Chilled-Water VAV Systems - HVAC.Amickracing

Primary System Componentsbarometric damper may be required if a tall chimney produces excessivedraft.• ControlsBecause airflow across the gas-fired burner varies, the heating capacity ofthe burner must be modulated to prevent the temperature rise throughthe burner from exceeding the maximum allowable limit. For most VAVapplications, this likely requires a burner with a 10:1 turndown ratio. Thismeans the burner can operate at a capacity as low as 10 percent of itsrated capacity. In addition, air temperature sensors on the entering andleaving sides of the burner are needed to protect it from damage.Finally, allow the supply fan to continue to operate for a period of timeafter the gas-fired burner has been shut off, allowing the heat exchangerto dissipate any residual heat. Consult the manufacturer for the length ofthis “cool-down” period.In VAV applications, the air velocity across a direct-fired burner mustremain within a specific range for safe operation. In this case,manufacturers typically provide an adjustable opening that automaticallyvaries the opening size as airflow changes, keeping the air velocityrelatively constant.Recovered heatIn some applications, heat may be recovered from another part of the HVACsystem. Common sources of recoverable heat include:• Warm condenser water leaving a water-cooled chiller or hot refrigerantvapor leaving the compressor in an air-cooled chiller (see “Condenserheat recovery,” p. 88)• Another air stream, or another location in the same air stream, using anair-to-air heat exchanger (see “Air-to-Air Energy Recovery,” p. 160)Fans (supply, return, relief)Fans are used to move air throughout the various components of a VAVsystem. Depending on the application, the system may include: 1) a supplyfan only, 2) a supply fan and a relief (or exhaust) fan, or 3) a supply fan and areturn fan.Supply fan onlyIn this configuration (Figure 19), the supply fan must create high enoughpressure at its outlet (A) to overcome the pressure losses associated withpushing the air through the main supply ductwork, VAV terminal units,supply-duct runouts, and supply-air diffusers. (Note: If the system uses seriesfan-powered VAV terminals, the small terminal fan is used to overcome thepressure losses between the terminal unit and the zone. For furtherdiscussion, see “Fan-powered VAV terminal units,” p. 58.)26 Chilled-Water VAV Systems SYS-APM008-EN