gallon of dispersiongenerated condensate. • Released heat, from steam changing to condensate in the dispersion assembly, is added to the airstream. This wastes energy by increasing the quantity of steam generated to meet the humidification load. The released heat also can have a ripple effect throughout the HVAC system. Cooling loads increase in applications that humidify and cool simultaneously, fan energy use increases to circulate more air, fans and pumps work harder and add more heat to the airstream, and even cooling tower size may need to increase. • The heat transfer rate from a dispersion tube to an airstream is not affected by humidi- fication load. At a steady-state operating temperature, a tube dispersing 10 lb/h transfers the same amount of heat and generates the same amount of condensate as that same tube dispersing 50 lb/h, given the same airstream temperature and velocity. • Condensate, not always returnable to the steam generator, is often hot water down the drain. In some jurisdictions, hot discharge water must be tempered with cold water before discharge into a sanitary system, wasting more energy and up to two times more water. These numbers add up quickly. Reducing dispersion-generated condensate can significantly reduce humidifier and some HVAC system operating costs (see Table 1). Condensate reduction The first tip is to reduce condensate with insulated dispersion tubes. Dispersion tube insulation can reduce dispersion-tubegenerated condensate production and its associated wasted energy by up to 85%. There are several ways to insulate dispersion tubes: • Steam jacket with insulation: A steam jacket surrounded by factory-installed fiberglass insulation protected by stainless steel (see Figure 1). • Ceramic insulation: A factory-applied 0.030-in.-thick thermal insulating coating. • Closed-cell foam insulation: A 0.125-in.-thick factory-applied polyvinylidene fluoride (PVDF) insulation with a 0/0 flame/smoke rating. All of these insulation types withstand the environmental extremes of steam humidification while meeting plenum requirements for smoke and flame. Consulting-Specifying Engineer • SEPTEMBER 2008 61

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Consulting-Specifying Engineer - September 2008 Contents Viewpoint Letters News M/E�Roundtable Selecting Appropriate Egress Strategies Commissioning On-Site Electrical Systems Mentoring Control Engineers and Technicians Casting Call for Cx Case Study New Products Equipment Lifecycles Advertiser Index Green Space

Consulting-Specifying Engineer - September 2008

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