This section of the exam guide book focuses on the Mechanical Equipment and Systems used in the HVAC and Refrigeration field. The equipment discussed in this section are the most common pieces of equipment and systems and include: air distribution equipment like ducts, fans and dampers and fluid distribution equipment like pipes, pumps and valves. Also included are other equipment include chillers, cooling towers, energy recovery devices, boilers, etc.
Air Supply Ducts
In the HVAC field, air distribution systems are used to supply cold/hot air to various spaces to keep the occupants comfortable and/or to keep equipment operational. Fresh air shall also be provided through the air distribution system to provide appropriate ventilation levels, in order to alleviate carbon dioxide (CO2) levels.
The method in which air is routed throughout a building is through the use of ducts, which can be constructed of metal, plastic or fiberglass. As a minimally competent engineer, one should be able to accomplish the following:
1) Determine the velocity in ducts.
2) Determine the pressure drop in ducts.
3) Size the duct based on required velocity or pressure drop.
Darcy Weisbach Equation
The equation used to determine the pressure drop in ducts is the Darcy Weisbach Equation.
Converting to more commonly used HVAC and Refrigeration units:
Although this equation is the governing equation for determining pressure drop, it is most often not used in the HVAC and Refrigeration field. In this field, airflow pressure drop calculations through are completed through the use of Friction Charts. Airflows, pressures, elevations and duct construction NOT normally encountered in the HVAC and Refrigeration should use the Darcy Equation.
Determining Velocity in Ducts for Pressure Calculations.
In the HVAC and Refrigeration field airflow is typically measured in cubic feet per minute or CFM. But velocity is the term that is required in determining the pressure drop of the air flow through a duct. In order to determine the velocity, the area of the duct must be found. Finding the area of ducts is a simple calculation for circular ducts, which are shown below.
For rectangular and oval ducts, the area CANNOT be calculated as shown below. The rectangular and oval duct dimensions MUST FIRST be converted to Equivalent Diameter. Remember, that the pressure loss calculations require a circular shape.
Determining Diameter of Duct
The Friction Charts and the Darcy Equation are typically a function of duct diameter. Thus no calculations are necessary for a circular duct. However, rectangular and oval ducts must be converted to an equivalent diameter circular duct before the equation can be properly completed. The equations for determining equivalent diameters are shown below.
A quicker way to determine equivalent diameter is to use the Equivalent Diameter Tables for Rectangular and Oval ducts shown in ASHRAE Fundaments.
Determining Pressure Drop in Ducts
Once the equivalent diameter of the duct is found and the CFM is known, then simply refer to the Friction Charts for Ducts and simply read the pressure drop. This process is detailed more in Fan Sizing, later on in this section.
Air Supply Fans
Diffusers, registers and grilles are at the end of ducts and serve as the distribution equipment to the conditioned space. The purpose of these mechanical pieces of equipment is to provide thermal comfort for the occupants of the space or to provide proper thermal conditions suitable for the equipment in the space.
Diffusers are defined as air terminal devices that distribute conditioned air in various directions through the use of its deflecting vanes. It is designed to promote the mixing of conditioned air with the air already in the space. It is important to properly mix the conditioned air into the space, in order to provide cooling/heating and to distribute fresh air to the entire space and to avoid stagnant air in the space. However, not all types of diffusers have the same performance in mixing the conditioned air in the space. Each diffuser will be provided with a table describing its performance similar to the one on the following page.
The values shown in tables similar to the one above are specific to a certain manufacturer’s type of diffuser and size. The third row indicates the total amount of CFM that is distributed through this diffuser. This term is the value with which the designer will begin. From this value, the velocity and pressure drop through the diffuser can be determined. It is also important to note that at higher velocities, the pressure drop increases and the NC or noise criteria increases. The NC rating corresponds to a curve of DB levels at various frequencies. This NC rating is used to rate the sound levels of air conditioning equipment and also used to rate the sound requirements of rooms. For example, a typical classroom will require a NC rating of 25-30. Using the table above, this corresponds to a maximum airflow somewhere between 85 and 95 CFM.
