Auto control for Commercial building P2

ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 31 Fig. 50. Rod-and-Tube Element. In a remote-bulb controller (Fig. 51), a remote capsule, or bulb, is attached to a bellows housing by a capillary. The remote bulb is placed in the controlled medium where changes in temperature cause changes in pressure of the fill. The capillary transmits changes in fill pressure to the bellows housing and the bellows expands or contracts to operate the mechanical output to the controller. The bellows and capillary also sense temperature, but because of their small volume compared to the bulb, the bulb provides the control. Fig. 51. Typical Remote-Bulb Element. Two specialized versions of the remote bulb controller are available. They both have no bulb and use a long capillary (15 to 28 feet) as the sensor. One uses an averaging sensor that is liquid filled and averages the temperature over the full length of the capillary. The other uses a cold spot or low temperature sensor and is vapor filled and senses the coldest spot (12 inches or more) along its length. Electronic temperature controllers use low-mass sensing elements that respond quickly to changes in the controlled condition. A signal sent by the sensor is relatively weak, but is amplified to a usable strength by an electronic circuit. The temperature sensor for an electronic controller may be a length of wire or a thin metallic film (called a resistance temperature device or RTD) or a thermistor. Both types of resistance elements change electrical resistance as temperature changes. The wire increases resistance as its temperature increases. The thermistor is a semiconductor that decreases in resistance as the temperature increases. Because electronic sensors use extremely low mass, they respond to temperature changes more rapidly than bimetal or sealed-fluid sensors. The resistance change is detected by a bridge circuit. Nickel “A”, BALCO, and platinum are typical materials used for this type of sensor. In thermocouple temperature-sensing elements, two dissimilar metals (e.g., iron and nickel, copper and constantan, iron and constantan) are welded together. The junction of the two metals produces a small voltage when exposed to heat. Connecting two such junctions in series doubles the generated voltage. Thermocouples are used primarily for high- temperature applications. Many special application sensors are available, including carbon dioxide sensors and photoelectric sensors used in security, lighting control, and boiler flame safeguard controllers. PRESSURE SENSING ELEMENTS Pressure sensing elements respond to pressure relative to a perfect vacuum (absolute pressure sensors), atmospheric pressure (gage pressure sensors), or a second system pressure (differential pressure sensors), such as across a coil or filter. Pressure sensors measure pressure in a gas or liquid in pounds per square inch (psi). Low pressures are typically measured in inches of water. Pressure can be generated by a fan, a pump or compressor, a boiler, or other means. Pressure controllers use bellows, diaphragms, and a number of other electronic pressure sensitive devices. The medium under pressure is transmitted directly to the device, and the movement of the pressure sensitive device operates the mechanism of a pneumatic or electric switching controller. Variations of the pressure control sensors measure rate of flow, quantity of flow, liquid level, and static pressure. Solid state sensors may use the piezoresistive effect in which increased pressure on silicon crystals causes resistive changes in the crystals. FLAPPER SPRING SIGNAL PORT BRASS TUBE INVAR ROD EXTENSION SPRING SENSOR BODY C2081 MECHANICAL OUTPUT TO CONTROLLER BELLOWS LIQUID FILL CAPILLARY CONTROLLED MEDIUM (E.G., WATER) BULB C2083 ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 32 FLOW SENSORS Flow sensors sense the rate of liquid and gas flow in volume per unit of time. Flow is difficult to sense accurately under all conditions. Selecting the best flow-sensing technique for an application requires considering many aspects, especially the level of accuracy required, the medium being measured, and the degree of variation in the measured flow. A simple flow sensor is a vane or paddle inserted into the medium (Fig. 53) and generally called a flow switch. The paddle is deflected as the medium flows and indicates that the medium is in motion and is flowing in a certain direction. Vane or paddle flow sensors are used for flow indication and interlock purposes (e.g., a system requires an indication that water is flowing before the system starts the chiller). MOISTURE SENSING ELEMENTS Elements that sense relative humidity fall generally into two classes: mechanical and electronic. Mechanical elements expand and contract as the moisture level changes and are called “hygroscopic” elements. Several hygroscopic elements can be used to produce mechanical output, but nylon is the most commonly used element (Fig. 52). As the moisture content of the surrounding air changes, the nylon element absorbs or releases moisture, expanding or