Effective Management of Nighttime and Holiday HVAC Loads

Requirements for night and holiday cooling/ heating loads are different from those of normal working hour loads in any type of building. Main air conditioning system of a building is selected for maximum occupancy and activity levels. After working hours building system is required to meet certain fixed loads. These may include electronic and telecommunication equipment which are required to operate twenty four hours.

HVAC system selection for these fixed year round loads requires exact estimation of after hour operating systems. These are all added to determine air conditioning loads required to meet this demand. Central plant system for normal working hour operations is evaluated for part load performance. When main plant part load analysis shows satisfactory operation at after hour load conditions, central plant is used for night or holiday operations.

Normally it becomes inefficient for the main building plant to operate at low load conditions, required for night operations. In this case dedicated system is selected for this load. During day operations these areas are served from central plant. For after hour operations, system switches over to night mode.

A separate small capacity chiller with pumping arrangement is used when these loads are substantial. Piping network is kept separate. Separate pipes from night operating chillers are connected to server rooms and other night loads. During day operations this same piping is connected to building system through valve arrangement.

Running separate piping is also advantageous to meet different indoor conditions of these areas. During some ambient conditions, one system may be on heating and other on cooling. In this case small chillers are kept operative during day operations when main system is on heating.

For small buildings multiple split direct expansion type units are used. These units connect eight indoor units with one outdoor unit. Low ambient kit is provided for satisfactory operation in low ambient conditions. These units have limitation of height and total length of refrigerant lines. These are planned accordingly.

In case of water cooled plant for building operations, cooling tower for building system can be used for water cooled units during night operations. Waterside economizer system is used for night operations. It saves substantial energies in areas where night conditions are favorable.