General HVAC equipment description and operation

The following chapters describe some of the equipment used in various combinations to produce an HVAC system. A description of the equipment and its operation is provided.

1. Refrigeration systems. Refrigeration systems are most often supplied skid mounted as a packaged unit. The assembled package includes all major components and controls mounted and pre-wired requiring only that they be anchored to a foundation, hooked up to the hydronic piping system, connected to a treated water supply and connected to power. There are two basic refrigeration cycles: one compresses the refrigerant gas and the other absorbs the refrigerant at very low pressures. Chillers which chill water are often called chillers. Chillers can be packaged units or field assembled units.
   1. In the compression refrigeration cycle, HCFC-22 and HFC-134a refrigerants remove heat from a medium by changing phases. The refrigerant is compressed in its gaseous phase and passed through a condenser. Outside air or water from a cooling tower passes over the condenser coil to remove heat in one of three ways as discussed below under heat rejection equipment. The refrigerant gas leaving the condenser is then expanded through a valve which lowers its pressure and temperature further until a phase change occurs and a liquid is formed. This now cool, liquid will boil at approximately 20o to 30°F. The liquid refrigerant then passes through an evaporator coil and boils. Water or air flowing through the evaporator gives up its heat to the cold refrigerant making it boil. Entering water temperature is usually 54°F. It is usually chilled to 44°F and is called chilled water. It is treated with cleaning, anti-scaling, anti-fouling, anti-corrosion and anti- freezing agents. The chilled water is pumped to coils in air handling units (AHU) and ducts where building air or room air is passed over the coil thus cooling the air. Chilled water is often called brine because of salts added to condition the water. When building/room air is passed directly over evaporator coils and the chilled water system eliminated, the system is referred to as direct expansion (DX). DX units are located near the area to be cooled and do not use water as an intermediate medium to condition the air.
   2. Drip pans are placed under cooling coils to collect condensate resulting from cooling building/room air and dropping its dew point temperature. Condensate flows by gravity to a sanitary drain or is collected and pumped to a drain. There are several types of chillers as discussed in the following paragraphs.
   3. Centrifugal chillers use centrifugal compressors to compress the refrigerant gas. Centrifugal compressors compress gas as it enters the center of a fluted casting, housing a rotating impeller. The impeller imparts kinetic energy to the gas which turns into potential energy as the gas velocity slows, thus increasing pressure. Compression is a continuous process. One, two, or three stages may be used. Centrifugal compressors are used in large systems.
   4. Reciprocating or positive displacement chillers use reciprocating or piston type compressors to compress the refrigerant gas. The gas enters a cylinder through a valve when a piston in the cylinder is going down. The valve closes when the piston starts to go up. As the piston approaches the top of the cylinder the gas is compressed by the decreasing volume. An exhaust valve opens when the piston is near the top of the cylinder allowing the compressed gas to exit. The cycle is then repeated.
   5. Rotary screw chillers use rotary helical screw compressors to compress refrigerant gas or increase the pressure of liquid refrigerant. They are positive displacement machines. The twin-screw compressor consists of accurately matched rotors [one lobe (male) and one helix (female)] that mesh when rotating within a close tolerance common housing. One rotor is driven and geared to the other, turning it in a counter-rotating motion.
   6. Scroll chillers employ a stationary scroll and a motor driven orbiting scroll to gather refrigerant gas at the circumference and move it through an ever- decreasing volume to the center of the scroll where it exists at a higher pressure.
   7. In absorption chillers the absorption cycle uses a heat-driven concentration difference to move refrigerant vapors (usually water) from the evaporator to the condenser. The high concentration side of the cycle absorbs refrigerant vapors, diluting the absorbent material, usually lithium bromide. Absorption chillers are typically found in large or small systems. Systems with medium cooling loads typically accomplish the demand through use of other types of cooling units.
   8. In the evaporator, water at about 40°F is evaporating off the chilled water tubes, thereby bringing the temperature down from the 54°F being returned from the air handlers to the required 44°F chilled water supply temperature. The water vapor is absorbed by the concentrated lithium bromide solution due to its hygroscopic characteristics. The heat of vaporization and the heat of solution are removed using cooling water. The solution is then pumped to a concentrator at a higher pressure where heat is applied (using steam or hot water) to drive off the water and thereby re-concentrate the lithium bromide.
   9. The water driven off by the heat input step is then condensed using cooling tower water, collected, and then flashed to the required low temperature of 40°F to complete the cycle. Since water is moving the heat from the evaporator to the condenser, it serves as the refrigerant in this cycle.