contracting, respectively. The movement of the element operates the controller mechanism. NYLON ELEMENT RELATIVE HUMIDITY SCALE C2084 LOW HIGH ON/OFF SIGNAL TO CONTROLLER SENSOR PIVOT FLOW PADDLE (PERPENDICULAR TO FLOW) C2085 Fig. 52. Typical Nylon Humidity Sensing Element. Electronic sensing of relative humidity is fast and accurate. An electronic relative humidity sensor responds to a change in humidity by a change in either the resistance or capacitance of the element. If the moisture content of the air remains constant, the relative humidity of the air increases as temperature decreases and decreases as temperature increases. Humidity sensors also respond to changes in temperature. If the relative humidity is held constant, the sensor reading can be affected by temperature changes. Because of this characteristic, humidity sensors should not be used in atmospheres that experience wide temperature variations unless temperature compensation is provided. Temperature compensation is usually provided with nylon elements and can be factored into electronic sensor values, if required. Dew point is the temperature at which vapor condenses. A dew point sensor senses dew point directly. A typical sensor uses a heated, permeable membrane to establish an equilibrium condition in which the dry-bulb temperature of a cavity in the sensor is proportional to the dew point temperature of the ambient air. Another type of sensor senses condensation on a cooled surface. If the ambient dry-bulb and dew point temperature are known, the relative humidity, total heat, and specific humidity can be calculated. Refer to the Psychrometric Chart Fundamentals section of this manual. Fig. 53. Paddle Flow Sensor. Flow meters measure the rate of fluid flow. Principle types of flow meters use orifice plates or vortex nozzles which generate pressure drops proportional to the square of fluid velocity. Other types of flow meters sense both total and static pressure, the difference of which is velocity pressure, thus providing a differential pressure measurement. Paddle wheels and turbines respond directly to fluid velocity and are useful over wide ranges of velocity. In a commercial building or industrial process, flow meters can measure the flow of steam, water, air, or fuel to enable calculation of energy usage needs. Airflow pickups, such as a pitot tube or flow measuring station (an array of pitot tubes), measure static and total pressures in a duct. Subtracting static pressure from total pressure yields velocity pressure, from which velocity can be calculated. Multiplying the velocity by the duct area yields flow. For additional information, refer to the Building Airflow System Control Applications section of this manual. Applying the fluid jet principle allows the measurement of very small changes in air velocity that a differential pressure sensor cannot detect. A jet of air is emitted from a small tube perpendicular to the flow of the air stream to be measured. ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 33 The impact of the jet on a collector tube a short distance away causes a positive pressure in the collector. An increase in velocity of the air stream perpendicular to the jet deflects the jet and decreases pressure in the collector. The change in pressure is linearly proportional to the change in air stream velocity. Another form of air velocity sensor uses a microelectronic circuit with a heated resistance element on a microchip as the primary velocity sensing element. Comparing the resistance of this element to the resistance of an unheated element indicates the velocity of the air flowing across it. PROOF-OF-OPERATION SENSORS Proof-of-operation sensors are often required for equipment safety interlocks, to verify command execution, or to monitor fan and pump operation status when a central monitoring and management system is provided. Current-sensing relays, provided with current transformers around the power lines to the fan or pump motor, are frequently used for proof-of- operation inputs. The contact closure threshold should be set high enough for the relay to drop out if the load is lost (broken belt or coupling) but not so low that it drops out on a low operational load. Current-sensing relays are reliable, require less maintenance, and cost less to install than mechanical duct and pipe devices. TRANSDUCERS Transducers convert (change) sensor inputs and controller outputs from one analog form to another, more usable, analog form. A voltage-to-pneumatic transducer, for example, converts a controller variable voltage input, such as 2 to 10 volts, to a linear variable pneumatic output, such as 3 to 15 psi. The pneumatic output can be used to position devices such as a pneumatic valve or damper actuator. A pressure-to-voltage transducer converts a pneumatic sensor value, such as 2 to 15 psi, to a voltage value, such as 2 to 10 volts, that is acceptable to an electronic or digital controller. CONTROLLERS Controllers receive inputs from sensors. The controller compares the input signal with the desired condition, or setpoint, and generates an output signal to operate a controlled