CHAPTER 4. HVAC DELIVERY SYSTEMS 4.1 Introduction 4.2 Centralized System versus Individual System 4.3 Heat Transfer Fluids 4.4 CAV versus VAV Systems 4.5 Common Systems for Heating and Cooling 4.6 Economizer Cycle 4.7 Energy Recovery Systems 4.8 Controls and Automation 4.1 Introduction Complete HVAC systems are made up of sub-systems that produce heating and cooling, move heat transfer fluids, and control delivery to a space to maintain stable conditions. Equipment to produce heat transfer fluids (Primary components): Refrigeration Equipment (Cooling) Boiler (Heating) Furnaces (Heating) Equipment to move heat transfer fluids (Secondary components): Air: Air handling unit, Fan, Duct, Grill, Diffuser Water and refrigerant: pump, pipe Equipment to control flow rate of heat transfer fluids (Secondary components): Air: fan(speed), damper(on/off, flow rate) Water: pump(speed), valve(on/off, flow rate)
4.2 Centralized vs Individual Systems Duct and Diffuser Fan Coil Unit Duct Pipes AHU A high-rise building with centralized HVAC system Boiler Chiller Central mechanical room Refrigerant A building with central heating and individual cooling systems Radiator Hot water or steam Individual space heating Boiler
4.3 Heat Transfer Fluid (1) All Air System: AHU, Duct, Fan, Diffuser (2) All Water System: Pump, Pipe, Radiator, Fan Coil Unit (3) Air and Water System: (1) + (2) (4) Direct Expansion(DX) System: Package Air-Conditioning, Heat Pump 4.4 CAV versus VAV Systems 4.4.1 Constant Air Volume (CAV) Systems In a Constant Air Volume (CAV) system, variations in the thermal requirements of a space are satisfied by varying the temperature of a constant volume of air delivered to the space. A constant fraction of outdoor air can be delivered to occupied spaces to satisfy ventilation standards. Variable Coil Temperatures ( o C) Constant Fan Speed (RPM) Constant Air Volume (m 3 /h) CAV systems are suitable for a large volume space and under ground spaces requiring high rates of ventilation. CAV systems are less energy efficient than VAV systems due to electric energy consumed by the fans. Q 0.29V t kcal/h V = constant air vol. (m 3 /h) t = variable air temp. ( o C) Variable Water Flow Rate (L/s) Variable Air Temperature
4.4.2 Variable Air Volume (VAV) Systems In a Variable Air Volume (VAV) ventilation system, variations in the thermal requirements of a space are satisfied by varying the volume of air that is delivered to the space at a constant temperature. Constant Coil Temperatures Variable Fan Speed Varying Duct Pressure Air Volume Control by VAV Box VAV systems reduce HVAC energy cost by 10-20% over CAV systems due to the reduced fan energy. Variable Air Volume (m 3 /h) VAV systems complicate the delivery of outdoor air. If the fraction of outdoor air is constant, the total volume of outdoor air will be reduced as the supply air volume is reduced. This would occur during partial-load conditions and cause indoor air quality problem. Q 0.29V t kcal/h V = variable air vol. (m 3 /h) t = constant air temp. ( o C) Constant Water Flow Rate (L/s) Constant Air Temperature 4.5 Common Systems for Heating and Cooling All Air Systems CAV Systems Single-Zone CAV System (4.5.1) Single-Zone CAV Reheat System (4.5.2) Multiple-Zone CAV Terminal Reheat System (4.5.3) Multiple-Zone CAV Dual-Duct System (4.5.4) CAV Multizone System (4.5.5) VAV Systems Single-Zone VAV System (4.5.6) Multiple-Zone VAV System (4.5.7) Multiple-Zone VAV Terminal Reheat System (4.5.8) Multiple-Zone VAV Dual-Duct System (4.5.9) VAV Multizone System (4.5.10) All Water Systems Fan Coil Units (FCU) (4.5.11) Radiant Panel Heating and Cooling (4.5.12) Direct Expansion(DX) Systems Package Terminal Air Conditioners (4.5.13) Heat Pumps (4.5.14)
4.5.1 Single-Zone CAV System Fans run continuously (constant volume) Coil temperatures are controlled by space thermostat to change flow rate of heating or cooling water. Application: Small scale commercial buildings. 4.5.2 Single-Zone CAV Reheat System This system is used when humidity control is especially important. A space humidistat is used for adjusting cooling coil temperature to condense moisture in the mixture air of OA and RA. Since the air temperature will generally be too cold to maintain a proper space temperature, a heating coil (reheat coil) is placed after the cooling coil.
