HEATING and COOLING
HEATING and COOLING
PSYCHROMETRIC CHART
Temperature In the HVAC area, we talk about two kinds of temperatures. One is called dry bulb (DB) temperature, a fancy name for the reading from an ordinary Mercury bulb thermometer or regular temperature sensor. The other is called the wet bulb temperature (WB). It is found by taking a standard Mercury bulb thermometer, covering the tip with cotton or material of some kind, wetting it thoroughly with water and moving it around in the air. As the water evaporates if it can it cools the tip of the therm-ometer and the reading is the wet bulb temperature.
Humidity Specific humidity is the actual amount of water vapor in 1 kg of dry air, whereas relative humidity is the ratio of the actual amount of moisture in the air to the maximum amount of moisture air can hold at that temperature.
Relative Humidity Relative humidity is the percent weight of water vapor that air is currently holding compared to the weight of water vapor it could potentially hold at its present temperature. m φ = m v g PV v = P V g / / RT RT Pv = P g Thus, relative humidity, or RH, is always less than or equal to 100 percent. When the RH is 100 percent, we say that the air is saturated that is, it cannot hold any more water vapor at its present temperature. Adding water vapor to air is called humidification. Removing water vapor from air is called dehumidification.
PSYCHROMETRIC CHART
PSYCHROMETRIC CHART
COMFORT ZONE
THE NEED FOR HVAC The primary purpose of the HVAC system is to provide a comfortable and safe environment for the occupants, equipment and processes being conducted. We would also like the HVAC system to accomplish its purpose with minimum energy use and minimum cost. The environmental factors that need to be controlled by the HVAC system for comfort and safety are: temperature, relative humidity, and air quality.
Air Quality - ASHRAE 62.1-2004 Air quality or indoor air quality (IAQ) is a complex issue in general, but at the Energy Manager level, we restrict our discussion to meeting the requirements of the ASHRAE ventilation standard ASHRAE 62.1-2004 Purpose: to specify minimum ventilation rates and indoor air quality that will be acceptable to human occupants Ventilation: 7 9 L/s outside air per person or CO 2 less than 1000 ppm
Power, energy and airconditioning For air-conditioning systems, the most common term for heat removal capacity of the HVAC system is the kw, which is a heat removal rate of 3600 kj per hour. One kw equals 3600 kj per hour. Since this is a rate of flow of energy (kj/h), it is a measure of power, not energy. The unit of energy in the HVAC system is the kw-hour, which is equal to 3600 kj. One kwh equals 3600 kj.
Example A large room in a commercial office building has a heat production rate of 600,000 kj/h from lights, equipment, people and heat flow from the outside. How many kw of airconditioning is required to remove this heat? How many kw is the power input if the COP is 3.0 Solution X kw = 600,000 kj 3,600 kj kw 1 =167 kw 167 kw X kw el = = 55. 67 kw 3. 00 1
COMFORT and SAFETY Typical Design Conditions: Temperature: 23 C 20 26 C Relative Humidity: 50% 40 60% Ventilation: 7 9 L/s outside air per person or CO 2 less than 1000 ppm (ASHRAE 62.1 2004 Ventilation Standard)
Types of HVAC Equipment Cooling Air based systems» Window units, split systems, rooftop units Water based systems» Chillers, Air Handling Units Heating Air based systems» Gas, oil and electric heaters; heat pumps Water based systems» Hot water boiler, Air handling units
CENTRAL HEATING Hydro-flow (or water circulation) Heating System Combi-boiler Radiators Shower Two-pipe system There is a flow and a return pipe.
CENTRAL HEATING
CENTRAL COOLING AIR BASED SYSTEMS: Ducted Split-unit Systems THE AIR CONDITIONER IS SPLIT INTO TWO UNITS Refrigerant Piping Outdoor Unit Indoor Unit
CENTRAL COOLING Ducted Split-unit Systems Outdoor Unit Indoor Unit
CENTRAL COOLING Ducted Split-unit Systems Ducting Supply grilles Outdoor unit Indoor unit
CENTRAL COOLING AIR BASED SYSTEMS: Roof-top A/C unit
CENTRAL COOLING Roof-top unit
CENTRAL COOLING Duct Configurations
Plans of an Amphi-theatre with the application of roof-top a/c E.F. Roof-top unit
Roof top unit Fresh air intake Duct
CENTRAL COOLING WATER BASED SYSTEMS: Chillers, Boilers, Air handling Units (AHUs)
CENTRAL COOLING FCU-1 FCU-2 FCU-3 AHU Flow Duct Pump Return Cool Air Flow water ~ 7 deg C Return water ~ 13 deg C
System Improvement Options Make building envelope improvements to reduce HVAC load insulation, high performance windows and roofs Replace old HVAC units and chillers with more efficient models Consider multiple chillers Consider a chiller with a variable speed drive
Consider installing a small chiller or separate HVAC system for 24/7 loads Use VSDs on pumps, cooling towers Replace constant volume systems with VAV - variable air volume systems. Get large fan law savings Consider adding a gas engine driven chiller with heat recovery for hot water Retrofit to DDC controls Use cooling towers where possible