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Chapter 11 HVAC Air Systems

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1 Chapter 11 HVAC Air Systems
HVAC Systems • Common HVAC System Designs • Air-Handling Unit Components • Air-Cleaning Methods • Ventilation Air • Common Air-Handling Systems • Facility Pressures

2 Chapter 11 HVAC Air Systems
HVAC Systems • Common HVAC System Designs • Air-Handling Unit Components • Air-Cleaning Methods • Ventilation Air • Common Air-Handling Systems • Facility Pressures

3 Most air distribution systems of commercial buildings operate for occupant needs 24 hr a day, 7 days a week. HVAC systems must be geared to meet both specific and varying application requirements. A commercial building usually contains public space, office space, computer space, retail space, cafeteria space, and/or restaurant space. The air distribution systems of commercial buildings usually operate from a minimum of 12 hr a day, 5 days per week, up to 24 hr per day, 7 days a week. To conserve energy, some buildings can use the main HVAC system at partial load and meet specific occupant needs through smaller supplemental systems. See Figure Despite the type of HVAC system used, the system will serve the same general purpose as all HVAC systems—to distribute conditioned air to building spaces for occupant comfort.

4 Recommended temperature ranges differ between the summer and winter seasons.
Temperatures in occupied building spaces should be as close to the design setpoints as possible. When people feel uncomfortably hot or cold, they are more likely to complain about IAQ problems. Many building leases now specify required ranges for temperature and relative humidity. See Figure 11-2.

5 When water is present, even a small collection of dirt and dust provides a breeding ground for microbial contaminants in HVAC equipment such as drip pans, humidifiers, ducts, and coils. The inside and outside of all HVAC equipment must be kept clean and dry. Cleaning minimizes dirt and dust buildup. Dust buildup can become airborne and be distributed by the HVAC system. In the presence of water, even a small collection of dirt and dust provides a breeding ground for microbial contaminants. Drip pans, humidifiers, duct, and coils must be periodically checked for microbial growth. See Figure 11-3.

6 HVAC systems that use 100% outside air do not use any return air because the return air may contain contaminants. One way to categorize air-handling systems is to describe the system as either a 100% outside-air (OA) system or as a mixed-air system. A 100% outside-air (OA) system is a system that does not recirculate any return air from building spaces. Instead, 100% outside-air systems use outdoor air from outside the building for heating and cooling purposes. See Figure Using 100% outside air is a way to prevent any contaminants already in the indoor air from being recirculated back into building spaces. Places where 100% outside-air systems may be used include cafeterias, laboratories, operating rooms in hospitals, and others.

7 Mixed-air systems allow a percentage of outside air to combine with return air for space use.
A mixed-air system is a unit or section of an HVAC system that brings a specified amount of return air into contact with the outside air entering the HVAC system for space use. See Figure Mixed-air systems assume that the return air is relatively free of contaminants. The amount of outside air mixed with return air is determined by the control system. Outside-air dampers and return-air dampers are positioned to obtain the proper amount of mixed airflow. A minimum open setting for the outside-air damper is used to ensure that a specified amount of outdoor air is introduced into the system for ventilation purposes. Some mixed-air systems can also be used as economizers.

8 A dry bulb economizer is an economizer that operates in proportion to the outside-air temperature, with no reference to the humidity values of the air. A dry bulb economizer is a type of economizer that operates strictly in proportion to the outside-air temperature, with no reference to humidity values. As long as the outside-air temperature is below a specific value, the air can be used for economizer cooling. See Figure Usually, the temperature of the outside air must be below 65°F. The exact setting is dependent upon the prevailing climate in specific geographical regions. When the outside-air temperature rises and is too warm for free economizer cooling, the economizer cycle is ended. Ending the economizer cycle forces the outside-air damper to a minimum value, which allows enough outside airflow for ventilation purposes only.

