CENTRAL HYDRONIC AIR CONDITIONING SYSTEMS

CENTRAL HYDRONIC AIR CONDITIONING SYSTEMS

Central hydronic air conditioning systems are also called central air conditioning systems. In a central hydronic air conditioning system, air is cooled or heated by coils filled with chilled or hot water distributed from a central cooling or heating plant. It is mostly applied to large-area buildings with many zones of conditioned space or to separate buildings. Water has a far greater heat capacity than air. The following is a comparison of these two media for carrying heat energy at 68°F (20°C):

The heat capacity per cubic foot (meter) of water is 3466 times greater than that of air. Transporting heating and cooling energy from a central plant to remote air-handling units in fan rooms is far more efficient using water than conditioned air in a large air conditioning project. However, an additional water system lowers the evaporating temperature of the refrigerating system and makes a small- or medium-size project more complicated and expensive. A central hydronic system of a high-rise office building, the NBC Tower in Chicago, is illustrated in Fig. A central hydronic air conditioning system consists of an air system, a water system, a central heating/cooling plant, and a control system.

Air System    

    An air system is sometimes called the air-handling system. The function of an air system is to condition, to transport, to distribute the conditioned, recirculating, outdoor, and exhaust air, and to control the indoor environment according to requirements. The major components of an air system

FIGURE 1.1 Schematic diagram of the central hydronic air conditioning system in NBC Tower.

are the air-handling units, supply/return ductwork, fan-powered boxes, space diffusion devices, and exhaust systems. An air-handling unit (AHU) usually consists of supply fan(s), filter(s), a cooling coil, a heating coil, a mixing box, and other accessories. It is the primary equipment of the air system. An AHU conditions the outdoor/recirculating air, supplies the conditioned air to the conditioned space, and extracts the returned air from the space through ductwork and space diffusion devices. A fan-powered variable-air-volume (VAV) box, often abbreviated as fan-powered box, employs a small fan with or without a heating coil. It draws the return air from the ceiling plenum, mixes it with the conditioned air from the air-handling unit, and supplies the mixture to the conditioned space. Space diffusion devices include slot diffusers mounted in the suspended ceiling; their purpose is to distribute the conditioned air evenly over the entire space according to requirements. The return air enters the ceiling plenum through many scattered return slots. Exhaust systems have exhaust fan(s) and ductwork to exhaust air from the lavatories, mechanical rooms, and electrical rooms. The NBC Tower in Chicago is a 37-story high-rise office complex constructed in the late 1980s. It has a total air conditioned area of about 900,000 ft2 (83,600 m2 ). Of this, 256,840 ft2 (23,870 m2 ) is used by NBC studios and other departments, and 626,670 ft2 (58,240 m2 ) is rental offices located on upper floors. Special air conditioning systems are employed for NBC studios and departments at the lower level. For the rental office floors, four air-handling units are located on the 21st floor. Outdoor air either is mixed with the recirculating air or enters directly into the air-handling unit as shown in Fig. 1.2. The mixture is filtrated at the filter and is then cooled and dehumidified at the cooling coil during cooling season. After that, the conditioned air is supplied to the typical floor through the supply fan, the riser, and the supply duct; and to the conditioned space through the fan-powered box and slot diffusers.

    Water System

 The water system includes chilled and hot water systems, chilled and hot water pumps, condenser water system, and condenser water pumps. The purpose of the water system is (1) to transport chilled water and hot water from the central plant to the air-handling units, fan-coil units, and fanpowered boxes and (2) to transport the condenser water from the cooling tower, well water, or other sources to the condenser inside the central plant. In Figs. 1.1 and 1.2, the chilled water is cooled in three centrifugal chillers and then is distributed to the cooling coils of various air-handling units located on the 21st floor. The temperature of the chilled water leaving the coil increases after absorbing heat from the airstream flowing over the coil. Chilled water is then returned to the centrifugal chillers for recooling through the chilled water pumps. After the condenser water has been cooled in the cooling tower, it flows back to the condenser of the centrifugal chillers on lower level 3. The temperature of the condenser water again rises owing to the absorption of the condensing heat from the refrigerant in the condenser. After that, the condenser water is pumped to the cooling towers by the condenser water pumps.               

Central Plant

The refrigeration system in a central plant is usually in the form of a chiller package. Chiller packages cool the chilled water and act as a cold source in the central hydronic system. The boiler plant, consisting of boilers and accessories, is the heat source of the heating system. Either hot water is heated or steam is generated in the boilers. In the NBC Tower, the refrigeration system has three centrifugal chillers located in lower level 3 of the basement. Three cooling towers are on the roof of the building. Chilled water cools from 58 to 42°F (14.4 to 5.6°C) in the evaporator when the refrigerant is evaporated. The refrigerant is then

compressed to the condensing pressure in the centrifugal compressor and is condensed in liquid form in the condenser, ready for evaporation in the evaporator. There is no boiler in the central plant of the NBC Tower. To compensate heat loss in the perimeter zone, heat energy is provided by the warm plenum air and the electric heating coils in the fanpowered boxes.

Control System

Modern air conditioning control systems for the air and water systems and for the central plant consist of electronic sensors, microprocessor-operated and -controlled modules that can analyze and perform calculations from both digital and analog input signals, i.e., in the form of a continuous variable. Control systems using digital signals compatible with the microprocessor are called direct digital control (DDC) systems. Outputs from the control modules often actuate dampers, valves, and relays by means of pneumatic actuators in large buildings and by means of electric actuators for small projects. In the NBC Tower, the HVAC&R system is monitored and controlled by a microprocessor-based DDC system. The DDC controllers regulate the air-handling units and the terminals. Both communicate with the central operating station through interface modules. In case of emergency, the

fire protection system detects alarm conditions. The central operating station gives emergency directions to the occupants, operates the HVAC&R system in a smoke control mode, and actuates the sprinkler water system.

Air, Water, Refrigeration, and Heating Systems 

Air, water, refrigeration, heating, and control systems are the subsystems of an air conditioning or HVAC&R system. Air systems are often called secondary systems. Heating and refrigeration systems are sometimes called primary systems. Central hydronic and space conditioning air conditioning systems both have air, water, refrigeration, heating, and control systems. The water system in a space conditioning system may be a chilled/hot water system. It also could be a centralized water system to absorb heat from the condenser during cooling, or provide heat for the evaporator during heating. For a unitary packaged system, it consists of mainly air, refrigeration, and control systems. The heating system is usually one of the components in the air system. Sometimes a separate baseboard hot water heating system is employed in the perimeter zone. A evaporative-cooling system always has an air system, a water system, and a control system. A separate heating system is often employed for winter heating. In an individual room air conditioning system, air and refrigeration systems are installed in indoor and outdoor compartments with their own control systems. The heating system is often a component of the supply air chamber in the room air conditioner. It can be also a centralized hot water heating system in a PTAC system. Air conditioning or HVAC&R systems are therefore often first described and analyzed through their subsystems and main components: such as air, water, heating, cooling/refrigeration, and control systems. Air conditioning system classification, system operating characteristics, and system selection must take into account the whole system. Among the air, water, and refrigeration systems, the air system conditions the air, controls and maintains the required indoor environment, and has direct contact with the occupants and the manufacturing processes. These are the reasons why the operating characteristics of an air conditioning system are esssentially represented by its air system.