Retrofit

Retrofitting Strategy	Conversion of VAV system to sensible space cooling with dedicated outdoor air system (DOAS)
	The latent load of the fresh air intake is removed by using desiccant dehumidification system. The FCU is designed to remove only the sensible load. The chilled water supply temperature to the FCU can be higher than the indoor dew point temperature and no condensation occurs in the chilled water coil. With higher chilled water temperature in the system, the chiller will be running at higher efficiency resulting in energy saving.
	Consideration
	The indoor environment can be improved as the humidity is controlled at a lower RH. Thermal comfort can be improved and a higher indoor temperature can be set with equivalent thermal comfort and hence reduce the cooling load (1°C increase in indoor temperature can reduce 3% of air-conditioning load). Since the FCU only cater for the sensible load, it can respond faster to maintain the set indoor temperature. Hence the thermal comfort can be improved. Space temperature can be better controlled with the FCU vs VAV boxes. The fan power of FCU is lower than AHU as the system pressure is much lower due to less ducting and accessories such as VAV box. Installation of power meter for FCU electrical circuits 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 DOAS (Figure 3). Installation of power meter, temperature sensors, and flowmeter or differential pressure sensors for monitoring the power, cooling energy and COP of chiller (Figure 1).
	Estimated Saving
	Waterside: (1/COP_CH_PRE – 1/COP_CH_POST) * QBLDG * OPHR,yr Where (COP_CH_POST – COP_CH_PRE)/ COP_CH_PRE = 1.25% per 1°C increase of TCHWS COP_CH_POST = [1.25%(TCHWS_POST – TCHWS_PRE) + 1]COP_CH_PRE COP_CH = 0.605COP_CH_SM + 0.395COP_CH_WN COP_CH_SM = (COPa_SM + COPb_SM)/2; COP_CH_WN = (COPa_WN + COPb_WN)/2 QBLDG = 0.605[0.77(0.88NCH_SMQCH_SM + 0.12NCH_WNQCH_WN)] + 0.395[0.596(0.184NCH_SMQCH_SM + 0.816NCH_SMQCH_SM)] Airside (AHU à iFCU): (SkW_AHU_PRE – SkW_FCU_POST)OPHR,yr Airside (PAU à DOAS): (SkW_PAU_PRE – SkW_DOAS_POST)OPHR,yr Where S kW_AHU_PRE = (0.605LF_SM,PRE + 0.395LF_WN,PRE )NAHUkW_AHU SkW_FCU_POST = (0.605LF_SM,POST + 0.395LF_WN,POST)NFCUkW_FCU S kW_PAU_PRE = (0.605LF_SM,PRE + 0.395LF_WN,PRE )NPAUkW_PAU SkW_DOAS_POST = (0.605LF_SM,POST + 0.395LF_WN,POST)NDOASkW_DOAS For CSD: LF_SM = LF_WN = 1, For 3-speed: LF_SM = 0.882 , LF_WN = 0.717 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.