   10. Lithium bromide is the most common absorbent used in commercial cooling equipment, with water used as the refrigerant. Lithium bromide has a very high affinity for water, is relatively inexpensive and non-toxic. However, it can be highly corrosive, and disposal is closely controlled. Smaller absorption chillers sometimes use water as the absorbent and ammonia as the refrigerant. Ammonia absorption is used in recreational vehicles and food processing plants.
   11. Absorption chillers must operate at very low pressures, about l/l00th of atmospheric pressure for the water to vaporize at the required ~ 40°F.
   12. Absorption chillers are available in two types, single effect (stage) units using low pressure (20 psig or less) as the driving force and 9 psig steam at the generator, and double effect (two-stage) Units available as gas-fired or steam-driven with high pressure steam (40 to 140 psig). To achieve their
2. Heating systems. To heat building/room air, heating water coils are mounted in ducts. The heating water receives its heat directly from a hot water boiler or from a heat exchanger. Entering the heat exchanger is high temperature hot water or steam which provides heat for the heating water system. Building/room air can also be heated directly in an air heating furnace or by passing over the evaporator turned condenser in a direct exchange heat pump system.
   1. Heating generators are most often supplied skid mounted as a packaged unit. The assembled package includes all major components and controls, mounted and pre-wired requiring only that they be anchored to a foundation, hooked up to the hydronic piping and duct systems, connected to a fuel supply and treated water supply, connected to a stack and connected to power. Following is a discussion of various heat generators.
   2. Boilers burn natural gas, propane, oil or coal or use electric to produce steam, high temperature water and hot water. A boiler can be provided in each building for heating or a central location for
   3. heating a complex of buildings. In a firetube boiler, gaseous products of combustion pass through tubes surrounded by the water to be heated. In a watertube boiler, water passes through the tubes as the products of combustion passes over them. In an electric boiler, electric heating elements are inserted directly into the water. Depending on temperature and pressure the water will vaporize  into steam, become high temperature water, or become hot water. Steam can be piped directly to individual room heaters of various types or can be used in a heat exchanger to condense, transferring the heat of vaporization to heating water. High temperature hot water boilers work in a similar fashion but produce high temperature water instead of steam. Hot water boilers  produce hot water from 140 to 1 80oF for direct use in heating coils to heat building or room air.
   4. Air heating furnaces burn natural gas, propane, or oil, or use electric to produce hot air to be used directly in heating building space or rooms. An air heating furnace is usually provided in each building for heating. When fuel is burned, the products of combustion are mixed with excess air and flow inside of a tube bundle or heat exchanger to heat the building/room air passing over the outside of the tube. When electric is used, heating elements are inserted directly into the air stream.
   5. Heat pumps are discussed below, Unitary heat pump/air-conditioning equipment.
3. Heat rejection equipment. Heat rejection equipment is most often supplied skid mounted as a packaged unit. The assembled package includes all major components and controls mounted and pre-wired requiring only that they be anchored to a foundation, hooked up to the piping system, connected to a treated water supply, and connected to power. A discussion of heat rejection equipment including cooling towers, evaporative condensers and air-cooled condensers to remove heat from the condenser coil of a refrigeration machine follows.
   1. Water from a cooling tower may be passed through the condenser to remove heat. Under full load conditions the design condenser cooling water inlet temperature is 85°F and leaving temperature is 95°F. The cooling tower water can then be directly exposed to outside air to reject heat from the water. This water is treated for corrosion, fouling, and scaling and is circulated from a basin in the bottom of the tower. Cooling towers may also use a closed loop for condenser water. A coil is installed in the tower; condenser water is pumped through it and therefore not exposed to the atmosphere. The water basin in the tower now provides water to spray over the tower coil. This with the assistance of a fan removes heat from the condenser water. This basin water is also treated for corrosion, fouling, and scaling.
   2. An evaporative condenser is placed directly over a basin, water from the basin is sprayed, and air is forced over the coils resulting in cooling of refrigerant in the coils. At full load conditions the design entering air dry bulb temperature is usually 95°F and entering air wet bulb temperature is 75°F.
   3. An air-cooled condenser has outside air passed over it, the refrigerant in the coil is cooled directly by the air. At full load conditions the design entering air dry bulb temperature is 95°F. Entering condenser refrigerant vapor temperature is 190°F. Leaving condenser refrigerant temperature is 125°F.
4. Air handling equipment and systems. Air handling equipment is designed to condition air in rooms or in designated areas in a building called zones. In an AHU air is moved through filters and over coils to clean, heat or cool it. Fluid flow in coils or air flow through the AHU can be adjusted for interior environmental control. Following are variations of AHUs.
   1. A single-zone AHU controls interior building air in one building zone. There may be several rooms in one zone.
   2. Multi-zone AHUs control interior building air in two or more building zones.
   3. A variable air volume (VAV) AHU controls interior building air by varying air flow over the coils as opposed to the usual way of varying the flow of the heating water and chilled water through the coils.
