HVAC Instrumentation and Controls

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HVAC Instrumentation and Control

Introduction

HVAC Instrumentation and Controls

The application of Heating, Ventilating, and Air-Conditioning (HVAC) controls starts with an understanding of the building and the use of the spaces to be conditioned and controlled. All control systems operate in accordance with few basic principles but before we discuss these, let’s address few fundamentals of the HVAC system first.

HVAC systems

HVAC systems are classified as either self-contained unit packages or as central systems. A unit package describes a single unit that converts a primary energy source (electricity or gas) and provides final heating and cooling to the space to be conditioned. Examples of self-contained unit packages are rooftop HVAC systems, air conditioning units for rooms, and air-to-air heat pumps.

With central systems, the primary conversion from fuel such as gas or electricity takes place in a central location, with some form of thermal energy distributed throughout the building or facility.

Boiler Operator’s Handbook

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Central systems are a combination of central supply subsystem and multiple end use subsystems. There are many variations of combined central supply and end use zone systems. The most frequently used combination is central hot and chilled water distributed to multiple fan systems. The fan systems use water-to-air heat exchangers called coils to provide hot and/or cold air for the controlled spaces. End-use subsystems can be fan systems or terminal units. If the end use subsystems are fan systems, they can be single or multiple zone type. The multiple end use zone systems are mixing boxes, usually called VAV boxes.

How does central air-conditioning system work?

Cooling Cycle (chilled water system): The supply air, which is approximately 20° F cooler than the air in the conditioned space, leaves the cooling coil through the supply air fan, down to the ductwork and into the conditioned space. The cool supply air picks up heat in the conditioned space and the warmer air makes its way into the return air duct back to the air handling unit. The return air mixes with outside air in a mixing chamber and goes through the filters and cooling coil. The mixed air gives up its heat into the chilled water tubes in the cooling coil, which has fins attached to the tubes to facilitate heat transfer. The cooled supply air leaves the cooling coil and the air cycle repeats.

The chilled water circulating through the cooling coil tubes, after picking up heat from the mixed air, leaves the cooling coil and goes through the chilled water return (CHWR) pipe to the chiller's evaporator. Here it gives up the heat into the refrigeration system. The newly "chilled" water leaves the evaporator and is pumped through the chilled water supply (CHWS) piping into the cooling coil continuously and the water cycle repeats.

The evaporator is a heat exchanger that allows heat from the CHWR to flow by conduction into the refrigerant tubes. The liquid refrigerant in the tubes "boils off" to a vapor removing heat from the water and conveying the heat to the compressor and then to the condenser. The heat from the condenser is conveyed to the cooling tower by the condenser water. Finally, outside air is drawn across the cooling tower, removing the heat from the water through the process of evaporation.

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