device. A sensor may be integral to the controller (e.g., a thermostat) or some distance from the controller. Controllers may be electric/electronic, microprocessor, or pneumatic. An electric/electronic controller provides two- position, floating, or modulating control and may use a mechanical sensor input such as a bimetal or an electric input such as a resistance element or thermocouple. A microprocessor controller uses digital logic to compare input signals with the desired result and computes an output signal using equations or algorithms programmed into the controller. Microprocessor controller inputs can be analog or on/off signals representing sensed variables. Output signals may be on/off, analog, or pulsed. A pneumatic controller receives input signals from a pneumatic sensor and outputs a modulating pneumatic signal. ACTUATORS An actuator is a device that converts electric or pneumatic energy into a rotary or linear action. An actuator creates a change in the controlled variable by operating a variety of final control devices such as valves and dampers. In general, pneumatic actuators provide proportioning or modulating action, which means they can hold any position in their stroke as a function of the pressure of the air delivered to them. Two-position or on/off action requires relays to switch from zero air pressure to full air pressure to the actuator. Electric control actuators are two-position, floating, or proportional (refer to CONTROL MODES). Electronic actuators are proportional electric control actuators that require an electronic input. Electric actuators are bidirectional, which means they rotate one way to open the valve or damper, and the other way to close the valve or damper. Some electric actuators require power for each direction of travel. Pneumatic and some electric actuators are powered in one direction and store energy in a spring for return travel. Figure 54 shows a pneumatic actuator controlling a valve. As air pressure in the actuator chamber increases, the downward force (F1) increases, overcoming the spring compression force (F2), and forcing the diaphragm downward. The downward movement of the diaphragm starts to close the valve. The valve thus reduces the flow in some proportion to the air pressure applied by the actuator. The valve in Figure 54 is fully open with zero air pressure and the assembly is therefore normally open. ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 34 Fig. 54. Typical Pneumatic Valve Actuator. A pneumatic actuator similarly controls a damper. Figure 55 shows pneumatic actuators controlling normally open and normally closed dampers. DIAPHRAGM AIR PRESSURE SPRING FLOW VALVE ACTUATOR CHAMBER F1 F2 C2086 NORMALLY OPEN DAMPER ACTUATOR ACTUATOR SPRING PISTON ROLLING DIAPHRAGM AIR PRESSURE AIR PRESSURE NORMALLY CLOSED DAMPER C2087 PUSH ROD CRANK ARM ACTUATOR DAMPER C2721 Fig. 55. Typical Pneumatic Damper Actuator. Electric actuators are inherently positive positioning. Some pneumatic control applications require accurate positioning of the valve or damper. For pneumatic actuators, a positive positioning relay is connected to the actuator and ensures that the actuator position is proportional to the control signal. The positive positioning relay receives the controller output signal, reads the actuator position, and repositions the actuator according to the controller signal, regardless of external loads on the actuator. Electric actuators can provide proportional or two-position control action. Figure 56 shows a typical electric damper actuator. Spring-return actuators return the damper to either the closed or the open position, depending on the linkage, on a power interruption. Fig. 56. Typical Electric Damper Actuator. AUXILIARY EQUIPMENT Many control systems can be designed using only a sensor, controller, and actuator. In practice, however, one or more auxiliary devices are often necessary. Auxiliary equipment includes transducers to convert signals from one type to another (e.g., from pneumatic to electric), relays and switches to manipulate signals, electric power and compressed air supplies to power the control system, and indicating devices to facilitate monitoring of control system activity. ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 35 CHARACTERISTICS AND ATTRIBUTES OF CONTROL METHODS Review the columns of Table 4 to determine the characteristics and attributes of pneumatic, electric, electronic, and microprocessor control methods. Table 4. Characteristics and Attributes of Control Methods. Pneumatic Electric Electronic Microprocessor Naturally proportional Requires clean dry air Air lines may cause trouble below freezing Explosion proof Simple, powerful, low cost, and reliable actuators for large valves and dampers Simplest modulating control Most common for simple on-off control Integral sensor/ controller Simple sequence of control Broad environmental limits Complex modulating actuators, especially when spring-return Precise control Solid state repeatability and reliability Sensor may be up to 300 feet from controller Simple, remote, rotary knob setpoint High per-loop cost Complex