   4. A terminal unit VAV (dual duct) unit is located at the end of a heating air and cooling air supply duct and controls interior building air by varying the air volume and mix.
   5. A terminal unit dual duct fan series AHU is located at the end of a heating air and cooling air supply duct and controls interior building air by varying the air volume with the use of local fan(s).
   6. Terminal units with cooling are located at the end of a heated or non-heated air supply duct and include a cooling coil for control of interior building air.
   7. Terminal units with and without fans are located at the end of a conditioned air supply duct and may or may not include a fan for control of interior building air.
   8. DX air units control interior building air by using a DX coil in the air stream. In the DX coil the fluid flowing inside of the coil is a refrigerant. Generally, this coil is for cooling, but it can be used for heating as well.
   9. Fan coil units are AHUs which control interior building air with a coil to change air temperature and a fan to force the air over the coil and into the building.
   10. Unit heaters usually employ a fan to circulate air within an area or zone and have no supply or return air duct or system of ducts. To control building air, steam coils, hot water coils, electrical elements, or direct gas fired heat exchangers may be used for heating. Radiant heaters use gas and sometimes electric to heat a ceramic surface. Heat is transferred by radiation to the floor and objects in the room which then heat air by free convection instead of forced convection as is accomplished in most AHUs.
   11. Base heater units consist of an electric element or a finned tube with hot water flowing inside the tube. These units heat interior air by free convection or sometimes incorporate a fan to heat air by forced convection. They heat air in the local vicinity of the heater.
   12. A computer room unit is prepared for controlling the environment in a computer room. This includes provisions for filtering, heating, cooling and humidity control. Humidity control is usually provided by increased cooling capacity and reheating of the air through additional heating capacity.
   13. A packaged roof top unit is an AHU with its duct, coils, fans and controls packaged in a weather proof box and is provided with weather proof components to withstand the elements. It is also designed to be supported on a roof with supply and return air ducts located on the bottom of the unit.
   14. An economizer is a gas-to-gas, liquid-to-gas or liquid-to-liquid heat exchanger which, when practical, is used to salvage waste heat from a process or part of an HVAC system. The waste heat is used to pre-heat heating air thereby reducing the amount and cost of heating. The heating system is therefore more economical to operate with an economizer installed.
   15. Variable speed fans are used in units where a variable volume of air flow is required. Variable speed fans may be provided in place of a central fan and may incorporate inlet guide vanes and/or modulating dampers.
5. Unitary heat pump/air-conditioning equipment. Unitary heat pumps are factory-packaged refrigerant based units. They can provide cooling or heating of interior building air. Both heating and cooling of air can be provided if a refrigerant reversing valve and associated controls are included. This valve reverses the flow of refrigerant transforming the evaporator into a condenser and the condenser into an evaporator. In this way heat can be moved (pumped) from inside building air to the outside in the summer, and from the outside to inside building air in the winter. Provisions are provided to collect condensate from cooling interior air and connection to a drain is needed. Heat pumps and air- conditioning equipment are available in a number of application categories which include the following.
   1. A packaged terminal heat pump (PTHP) unit is designed to be installed at the end of a building interior air supply duct. It includes appropriate duct connections, coils, fans and controls packaged in one box. It is also designed to be supported from overhead.
   2. Single packaged units have their duct, coils, fans and controls packaged in one weather proof box and are provided with weather proof components to withstand the elements. They are designed to be located outside and supported on a concrete pad or steel frame from the bottom with supply and return air ducts located horizontally or vertically. Console under window units are also available without duct connections.
   3. A roof-top packaged unit has its duct, coils, fans and controls packaged in a weather proof box and is provided with weather proof components to withstand the elements. It is also designed to be supported on a roof with supply and return air ducts located on the bottom of the unit.
   4. Split heating, air conditioning, and heat pump units have their ducts, coils, fans and controls packaged in two boxes. A weather proof exterior box for the compressor, condenser coil, condenser fan and controls are designed to be supported on a concrete pad or steel frame from the bottom. An interior box is provided with supply and return air duct connections, fan and drip pan. Connections for refrigerant tubing are provided on both boxes. This tubing is routed as needed and insulated to prevent heat loss or gain in the refrigerant and to prevent condensate from forming on the tubing.
   5. Ground source heat pumps (GSHP) are typically installed indoors. One box contains the compressor, pump, fans, duct connections and controls, but often components are not included in the central box to adapt the system for the user’s needs. The box is connected to underground piping networks that exchange heat with the ground. These pipes can be closed loops, buried only feet underground or connected to sources of water such as wells or ponds. A desuperheater can also be employed during the summer months to extract heat from the conditioned air for hot water heating.