4.5.3 Multiple-Zone CAV Terminal Reheat System This system is similar in principle to single-zone CAV reheat system, except that multiple zones of temperature and humidity controls can be achieved. The terminal box containing reheat coil is installed for each zone. The AHU unit is used mainly for humidity control, while the terminal unit for temperature control. 4.5.4 Multiple Zone CAV Dual-Duct System Warm and chilled air streams from AHU are sent through a pair of main trunk ducts and the two air streams are mixed in the mixing box installed for each zone or space. Effectively controls air temperature of each zone having different heating or cooling load. Mixing boxes are controlled with dampers to vary the quantity of warm and chilled air in response to the zone thermostat. The total airflow, consisting of warm and chilled air, stays fairly constant, since each damper closes as the other opens. This system waste much energy because warm and cold air are mixed.
Typical dual-duct terminal and its internal section 4.5.5 CAV Multizone System This system is very similar in operation to dual-duct system. The mixing is done by dampers located at the AHU rather than at terminals located near the spaces. This system also waste energy because it mix warm and cold air. Multizones can have two sets of dampers (warm air and cold air) or three sets (warm, cold, and bypass). The latter, called triple-deck multizone, saves energy by mixing warm and bypass or cold and bypass rather than warm and cold air. A single multizone unit can serve up to approximately 12 zones. CAV multizone system (triple-zone, double-deck multizone unit)
The triple-deck multizone unit offers significant energy conservation by allowing return or outside air to bypass both coil, and the thermal inefficiency of mixing heated and cooled air is eliminated. 4.5.6 Single-Zone VAV System Airflow of constant temperature is increased or decreased in response to the space thermostat by adjusting dampers or the speed of the fans. This system gained popularity during the mid-1970s as an energy-efficient alternative to CAV terminal-reheat and CAV dual-duct system. No energy is wasted by reheating or mixing warm and chilled air. In addition, varying the air quantities offers energy savings in terms of operation of the fan, in comparison with CAV system. As the heating or cooling load diminishes, such as in spring and fall seasons, the airflow is reduced accordingly, and indoor air quality will also be deteriorated. Because the unit can serve just one zone, it is suitable only for small and simple buildings. Large buildings require multiples of this unit.
VAV Box 4.5.7 Multiple-Zone VAV System This system uses a VAV AHU to supply chilled air into a main trunk duct feeding multiple VAV terminals for individual zones. VAV terminals are designed to provide only cooling. For buildings with heating loads, this system must be used in combination with other devices to provide heat. Interior spaces of large buildings need cooling year-round, and VAV systems often meet this need. Perimeter spaces need heating during cold weather and can be served by convectors or fan coil units (FCU)
Typical convector installed at the base of a perimeter wall to overcome the downdraft during cold weather Multiple VAV Interior and hot-water convector perimeter zone system Cutaway view of a hot-water convector showing the finned tube and supply pipe. Convectors may use steam or electric power and come in various designs and heights. 4.5.8 Multiple-Zone VAV Terminal Reheat System This system is equipped with electric or hot water heating coils and is similar to CAV reheat terminals except for control of the airflow. VAV reheating is generally less expensive and does not require locating equipment on the floor, so that space can be used more flexibly. OA
Construction of an electrical reheat coil in the ductwork. The coil may be a hot-water or an electric coil. 4.5.9 Multiple-Zone VAV Terminal Dual-Duct System VAV dual-duct terminals are arranged for operation at a variable rather than a constant air volume in order to conserve energy. Two air dampers (chilled air and warm air) in a VAV box are independently operated. In cooling mode, only chilled-air damper is open and the warm-air damper remains closed. As the cooling load diminishes, the amount of chilled air is reduced accordingly. When heating is required, the chilled air damper will be at or near closed position, and the warm-air damper will start to open and increase the flow of warm air as the heating load increases. With this mode of operation, there is minimal overuse of energy due to mixing of warm and cold air at the terminal.
