Retrofit

Retrofitting Strategy	Conversion of constant-air-volume (CAV) to variable-air-volume (VAV) system
	The air conditioning system operates at part load for most of the time. Common CAV system delivers a constant air flow all the time while changing the supply air temperature to control the temperature of the space. By changing the fan to variable speed, the space temperature can be controlled by varying the supply air flow rate at constant temperature. This can significantly reduce the fan power during part load. For a simple ducting system supplying conditioned air to a small part of a shopping centre, a frequency inverter can be added to change the fan speed. For an AHU system supplying air-conditioned air to an office where temperature of different zones can have individual temperature control, the ducting needs to be rearranged with VAV box to control each zone’s temperature. VAV systems offer superior performance in applications where the conditioned space is subject to frequent part-load conditions. VAV systems typically offer energy savings above 30% compared with CAV systems.
	Consideration
	The constant speed fans can be converted to variable speed by adding frequency inverters or replaced by EC plug fans which are more energy efficient (refer to the above). The fan speed is controlled by the duct static pressure which can be reset according to the damper positions of VAV box to reduce the fan’s energy consumption. Need to ensure that there is enough fresh air delivered to the conditioned space when the supply flow rate reduces due to low load conditions. Installation of power meter, temperature sensors together with airflow measuring station and/or VSD devices (induction motor only) for monitoring the power and W/L/s of AHU (Figure 3).
	Estimated Saving
	(SkW_AHU_PRE – SkW_AHU_POST) OPHR,yr Where S kW_AHU_PRE = NAHU_PREkW_AHU_PRE SkW_AHU_POST = (0.605LF_SM,POST + 0.395LF_WN,POST)NAHU_POSTkW_AHU_POST For CSD: LF_SM = LF_WN = 1, For VSD: LF_SM = 0.795, LF_WN = 0.562; For EC or DC-motor: LF_SM = 0.71, LF_WN = 0.38 OPHR,yr = hr/dayday/wk52 Please click HERE to view the list of terminology.