9 Enthalpy economizers use temperature and humidity levels of the outside air to control the operation of the HVAC system. An enthalpy economizer is a type of economizer that uses temperature and humidity levels of the outside air to control operation. Enthalpy is the total heat content of a substance. For enthalpy control to work, both temp-erature and humidity sensors are mounted in the outdoor air. However, due to the problems associated with humidity sensors in the outdoor environment, humidity readings are usually taken in the mixed-air airstream using new algorithms for the controller. See Figure A controller uses the temperature and humidity values to calculate the current enthalpy of the outside air in British thermal units per pound (Btu/lb). When the enthalpy of the outside air is low enough, the economizer cycle of the HVAC system is used.

10 Air-handling units are found in many designs, but small units are usually known as terminal units, and large units are usually known as makeup air units. An air-handling unit is the part of an HVAC system that operates to distribute air throughout a building. An air-handling unit consists of several sections including a fan (blower), filter racks, a plenum, heating coils and/or cooling coils, and damper sections. Air handlers are found as small units (terminal units) that usually include a filter, coil, and blower, or large units (makeup air units) that condition 100% of outside air for use. See Figure Air-handling units designed for outdoor use are usually installed on roofs and are therefore known as rooftop units (RTUs).

11 Fans provide the mechanical energy to move air through an HVAC system and to building spaces.
Fans that are designed to operate at a constant speed are usually belt driven because the standard motor speed (with pulley ratios) is enough to overcome system static pressures (resistances to flow). Motors and belts that are designed to be inside the duct or fan housing can emit odors or belt material into the airstream. The bearings, shaft, and sheave wear out. Also, excessive operating temperatures can generate airborne contami- nants as well. See Figure 11-9.

12 Due to the amount of contaminants that can be present in inside air, the American National Standards Institute (ANSI) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have set ventilation standards as a guide for maintaining proper indoor air quality. The American National Standards Institute (ANSI) and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) have set ventilation standards as a guide for maintaining proper indoor air quality. The idea behind the standards is to dilute any contaminants by introducing the proper amount of outside air into a space. Outside air can never be assumed to be free of pollutants because outdoor air may contain vehicle exhaust fumes, boiler or building exhaust gases, acid-laden air, construction dust, trash compactor odors, and a host of other contaminants and irritants. See Figure It is important to perform air-sample testing of the outside-air intake grills of the system before evaluating the potential problems with the system itself.

13 Proper operation of all HVAC system dampers must be periodically checked to ensure proper indoor air quality and energy efficiency. The function of outside-air (OA), return-air (RA), and exhaust-air (spill) dampers is an interrelated operation requiring periodic inspection, maintenance, and testing for proper operation. See Figure

14 The formula used to determine the percentage of outside air being used by air temperature varies between summer and winter. A building operating in the summer has a mixed-air temperature of 85°F, a return-air temperature of 75°F, and an outside-air temperature of 100°F. What is the percentage of summer outside air being distributed to building spaces? See Figure See calculation on page 210.

15 When the temperature of the outdoor air and indoor air are too close to truly determine, measuring CO2 becomes the pre-ferred method for determining the percentage of outdoor air being introduced into a system. A building operating in the summer has a mixed-air amount of CO2 at 845 ppm, a return-air amount of CO2 at 1100 ppm, and an outside-air amount of CO2 at 530 ppm. What is the percentage of outside air being distributed to building spaces? See Figure See calculation on page 210.

16 Electronic air cleaners effectively separate out dirt from the air for small air volumes, but large air volumes require that multiple units be used, which consumes large amounts of energy. An electronic air cleaner is a type of air cleaner in which particles in the air are positively charged in an ionizing section and then the air moves to a negatively charged collecting area (i.e., plates) to separate out the particles. As the electrostatically charged particles pass through the negatively charged collecting area, the positively charged dirt particles are attracted to the negatively charged collector plates. See Figure Depending on the system, the particles clinging to the collector plates are washed off with water or blown off with compressed air.