actuators and controllers Precise control Inherent energy management Inherent high order (proportional plus integral) control, no undesirable offset Compatible with building management system. Inherent database for remote monitoring, adjusting, and alarming. Easily performs a complex sequence of control Global (inter-loop), hierarchial control via communications bus (e.g., optimize chillers based upon demand of connected systems) Simple remote setpoint and display (absolute number, e.g., 74.4) Can use pneumatic actuators ENGINEERING MANUAL OF AUTOMATIC CONTROL CONTROL FUNDAMENTALS 36 PSYCHROMETRIC CHART FUNDAMENTALS 37 ENGINEERING MANUAL OF AUTOMATIC CONTROL Psychrometric Chart Fundamentals Contents Introduction 38 Definitions 38 Description of the Psychrometric Chart 39 The Abridged Psychrometric Chart 40 Examples of Air Mixing Process 42 Air Conditioning Processes 43 Heating Process 43 Cooling Process 44 Humidifying Process 44 Basic Process 44 Steam Jet Humidifier . 46 Air Washes 49 Vaporizing Humidifier 50 Cooling and Dehumidification 51 Basic Process . 51 Air Washes 51 Dehumidification and Reheat 52 Process Summary . 53 ASHRAE Psychrometric Charts 53 PSYCHROMETRIC CHART FUNDAMENTALS 38 ENGINEERING MANUAL OF AUTOMATIC CONTROL INTRODUCTION This section provides information on use of the psychrometric chart as applied to air conditioning processes. The chart provides a graphic representation of the properties of moist air including wet- and dry-bulb temperature, relative humidity, dew point, moisture content, enthalpy, and air density. The chart is used to plot the changes that occur in the air as it passes through an air handling system and is particularly useful in understanding these changes in relation to the performance of automatic HVAC control systems. The chart is also useful in troubleshooting a system. For additional information about control of the basic processes in air handling systems, refer to the Air Handling System Control Applications section. DEFINITIONS To use these charts effectively, terms describing the thermodynamic properties of moist air must be understood. Definition of these terms follow as they relate to the psychrometric chart. Additional terms are included for devices commonly used to measure the properties of air. Adiabatic process: A process in which there is neither loss nor gain of total heat. The heat merely changes from sensible to latent or latent to sensible. British thermal unit (Btu): The amount of heat required to raise one pound of water one degree Fahrenheit. Density: The mass of air per unit volume. Density can be expressed in pounds per cubic foot of dry air. This is the reciprocal of specific volume. Dew point temperature: The temperature at which water vapor from the air begins to form droplets and settles or condenses on surfaces that are colder than the dew point of the air. The more moisture the air contains, the higher its dew point temperature. When dry-bulb and wet-bulb temperatures of the air are known, the dew point temperature can be plotted on the psychrometric chart (Fig. 4). Dry-bulb temperature: The temperature read directly on an ordinary thermometer. Isothermal process: A process in which there is no change of dry-bulb temperature. Latent heat: Heat that changes liquid to vapor or vapor to liquid without a change in temperature or pressure of the moisture. Latent heat is also called the heat of vaporization or condensation. When water is vaporized, it absorbs heat which becomes latent heat. When the vapor condenses, latent heat is released, usually becoming sensible heat. Moisture content (humidity ratio): The amount of water contained in a unit mass of dry air. Most humidifiers are rated in grains of moisture per pound of dry air rather than pounds of moisture. To convert pounds to grains, multiply pounds by 7000 (7000 grains equals one pound). Relative humidity: The ratio of the measured amount of moisture in the air to the maximum amount of moisture the air can hold at the same temperature and pressure. Relative humidity is expressed in percent of saturation. Air with a relative humidity of 35, for example, is holding 35 percent of the moisture that it is capable of holding at that temperature and pressure. Saturation: A condition at which the air is unable to hold any more moisture at a given temperature. Sensible heat: Heat that changes the temperature of the air without changing its moisture content. Heat added to air by a heating coil is an example of sensible heat. Sling psychrometer: A device (Fig. 1) commonly used to measure the wet-bulb temperature. It consists of two identical thermometers mounted on a common base. The base is pivoted on a handle so it can be whirled through the air. One thermometer measures dry-bulb temperature. The bulb of the other thermometer is encased in a water-soaked wick. This thermometer measures wet-bulb temperature. Some models provide slide rule construction which allows converting the dry-bulb and wet-bulb readings to relative humidity. PSYCHROMETRIC CHART FUNDAMENTALS 39 ENGINEERING MANUAL OF AUTOMATIC CONTROL Fig. 1. Sling Psychrometer. C1828 RELATIVE