VAV dual-duct terminal 4.5.10 VAV Multizone System This is similar in operation to VAV dual duct system. Separate dampers are used for the warm and cold portions of the mixing section. The cold air damper is almost closed before the warm air damper is allowed to open. This prevents energy waste due to mixing.
4.5.11 Fan Coils Units A fan coil unit consists of a filter, a cooling and/or heating coil, a fan, and controls. Units can be located on the floor, above or below the ceiling, on the wall, or in the wall cabinets. Fan coil units can be designed with two or four pipes: A two-pipe unit has a set of supply and return pipes that can carry either hot or chilled water. A four-pipe unit has two sets of piping one set of supply and return for chilled water and another set for hot water. The four-pipe system is more flexible than the two-pipe system but more expensive to install. Fan coil units can be used at building perimeters in combination with VAV cooling-only systems serving interior spaces in large-scale commercial buildings. Exterior view of a FCU (vertical floor-mounted type) A close-up view of the valve assembly showing control and shutoff valves.
4.5.12 Radiant Panel Heating and Cooling Radiant heating panels can be used as perimeter heating devices in conjunction with coolingonly VAV systems, or as heating/cooling devices in conjunction with reduced-flow ventilation systems. Heating-only radiant panels can be electric or hydronic. Electric panels use heating wire or tape on the back side of the panel. Hydronic radiant ceiling panels are generally constructed of aluminum with copper tubing bonded to the back surface. Radiant heating/cooling systems are not appropriate for buildings with high ventilation needs, such as classroom buildings and laboratories.
Radiant cooling panels must be used in combination with ventilation air systems, which provide dehumidification and supplementary sensible cooling. Radiant heating/cooling panels is energy-efficient because of the reduced fan power for the supplementary ventilation systems. Also, the advantage of radiant heating panels is reduced transmission heat loss through building envelopes because the indoor air temperature can be maintained at a lower temperature. 4.5.13 Package Terminal Air Conditioners PTACs are window or through-wall units containing their own compressors and air-cooled condensers. PTACs are limited to room with outside exposure or suitable adjacent places to reject heat.
4.5.14 Heat Pumps The electric heat pump utilizes electrical energy to transport heat energy from the building exterior to the building interior The process is not an energy conversion process but rather a transfer process Types of Heat Pumps Air-to-Air Heat Pump Water-to-Air Heat Pump Water-to-Water Heat Pump Ground-Coupled Heat Pump
1) Air-to-Air Heat Pumps The most common type. Particularly suitable for factory-built unitary heat pumps. Widely used in residential and commercial applications. 2) Water-to-Air Heat Pumps Rely on water as the heat source and sink, and use air to transfer heat to, or from, the conditioned space
3) Water-to-Water Heat Pumps Use water as the heat source and sink for both cooling and heating. Heating-cooling changeover is usually done by switching the water circuits. 3) Ground-Coupled Heat Pumps Use the ground as a heat source and sink.
4.6 Economizer Cycle Economizer cycle is a AHU control method to save cooling energy by using cool outside air as a means of cooling the indoor space. When the enthalpy of the outside air is less than the enthalpy of the return air, conditioning the outside air is more energy efficient than conditioning the return air. Economizer cycle can reduce HVAC energy costs in cold and temperate climates while potentially improving IAQ, but are not appropriate in hot and humid climates. Economizer cycle can also be operated during nighttimes to cool indoor surfaces. 4.7 Energy Recovery Systems Run-Around Coil (4.7.1) Heat Pipe (4.7.2) Plate Heat Exchanger (4.7.3) Energy Transfer Wheel (4.7.4)
4.7.1 Run-Around Coil (sensible heat) The run-around coils are finned-tube copper coils placed in supply and exhaust airstreams. The coils of the run-around system are via piping, and a pump circulates water, glycol or other thermal fluid solution. The system utilizes the energy from the exhaust air stream to pre-condition the outdoor air. In summer, the exhaust air from the air-conditioned space cools the circulating fluid in the coil. The cooled fluid is then pre-cool the outdoor air. In winter, the process is reversed; heat is extracted from the exhaust air and then transferred to the make-up air. A high-performance, run-around energy exchanger can provide a large increase in overall HVAC system effectiveness from 50 percent to nearly 70 percent. Advantages This system does not require that the two air streams be adjacent to each other, several air streams can be used. It has relatively few moving parts - a small pump and control valve. There is no cross-contamination between air streams. 