TECHNICAL FEATURE Process Side Reactivation Side Process Side Reactivation Side instead of a constant/continuous type compressor to closely match 13 145 L/s the load profile and precise controls 25°C DB 9.7 g/kg in conjunction with hot-gas reheat Space Temp.: 28°C, ±2°C DBT condenser modulation to consistently Exhaust Air RH: 50%, ±5% maintain humidity levels as required Post-Cool Option 3: Desiccant System AH: 11.4 g/kg 3100 L/s CHW Coil 13 145 L/s for Latent Load and for the dehumidification process. 28°C DB 36.2°C DB CHW Coil for Sensible 11.4 g/kg 9.7 g/kg Another downside to this option can Return Air Cooling Hygroscopic 7645 L/s be the less efficient DX system comWheel 28°C DB pared to the building chiller used Enthalpy Wheel 11.4 g/kg Reactivation Fan for latent load; this will be evaluated Reactivation Air Exhaust Inlet 1815 L/s Air further in coordination with free 2400 L/s Exhaust Reactivation energy provided by the hot-gas reheat 28°C DB Air Fan Heater 11.4 g/kg condenser. Fresh Air Option 3 and Option 4 use a desicIntake 5500 L/s 5500 L/s 3255 L/s 3100 L/s 47.6°C DB 28.8°C DB 33°C DB 29.5°C DB cant dehumidification system based 7.3 g/kg 12.8 g/kg 22.2 g/kg 13.8 g/kg on a hygroscopic wheel using silica gel as the desiccant media. It removes 13 145 L/s 25°C DB moisture content in the air by adsorp9.7 g/kg Space tion (moisture is taken up by the Temp.: 28°C, ±2°C DBT Exhaust Air surface of its numerous pores rather RH: 50%, ±5% Post-Cool Option 4: Hybrid Desiccant 13 145 L/s AH: 11.4 g/kg than being taken up by volume in the 3100 L/s CHW Coil System and Precool CHW Coil for 27.6°C DB 28°C DB Latent Load and Post-Cool CHW case of absorption). This desiccant 9.7 g/kg 11.4 g/kg Return Air Coil for Sensible Load media typically has the capability to 10 045 L/s Hygroscopic 28°C DB Wheel hold moisture quantity many times Enthalpy Wheel 11.4 g/kg Reactivation Fan its actual weight. Since this principle Reactivation Air Exhaust Inlet of dehumidification does not depend 715 L/s Air Exhaust on dew-point temperature for offReactivation Air Fan Heater 2100 L/s system moisture content, it can be 3100 L/s 3100 L/s 12.8°C DB Fresh Air crucial for conditions requiring lower 8.8 g/kg 29.5°C DB Intake 3100 L/s 2100 L/s 12.8°C DB 3255 L/s 13.8 g/kg 8.8 g/kg 33°C DB 26.5°C DB 33°C DB humidity levels and can result in 1000 L/s 5.1 g/kg 3.3 g/kg 22.2 g/kg Precool CHW 12.8°C DB reducing the volume of air treated to Coil 8.8 g/kg Internal Bypass meet required latent loads. However, although the volume of complementing the DX dehumidification process by treated air can be reduced by using giving the heat of rejection (latent energy gained from this principle, the total volume of air in the system the evaporator plus the compressor heat) back to the air- would still be defined by required ventilation rates and stream. A backup condenser is also used to control the air changes necessary to maintain acceptable indoor air amount of heat rejected to dehumidified air; excess heat quality as per referenced standards. For this purpose, a is rejected to atmosphere using a backup condenser. hygroscopic wheel is used in Option 3 and Option 4 in The first downside to this system is due to the type of different system configurations. compressors usually used in DX systems. The constant Option 3 is a basic desiccant-based system configuranature of these compressors adversely affects part-load tion in which a hygroscopic wheel reduces the moisture dehumidification and gives sort of "gray regions" or content of process air after energy is recovered using the "dead bands" in the dehumidification process during enthalpy wheel. The hygroscopic wheel is used solely to compressor off-cycles. However, this can be mitigated process the air required to meet the latent load correby using a modulating compressor with many steps sponding to a reduction of moisture content by 5.5 g/kg O CTO B E R 2020 ashrae.org ASHRAE JOURNAL 29https://www.ashrae.org/