Engineered Systems - February 2008 - (Page 58) How our propeller fans build value in Variable Refrigerant Flow [ a i r ]. Greenheck offers the world’s largest selection of direct drive or belt drive propeller fans. Sidewall Propeller Fans offer adjustable speeds plus low pressure and high pressure props for many applications. Figure 2. Major components of a VRF system including the range of possible indoor unit configurations and a typical outdoor unit. (Figure courtesy of Daikin.) Propeller Upblast Roof Fans efficiently discharge contaminants or emergency smoke and are designed with the motor out of the airstream for easy access. Hooded Propeller Roof Fans offer an extended base for easy servicing and damper access. But that’s just a sample. Find out more at your nearby Greenheck rep or our Web site. 715.359.6171 greenheck.com FREE INFO: 43 Other features include the addition of concealed ducted units and ceiling cassette configurations to the traditional wallmounted units. Refrigerant piping runs of more than 200 ft are possible, and outdoor units are available in sizes up to 240,000 Btuh. Figure 2 shows a single outdoor unit and a general schematic of multiple indoor units in a VRF heat pump system. The indoor units include wall-mounted, floormounted, ceiling-cassette, and concealed ducted configurations. The term VRF refers to the ability of the system to control the amount of refrigerant flowing to each of the evaporators, enabling the use of many evaporators of differing capacities and configurations, individualized comfort control, simultaneous heating and cooling in different zones, and heat recovery from one zone to another. Most VRF condensers use VFDs to control the flow of refrigerant to the evaporators. Refrigerant flow control is the key to many advantages as well as the major technical challenge of VRF systems (Goetzler 2007). In most cases, two-pipe systems can be used effectively (in VRF heat pump systems) when all the zones in the facility will require cooling or all will require heating during the same operating period. Three-pipe (a heating pipe, a cooling pipe, and a return pipe) systems work best when there is a need for some of the spaces to be cooled and some of them to be heated during the same period. (This often occurs in the winter in medium-sized to largesized buildings with a substantial core.) One manufacturer has a two-pipe system than can be used to provide simultaneous heating and cooling as well as heat recovery operations. Heat recovery can be accomplished by transferring heat between the pipes providing refrigerant to the cooling and heating units. One way is to use heat exchangers to extract the superheat from the units in the cooling mode and route it into refrigerant entering a heated zone. One manufacturer sends the refrigerant first to the units that require heating, allows the refrigerant to condense, collects it at a central point, and then sends it to the indoor evaporators to do cooling. Most manufacturers have a proprietary design for heat recovery plumbing and operation with special valving arrangements, heat exchangers, controls, receivers, and distribution boxes. Ventilation can be integrated with the VRF system in several ways. A dedicated VRF indoor unit could be used in a ducted configuration to condition the ventilation air. A separate ventilation system and conditioning unit could be installed using conventional technology and the VRF system function would be restricted to the recirculation air. Some VRF units have the ability to handle some outside air and could be used accordingly. Bringing the outside air into the room and then conditioning 58 En gi neer ed S y stem s February 2008 http://greenheck.com http://greenheck.com
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