4.7.2 Heat Pipe (sensible heat) A heat pipe is a sealed self-contained, liquid evaporating and condensing system. It consists of evaporator and condenser sections. Hot air passes through the evaporator section and the cold air passes through the condenser side in a counter-flow arrangement. A typical heat pipe consists of a sealed pipe or tube made of a material with high thermal conductivity such as copper or aluminum at both hot and cold ends. A vacuum pump is used to remove all air from the empty heat pipe, and then the pipe is filled with a fraction of a percent by volume of working fluid (or coolant) chosen to match the operating temperature. Examples of such fluids include CFCs, water, ethanol, acetone, sodium, or mercury. The working fluid in HVAC system is normally Class I refrigerant (CFC). Heat is transferred from the hot incoming gas to the evaporator section of the heat pipe. The local vapor pressure of the evaporator section increases and evaporated fluid moves along the pipe to the condenser section. As the condenser section s temperature is low due to the cold air passing around it, the migrated vapor condenses and the heat is released to the cold air. The liquid then returns to the evaporator section by a combination of capillary action and gravity. Wick Copper tube
Advantages Passive Operation No energy input is required to operate the heat pipes. Long Life There is nothing in the heat pipes to wear out. The heat pipes are passive and have no moving parts. To guard against corrosion, the heat pipes can be ordered with a baked epoxy coating or other protective coating. Isolated Air Streams Dividing plates to separate the two air streams are an integral part of heat pipe heat recovery systems, assuring an excellent seal to prevent cross contamination.. Minimum Maintenance Since the Heat Pipes have no moving parts (apart from any dampers), the only maintenance recommended is periodic cleaning. A coil cleaner may be applied for this purpose just as for any cooling coil. Summer and Winter The same heat pipe can be used for both summer cooling and winter heat recovery without any changeover mechanism. The temperature difference between the air streams activates the energy recovery. 4.7.3 Plate Heat Exchanger (sensible heat) Two airstreams are separated by metal plates, which are usually augmented with fins. The heat exchange principle is simple: warm exhaust air heats a fixed plate, which, in turn, heats the incoming cool outdoor air on the other side of the plate. Two airstreams are separated by the plates to ensure no cross contamination. The effectiveness is a function of plate gaps and lengths, and air flow rate. A minimum effectiveness is 50%.
4.7.4 Energy Transfer Wheel (sensible heat and/or latent heat) Energy transfer wheel is a heat transfer device with a rotating wheel, which allows the sensible and/or latent heat transfer between the incoming outdoor air and exhaust air in an HVAC system. The air duct connections are arranged so that each of the airstreams flow axially through approximately one half of the wheel in a counter-flow pattern. Energy Transfer Wheel SENSIBLE WHEEL - This wheel is not coated with a desiccant and therefore transfers only sensible heat. The wheel can be constructed of almost any material (paper, metal or plastic) and transfers energy between two air streams as the mass of the material gains or loses heat to the opposite air stream. The wheel rotates at a speed of 25 to 50 revolutions per minute. ENTHALPY WHEEL - It is similar to the sensible wheel except that a desiccant material such as silica gel is coated to the wheel s surface. As the wheel rotates, it can transfer latent heat (moisture) as well as sensible heat simultaneously. This wheel also rotates at 25 to 50 revolutions per minute. The range of heat recovery effectiveness is 70% to 80%. Cross leakage through the energy recovery wheels ranges from 2% to 5% between the supply and exhaust air streams
Pre- heating and humidification of OA in winter Warm & Humidified RA From Room Warm & Humidified SA To Room Air Filter Energy Transfer Wheel Cooled & Dry EA Cold & Dry OA Hot Water From Boiler Air Filter OA Damper RA: Return Air SA: Supply Air EA: Exhaust Air OA: Outdoor Air Pre- cooling and dehumidification of OA in summer Air Filter Energy Transfer Wheel Cool & Dry RA From Room Cool & Dehumidified SA To Room Warm & Humid EA Hot& Humid OA Chilled Water From Chiller Air Filter OA Damper RA: Return Air SA: Supply Air EA: Exhaust Air OA: Outdoor Air
4.8 Controls and Automation Overview (4.8.1) Basic Control Systems and Devices (4.8.2) Building Energy Management, BMS (4.8.3) 4.8.1 Overview Heating and cooling loads vary with time; therefore, the amount of heating or cooling supplied to a space must vary to keep the temperature and humidity within certain limit (comfort range). Controls and automation provide the intelligence of mechanical and electrical systems. Automation is the function of having equipment react, without any intervention of an operator, to satisfy preset conditions. Control occurs when a signal to the equipment cause the movement or adjustment of a component to produce the desired result.
