Consulting-Specifying Engineer - September 2008 - (Page 62)

Equipment Lifecycles Steam-filled jacket Humidification steam exits dispersion tube here Fiberglass-filled jacket without obstructing airflow. Absorption distance is, in part, determined by how well air and steam mix. The more steam discharge points or tubes spanning a duct or AHU, the shorter the absorption distance. Short-absorption dispersion panels can achieve absorption within 3 in. with multiple, closely spaced tubes. However, when not insulated they can condense significant amounts of steam, heating the air stream. Tubes insulated with ceramic or closed-cell foam insulation have a slender profile and, in most cases, do not cause an excessive pressure drop across the dispersion assembly when tubes are closely spaced, making it possible to simultaneously achieve short absorption and energy efficiency. (The typical pressure drop of a short-absorption dispersion panel with closed-cell foam insulation is 0.03 to 0.12 in. water column when tubes are installed 3 in. on-center in airflows of 500 to 1,000 fpm.) The key difference between ceramic and closed-cell foam insulation is how well they reduce dispersion tube heat loss. As you might expect, closed-cell foam performs better than ceramic insulation. The ceramic coating would have to be about 0.375 in. thick—three times as thick as the 0.125in.-thick closed-cell foam insulation—to achieve the same insulating capability. Condensate return The second tip is to return condensate to the boiler. All steam dispersion tubes, in the simplest terms, are devices with numerous holes through which steam exits, so they operate at just above atmospheric pressure. For this reason, systems delivering pressurized steam to dispersion tubes often waste generated condensate to an open drain, because returning nonpressurized condensate to the condensate return main requires purchasing, install- Figure 1 This illustration depicts the insulated steam-jacket dispersion tube profile. Source: DRI-STEEM Corp. Steam-jacketed dispersion tubes are a good solution for applications not requiring a short absorption distance. Their profile, especially when insulated, limits the number of tubes that can be installed across a duct or air handling unit (AHU) Selecting dispersion panels Table 1 (below) illustrates the amount of water and energy wasted or saved when using three different types of dispersion panels operating under the same conditions: Dispersion panel No. 1 (baseline): A 72 in. wide x 48 in. high steam dispersion panel with dispersion tubes fabricated at 3 in. on-center, operating 2,000 h/yr in a duct with 50 F air at 1,000 fpm air speed. There is no tube insulation and no integral heat exchanger in the header on this panel (condensate Dispersion panel Energy wasted in condensate Energy saved over dispersion panel No. 1 NA wastes to drain). Dispersion panel No. 2: The same conditions as No. 1, except the dispersion assembly has dispersion tubes insulated with closed-cell foam (condensate wastes to drain). Dispersion Panel No. 3: The same conditions as No. 1, except the dispersion assembly has dispersion tubes insulated with closed-cell foam and an integral heat exchanger in the header (condensate is returned to boiler). Water saved over dispersion panel No. 1 NA Heat added to downstream air by dispersion tubes 2.58 F Cooling load to offset heat added to downstream air 140,208,000 Btu/yr 70,104 Btu/h or 5.8 tons Water wasted in condensate 17,359 gal/yr Dispersion panel No. 1: No tube insulation; no heat exchanger Dispersion panel No. 2: PVDF tube insulation; no heat exchanger Dispersion panel No. 3: PVDF tube insulation; heat exchanger Total: 162,910,200 Btu/yr (140,208,000 Btu/yr duct air heat gain from condensing steam; 22,702,200 Btu/ yr hot condensate) Total: 40,580,600 Btu/yr (34,960,000 Btu/yr duct air heat gain from condensing steam; 5,620,600 Btu/ yr hot condensate) Total: 34,960,000 Btu/yr (34,960,000 Btu/yr duct air heat gain from condensing steam) 122,329,600 Btu/yr (75%) 4,298 gal/yr 13,061 gal/yr (75%) 0.65 F 34,960,000 Btu/ yr 17,480 Btu/h or 1.5 tons 127,950,200 Btu/yr (79%) 0 gal/yr 17,359 gal/yr (100%) 0.65 F 34,960,000 Btu/ yr 17,480 Btu/h or 1.5 tons 62 Consulting-Specifying Engineer • SEPTEMBER 2008

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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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