17 Ultraviolet (UV) air-cleaner systems kill biological contaminants using a specific light wavelength (approximately 450 nanometers) not visible to the human eye. An ultraviolet (UV) air-cleaner system is an air-cleaning system that kills biological contaminants using a specific light wavelength. Ultraviolet systems utilize UV light-generating equipment. Nonbiological pollutants such as particulates and VOCs are not affected by UV systems. Again, ultraviolet systems are better suited for smaller systems or in spot use, such as in a condensate pan or condenser water system where biological growth com-monly occurs. See Figure

18 A carbon filter uses an activated carbon-filtering medium to remove most odors, gases, smoke, and smog from the air by means of an adsorption process. A carbon filter is an air-cleaning method where an activated carbon-filtering medium is used to remove most odors, gases, smoke, and smog from the air by means of an adsorption process. See Figure Using carbon filters, however, usually results in modifications to the filter section, higher filter costs, increased maintenance time, high-pressure drops, and increased energy costs. When an indoor-air source is the problem, the con-taminants must be removed at the source using localized exhaust. By using exhaust fans, the problem can be solved without having to redesign the air-handling system.

19 In order to not contaminate building spaces, filters must be inspected periodically, of the proper thickness, stored and used properly, correctly installed, and handled correctly when replacing. Mechanical filters contain a fiber that has two purposes. The first purpose is to trap any particulate matter, while the second purpose is for the particulates to remain attached to the fibers of the filter medium. The filter medium traps particulate matter in several different ways. The fibers may be arranged in layers, or interwoven using alternating thicknesses of fiber. Sometimes ad-hesives are applied during the manufacture of the filter or applied as a spray product at the time of installation. See Figure

20 Filters are rated for efficiency (MERV rating), airflow resistance, and dust-holding capacity.
The quality of filtered air can directly affect IAQ, depending on the type and efficiency of the filtering medium. Filters are rated for efficiency, airflow resis-tance, and dust-holding capacity. Filter efficiency (i.e., MERV rating) indicates the ability of a filter to remove particulate matter from air. See Figure

21 The pressure drop across a filter is checked by an electronic manometer to determine if a filter is clogged, which would result in filter replacement. Filters are usually scheduled to be replaced when the filter has performed a specific amount of work (i.e., time in use). To determine whether a filter must be replaced, an electronic manometer is used to measure the increase in pressure drop across the filter and/or a visual inspection is performed to assess the amount of dirt clogging the filter. See Figure

22 The proper sizing of ducts and the location and design of diffusers, registers, and grills affect the amount of air movement in building spaces. An air-handling unit must be connected to a good distribution system (ducts) in order to perform properly. Space limitations have been known to compromise the design of air distribution ducts. Control devices, such as main and branch volume dampers or blowers controlled by variable frequency drives, help distribute the air according to the design engineer’s load calculations. The proper sizing, location, and aerodynamic design of supply diffusers help to distribute air evenly throughout spaces. The proper location of return-air grills ensures a sweeping effect of the air, negating short cycling of the conditioned supply air. See Figure

23 Cooling coils usually consist of a finned coil, copper tubes, and aluminum fins that allow chilled water to be used for cooling air. The cooling coil of an HVAC system is a heat exchanger that allows chilled water to be used for cooling air. Cooling coils usually consist of a finned coil, copper tubes, and aluminum fins. See Figure The air is cooled using chilled water or a refrigerant. While the cooling coil is cooling the air, it is also removing moisture from the air (condensate) at the same time. When the air leaves the cooling coil, it is at a very high relative humidity.

24 Drain pans must be pitched to allow drainage and must be periodically checked for blockages in the pan and/or drain pipes. Periodic inspection and cleaning of the interior parts of the cooling coil section, including the main drain pan, intermediate coil drain pans, water traps, and cooling coils are essential. See Figure Drain pans must be pitched properly to allow for positive drainage. Even when drain pans are properly positioned, the condensate water trap must prevent air from entering a coil section that is under vacuum and from escaping a coil section that is under pressure.

25 Humidifiers use various design methods to introduce water vapor into the duct airstream for occupant comfort. Most air-handling systems use some form of humidity control during the winter heating periods. This is generally accomplished with the introduction of water into the airstream on the output side of the heating coil. See Figure Steam from boilers must not be introduced into the HVAC airstream under any circumstances.