HUMIDITY SCALE HANDLE PIVOT WET-BULB THERMOMETER DRY-BULB THERMOMETER WATER-SOAKED WICK Although commonly used, sling psychrometers can cause inaccurate readings, especially at low relative humidities, because of factors such as inadequate air flow past the wet-bulb wick, too much wick wetting from a continuous water feed, thermometer calibration error, and human error. To take more accurate readings, especially in low relative humidity conditions, motorized psychrometers or hand held electronic humidity sensors are recommended. Specific volume: The volume of air per unit of mass. Specific volume can be expressed in cubic feet per pound of dry air. The reciprocal of density. Total heat (also termed enthalpy): The sum of sensible and latent heat expressed in Btu or calories per unit of mass of the air. Total heat, or enthalpy, is usually measured from zero degrees Fahrenheit for air. These values are shown on the ASHRAE Psychrometric Charts in Figures 33 and 34. Wet-bulb temperature: The temperature read on a thermom- eter with the sensing element encased in a wet wick (stocking or sock) and with an air flow of 900 feet per minute across the wick. Water evaporation causes the temperature reading to be lower than the ambient dry-bulb temperature by an amount proportional to the moisture content of the air. The temperature re- duction is sometimes called the evaporative effect. When the reading stops falling, the value read is the wet-bulb temperature. The wet-bulb and dry-bulb temperatures are the easiest air properties to measure. When they are known, they can be used to determine other air properties on a psychrometric chart. DESCRIPTION OF THE PSYCHROMETRIC CHART The ASHRAE Psychrometric Chart is a graphical represen- tation of the thermodynamic properties of air. There are five different psychrometric charts available and in use today: Chart No. 1 — Normal temperatures, 32 to 100F Chart No. 2 — Low temperatures, –40 to 50F Chart No. 3 — High temperatures, 50 to 250F Chart No. 4 — Normal temperature at 5,000 feet above sea level, 32 to 120F Chart No. 5 — Normal temperature at 7,500 feet above sea level, 32 to 120F Chart No. 1 can be used alone when no freezing temperatures are encountered. Chart No. 2 is very useful, especially in locations with colder temperatures. To apply the lower range chart to an HVAC system, part of the values are plotted on Chart No. 2 and the resulting information transferred to Chart No. 1. This is discussed in the EXAMPLES OF AIR MIXING PROCESS section. These two charts allow working within the comfort range of most systems. Copies are provided in the ASHRAE PSYCHROMETRIC CHARTS section. PSYCHROMETRIC CHART FUNDAMENTALS 40 ENGINEERING MANUAL OF AUTOMATIC CONTROL THE ABRIDGED PSYCHROMETRIC CHART Figure 2 is an abridged form of Chart No. 1. Some of the scale lines have been removed to simplify illustrations of the psychrometric processes. Smaller charts are used in most of the subsequent examples. Data in the examples is taken from full-scale charts The major lines and scales on the abridged psychrometric chart identified in bold letters are: —Dry-bulb temperature lines —Wet-bulb temperature lines — Enthalpy or total heat lines — Relative humidity lines — Humidity ratio or moisture content lines —Saturation temperature or dew point scale —Volume lines in cubic feet per pound of dry air The chart also contains a protractor nomograph with the following scales: — Enthalpy/humidity ratio scale — Sensible heat/total heat ratio scale When lines are drawn on the chart indicating changes in psychrometric conditions, they are called process lines. With the exception of relative humidity, all lines are straight. Wet-bulb lines and enthalpy (total heat) lines are not exactly the same so care must be taken to follow the correct line. The dry-bulb lines are not necessarily parallel to each other and incline slightly from the vertical position. The purpose of the two enthalpy scales (one on the protractor and one on the chart) is to provide reference points when drawing an enthalpy (total Fig. 2. Abridged Chart No. 1. S E N S I B L E H E A T T O T A L H E A T ∆H S ∆H T = 1.0 0.8 0.6 0.5 0.4 0.3 0.2 0.1 5000 3000 2000 1500 1000 500 -1000 -0.1 -0.3 -0.5 -1.0 -2.0 -4.0 1.0 2.0 E N T H A L P Y H U M I D I T Y R A T I O = ∆ h ∆ W 4.0 0 ∞ 15 20 25 30 35 40 45 50 35 35 40 45 50 50 55 55 60 65 70 75 80 85 85 80 75 70 60 55 50 45 40 .030 .026 .022 .020 .016 .012 .006 90 100 110 10 15 20 25 ENTHALPY— BTU PER POUND OF DRY AIR 60% 40% 14.0 14.5 15.0 30 SATURATIO N TEM PERATURE - °F DRY BULB - °F WET BULB - °F REALTIVE HUMIDITY - % VOLUME-CU FT PER POUND OF DRY AIR HUMIDITY RATIO(W) - POUNDS OF MOISTURE PER POUND OF DRY AIR M10306 .028 .024 .018 .014 .010 .008 .004 .002 80% 20% 13.0 13.5 45 60 95 105 120 12.5 115 40 35 40 45 50 55 60 65 70 75 80 85 65 35 . actuators for large valves and dampers Simplest modulating control Most common for simple on-off control Integral sensor/ controller Simple sequence of control. to the performance of automatic HVAC control systems. The chart is also useful in troubleshooting a system. For additional information about control of