6. HVAC control systems. HVAC control systems are used to control heating, cooling, and ventilating of interior building/zone/room air and are also used in fire safety control schemes
   1. Direct digital control (DDC) HVAC control systems use sensors, electric actuators, and microprocessors to provide a marked upgrade in system functionality over that attainable with pneumatic control. DDC systems provide operators the ability to remotely monitor existing conditions, change setpoints, diagnose and sometimes fix problems from a workstation or laptop PC. A single operator could monitor many buildings with DDC control, in addition to performing other operating and maintenance (O & M) tasks. Energy management and reporting functions improve operational efficiency and cost savings. Operational problems can be identified early and fixed before they become larger and more expensive. DDC systems involve vendor- specific application software and a communications network. Each software application provides varying degrees of flexibility and a unique software approach to DDC. The systems are often incompatible.
   2. Fire safety air moving systems provide fire safety to air zones and to stairwells by moving air or smoke. These systems provide for smoke removal only or air pressurization only to help keep areas clear of smoke and provide fire control. They may also use a combination of smoke removal with air pressurization to isolate an air zone for fire safety control.
   3. EMCS manage heating and cooling in a building or zone of a building according to programmed time and temperature schemes for efficient use of energy.
   4. Hydronic systems supply controlled chilled and heated water to cooling or heating coils. In hydronic systems an air separator is incorporated to remove air from the water and an expansion tank is provided to account for changing density in the water. Secondary piping systems are used to circulate water in secondary loops located in different buildings or zones which branch off of the main or primary heating or cooling water supply loop.
   5. Ventilation systems provide circulation of controlled conditioned and filtered air to building zones or rooms and incorporate exhaust air relief and make-up outside air.
7. Smoke management systems. HVAC systems can interface with fire detection/alarm systems. When HVAC control systems interface with fire systems, signals from the fire system are used to take action to minimize the fire and/or provide safe shut down of equipment.
   1. Smoke management systems can use smoke dampers to inhibit the passage of smoke from one zone to another. By appropriate opening and closing of smoke dampers in HVAC ducts, building zones free of fire can be pressurized with air while the zone with the fire can have smoke pulled from it.
   2. Fire dampers are dampers which close when a fuseable link melts and permits the damper to close. This prevents air movement in the duct which helps limit the fire and prevents smoke from flowing in the duct.
   3. Fire stops and smoke barriers are fixed obstacles provided to stop the movement of fire or smoke in walls or ceilings.
   4. Automatic and course control can be used in HVAC control systems for fire safety and smoke management. Air distribution systems have manually operated devices that stop the operation of supply, return and/or exhaust air and fans in an emergency. Automatic shutdown capability automatically, according to a predetermined fire management plan, closes dampers and shuts down fans when detectors located in the supply ducts, return ducts or the building fire protection system detect a fire.
   5. Proper gasketing and sealing of doors is needed to prevent air infiltration into a zone during a fire.
   6. Door release hardware and automatic door openers provide for opening doors for human egress and to control fire and smoke as predetermined in a fire management plan.
8. Ducting. HVAC ducts come in various configurations including round, square, rectangular and round oval. They are made from iron, concrete, galvanized steel, rigid fibrous and flexible materials. Metal ducts which supply cooling air must be insulated. Duct systems must be tested for leaks and cleaned before being put into service. Ducts can incorporate heating coils, cooling coils, DX coils, flow switches, pressure switches and gages, smoke detectors, heat detectors and dampers for system control.
9. Piping systems. Piping systems supply steam, high temperature water and hot water for heating and chilled water for cooling. These systems also provide make-up water to boilers and return steam condensate to boilers. Condensate from cooling coils is taken to a drain by pipe systems. Pumps move fluids, and steam is moved by pressure differential.
   1. Pipe systems may include steam/hot water converters which heat water with steam by using a heat exchanger, finned tube radiators which heat room air by natural convection, control valves operated by the HVAC control system for shutting off and controlling fluid flow, solenoids electrically actuated for stopping fluid flow, and relief valves to prevent over pressure of vessels and systems. Hydronic piping systems incorporate air separators and expansion tanks and must be adjusted and balanced before being put into service. Piping systems must be hydrostatically tested for leaks before being put into service. Piping can incorporate flow switches, pressure switches and gages, thermometers and thermo-wells, solenoids and relief valves for system control.
   2. Piping systems come in four types: 1-pipe, 2-pipe, 3-pipe and 4-pipe. The limitation of 1 and 2- pipe systems is that they can only operate in one mode at a time; heating is typically provided for a portion of the year while cooling is provided for the remainder. In 3 and 4-pipe systems, different users in the same building can call for heat or cooling at the same time, Two and 4- pipe systems are more efficient and consequently more expensive that their respective 1 and 3- pipe pairs.