26 Single-zone air-handling units provide primary HVAC capability to one zone or area.
A single-zone system is a type of air system that serves a single temperature control zone and is the simplest form of air system. See Figure Single-zone systems are completely responsive to the requirements of the space. Well-designed air systems maintain tem-perature and humidity efficiently and can be shut down without affecting the operation of adjacent areas.

27 Multizone air-handling units use mixing dampers in the air handler to provide HVAC to a small number of zones. A multizone system is a type of air system that serves a relatively small number of zones from a single, central air-handling unit. See Figure The system delivers a constant volume of air. In response to zone ther- mostats, the requirements of the various zones are met by mixing cold and warm air through zone dampers at the central air handler. The cold deck and hot deck tem-peratures may be varied in response to zone loads. The mixed conditioned air is distributed throughout the building by a system of single-zone ducts. All of the various economizer control arrangements can be used with a multizone system.

28 Dual-duct systems use hot ducts and cold ducts to carry conditioned air to mixing boxes in several zones to provide whatever comfort is required in a specific zone. A dual-duct system is a type of air system in which all the air is conditioned in a central fan system and distributed to conditioned spaces through two parallel main ducts. One duct carries cold air and the other carries warm air, thus providing air sources for both heating and cooling at all times. Dual-duct systems are used in buildings that also have multiple zones and can serve a wide variety of loads, but are considered to be energy inefficient and are very expensive to install. See Figure

29 Reheat systems utilize independent heating coils in the duct of each zone controlled by a thermostat in the zone. A reheat system is a type of air system that permits zone or space control for areas of different exposure, provides heating or cooling of perimeter areas of unequal exposure, or provides process or comfort control where close temperature control is required. See Figure Unfortunately, reheat systems are also known for being energy inefficient.

30 VAV air-handling units usually use a variable-frequency drive-controlled fan to vary air volumes in a duct in order to maintain the desired static pressure. A variable-air-volume (VAV) system is an air-handling unit that provides air at a constant temperature but varies the amount delivered to various loaded zones. See Figure VAV systems usually use a variable-frequency drive or variable-pitch fan blades to vary the amount of air. A central fan system delivers air at a constant temperature.

31 Even though variable-air-volume terminal boxes can throttle airflow from 100% to 0% of design flow, all VAV terminal boxes have a minimum flow setting to maintain ventilation standards. A VAV terminal box can throttle airflow from 100% to 0% of the design flow. A reduction in airflow can reduce ventilation airflow to a space below an acceptable quantity when the space temperature is satisfied and the damper is fully closed. Having the dampers 100% closed on a variable-air-volume terminal box does cause many air quality complaints. See Figure

32 The stack effect in a building occurs when hot air rises up through the building elevator shafts, stairwells, and service columns, similar to the operation of a boiler chimney. Facilities are usually operated at a slightly positive pressure so that contaminants do not leak into the building. A positive pressure is especially important on lower floors due to the stack effect of the building’s design. The stack effect occurs when hot air rises through the facility elevator shafts, stairwells, and service columns. See Figure

33 All pan and floor drain traps must be filled with water to prevent sewer gases from entering a building or HVAC system. To prevent odors, all pan drain traps must be filled with water to prevent sewer gases from entering the building or HVAC systems. Vacant floor plumbing traps that may have lost their water seal due to evaporation over a period of time must be inspected and filled. See Figure All drain and sewer line leaks must be repaired immediately.

34 One-pipe hydronic systems have a single pipe that acts as the supply pipe and return pipe for the flow loop, connecting one terminal unit to the next terminal unit. A one-pipe hydronic system is a type of hydronic piping system that uses a single pipe as both a supply pipe and return pipe. See Figure One-pipe systems are limited to residential and small commercial systems. A major disadvantage of the one-pipe system is that each terminal unit uses the water discharged from the previous terminal unit. Thus, the system rapidly loses efficiency.