4.8.2 Basic Control Systems and Devices The control systems in HVAC systems are designed to control the following properties of the energy transporting medium, such as air and water, and the related equipment. Temperature: with sensors set for an operating temperature, a differential, or temperature limits. Pressure: with sensors set for an operating pressure, a differential, or pressure limits. Flow rate: with sensors set for an operating rate, a differential, or flow rate limits. Humidity: with sensors set for an operating level, a differential, or humidity limits. Speed: with sensors to control the equipment so that it is either on or off or has variable or multiple speed Time: with a clock or a program to control the duration of operation of the equipment. 1) Basic Control Systems Control systems may be electric, electronic, pneumatic, direct digital, or a combination of these. Electric controls: Use line voltage (110 or 220V) or low voltage (12 to 24 V) to perform the basic function. A low-voltage control system is more sensitive and is preferred. Pneumatic controls: Use 5- to 30-psi (approximately 35 to 210 kpa) compressed air and receiver controllers with force-balance (usually springs) mechanism. Electronic controls: Use a similar form of controller, except that the signals are electronic rather than pneumatic. DDC controls: DDC system controllers contain microprocessors that are programmed to interpret the input signal, process the data in resident programs, and intelligently decide on the appropriate response.
2) Basic Control Devices The basic control devices include sensors, controllers, and actuators. Sensors: Measure the monitored or controlled variable. The signal from the sensors is input to a controller for processing and decision making. Controllers: Determine if a signal be sent to a monitoring station or to an actuator. The Controllers determine if the input or output signal is two-position or proportional, or direct- or reverse-acting. Actuators: Manipulate the equipment to meet the desired set point or the controlled variable.
Commonly used HVAC sensors: Thermostats: Sense and respond to temperature. Humidistats: sense and respond to humidity (either relative or absolute). Pressure sensors: sense and respond to pressure. Current sensors: sense and respond to electric current. Gas sensors: sense concentrations of CO and CO 2, and refrigerant Flow sensors: sense and respond to rates of flow Pneumatic Actuators
Electric Solenoid Actuators (on/off control) A typical electric solenoid consists of a coil, armature, spring, and stem. The coil is connected to an external current supply. The spring rests on the armature to force it downward. The armature moves vertically inside the coil and transmits its motion through the stem to the valve. When current flows through the coil, a magnetic field forms around the coil. The magnetic field attracts the armature toward the center of the coil. As the armature moves upward, the spring collapses and the valve opens. When the electric current stops, the magnetic field disappears. This allows the spring to expand and shut the valve. Electric Motorized Actuators (proportional control with position sensor)
Signals Two-position signals Input signals from sensors: indicate the operating status of the equipment (on/off, normal/alarm, open/close) Output signals from controllers: start/stop or open/close the controlled equipment. Proportional signals Monitor and control temperature or pressure, or flow. Provide multiple levels of control and alarm. 4.8.3 Building Energy Management System (BEMS) Building energy management is typically a function of the microprocessor-based DDC controllers. Automation and controls play a major role in managing a building s energy costs. In order to manage energy usage effectively, the controllers of the energy management system must be able to speak to one another. Addition of a local area network for dedicated communication among all controllers is the most common arrangement. The most basic option for managing energy is to shut down unnecessary equipment. Operating equipment at optimum conditions is another way to save energy. An excellent example is operating multiples of a single type of equipment. Energy can also be saved through monitoring demand and shutting down equipment to prevent consumption from exceeding a preset limit. Another method of energy management is to record equipment trend. By tracking the energy usage of the equipment, it is possible to identify when operations are outside the normal range, and then to perform the required maintenance.
Occupancy Sensor Systems Occupancy sensor system can be used to determine on/off control of HVAC operation to save unnecessary energy consumption. Configuration of a hierarchical control system
Intelligent Building System (IBS)