35 Two-pipe hydronic systems have a separate supply pipe and return pipe at each terminal unit.
A two-pipe hydronic system is a type of hydronic piping system that has a separate supply pipe and return pipe. See Figure Each coil has a separate take off from the supply line, ensuring that adequate water flow is available. The hot and chilled water sources are con-nected to a common return pipe and valves. Manual valves, automatic valves, and balancing valves are used to ensure adequate flow and control.

36 Three-pipe hydronic systems have a hot-water loop and a cold-water loop so that hot or cold water can be introduced to any terminal unit at any time. A three-pipe hydronic system is a type of hydronic piping system where hot or cold water can be introduced to a terminal unit (e.g., coil). Some three-pipe systems switch between hot water in the winter and chilled water in the summer. During a change in seasons, a three-pipe system is utilized so that either water source can be selected for the specific situation. See Figure This may be accomplished by using a manual system, outside-air thermostat, mixing valves, or other methods. Care must be used with changeover systems because introducing warm water to the evaporator of a chiller will cause high-head pressures and mechanical problems in the chiller.

37 A four-pipe hydronic system uses supply and return heating piping and supply and return cooling piping. A four-pipe hydronic system uses separate piping for heating and cooling. The terminal units are connected to both heating and cooling pipes. Water flow to the terminal units is controlled by mixing and diverting valves that allow either hot or cold water to flow through any terminal unit. Four-pipe hydronic systems are expensive to install but provide excellent control of air temperature. Four-pipe hydronic systems are more economical to operate than three-pipe hydronic systems. See Figure

38 Steam heating systems consist of a high-pressure boiler, fittings, accessories, steam supply piping, ter-minal units, a condensate return system, and controls. A steam heating system is a heating system that uses steam to carry heat from the point of generation to the point of use. Steam heating systems are used because steam carries more British thermal units per pound of steam than water, allowing steam to be used where there is a limited amount of room for the distribution system. Another advantage to steam heating systems is that the steam does not need to be pumped to the location of use. Steam is a vapor that flows any time there is a difference in pressure from one area (boiler) to another area (terminal unit). The boiler is located in a central plant and the steam is distributed to individual terminal units or buildings. See Figure

39 In direct-return systems, the shortest supply line has the shortest return. In reverse-return systems, the shortest supply line has the longest return. Another design consideration for water distribution systems is the routing of the supply and return piping. A direct-return system is a system that is piped so that the terminal unit with the shortest supply pipe also has the shortest return pipe. See Figure This means that the total piping is less, which means that the total resistance is lowest and the flow the greatest. A reverse-return system is a system consisting of pipes in which the coil with the shortest supply pipe also has the longest return pipe. This means that the relative pipe runs to each coil are about the same. This helps with water-balancing issues.

40 Large complexes such as hospitals and university campuses use primary/secondary systems to distribute water to buildings. In large multibuilding facilities, primary/secondary systems are often used. See Figure In primary/secondary systems, the hot or chilled water source has the supply connected to the return with its own pumping system. This is the primary loop. A separate supply take off and return pipe is available at each building with a separate pump. This is known as the secondary loop. There may be multiple secondary loops used, allowing each zone the capability of using as much water as required.

41 Radiant heat panels have resistance heating elements and radiate heat directly to the area below.
An electric radiant heating system is a heating system that transfers heat from an electric resistance heating element to air using radiant energy waves. Radiant heating is heating that occurs when a surface (resistance heating element) is heated and the surface gives off heat in the form of radiant energy waves. As the heating element heats up, the wire gives up heat to the air passing over the element, which raises the temperature of the air. See Figure Different versions of electric radiant heat panels are made for indoor use or outdoor use, with designs available for terminal unit use.

42 Electric baseboard heaters are smaller than electric space heaters, and are less expensive to install but more costly to operate than other heating systems. An electric baseboard heating system is an electric heater that is enclosed in a low cabinet that fits along the baseboard of a space. Electric baseboard heaters are smaller than typical electric space heaters and use a section of fins to transfer heat. The fins provide a large heat transfer surface area. Electric baseboard heaters can be 12″ high and are available in any length. Electric baseboard heaters are installed at floor level along the outside walls of building spaces. See Figure


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