Throw is defined as the horizontal distance from a diffuser at a specified velocity. For example, T50 = 15’, indicates that at a distance of 15’ from the diffuser, the velocity of the air will be 50 feet per minute. T100 = 10’, indicates the distance at which the air velocity is 100 feet per minute and T150 = 5’, indicates the distance for 150 feet per minute. Often times throw is shown simply in the following format, [T150 -T100 - T50]. For example, in the table above an airflow of 60 CFM results in a velocity of 150 fpm at 7’ from the diffuser, a velocity of 100 fpm at 9’ from the diffuser and a velocity of 50 fpm at 12’ from the diffuser. Refer to the following figure for a graphical explanation.
Typically in diffuser layout design for occupied areas, it is required to locate diffusers so that the T50 length is nearly equivalent to the characteristic length. The characteristic length is defined as the distance by one of the following:
1. Perpendicular distance between the center line of the diffuser and the wall.
2. Midpoint between the centerline of two diffusers.
Grilles are defined as air devices that consist of an opening with a covered grating or screen. Grilles are often used to return air back to the fan or to exhaust air from a space. Grilles are not typically used to supply air because there is an inability to accurately control the amount of air being supplied.
Registers are simply grilles with a damper that is used to restrict the amount of air flow required to be returned, supplied or exhausted.
Types of Fans
Fans are provided in HVAC and Refrigeration systems to distribute conditioned air, to provide ventilation or to exhaust un-wanted air.
Mechanical Horsepower (MHP): Mechanical horsepower is the measure of the power produced by the fan, a function of the air flow rate measured in cubic feet per minute (CFM) and the total static pressure (TSP) measured in “in. wg.”.
Brake Horsepower (BHP): Brake horsepower is the measure of the power drawn by the motor to turn the fan, a function of the fan efficiency and the mechanical horsepower.
Horsepower (HP): Horsepower is the motor size. It is a function of the BHP and the motor efficiency.
*upsize HP to nearest motor size
Velocity Pressure (VP): Velocity pressure is defined as the pressure caused solely by moving air.
Static Pressure (SP): Static pressure is the pressure caused solely by compression, the outward force on a duct.
Total Pressure (TSP): Total static pressure is the sum of the velocity pressure and the static pressure at any point.
The professional engineer must be able to properly size a fan. There are two main parameters that must be determined, (1) Volumetric Flow Rate [CFM] and (2) Static Pressures.
DETERMINING VOLUMETRIC FLOW RATE [CFM]
In order to find the volumetric flow rate of air that the fan must blow, will depend on the following factors, (1a) heat/cooling load, (1b) ventilation/exhaust or (1c) velocity.
(1a) First in the HVAC and Refrigeration field fans are used to provide cool/hot air to properly control the temperature of the space. The amount of air required is determined by the cooling/heat load and the desired temperature and the supply air temperature.
(1b) Second in the HVAC and Refrigeration field fans are used to provide ventilation to adequately remove noxious fumes, like carbon dioxide from occupied spaces. The amount of ventilation or exhaust is determined by researching ASHRAE 62.1 for the required factor. This factor could be a person factor, for example, “Provide 15 CFM per person” or it could be an area factor, for example, “Provide 1 CFM per square foot of area”.
(1c) Volumetric flow rate (CFM) can also be determined by the required velocity. This method is typically used in industrial ventilation situations and in kitchens. A high velocity is required in a system in order to keep particles suspended in the air so that they may be exhausted out of the space.
DETERMINING TOTAL STATIC PRESSURE [IN. WG]
The second parameter that must be found in order to size a fan is to determine the total static pressure. This is the total pressure that the fan must overcome in order to deliver the required CFM to the required location. The total static pressure is a function of the (2a) duct friction losses, (2b) duct fitting losses and (2c) miscellaneous equipment losses.
(2a) Duct Friction Losses: Straight lengths of duct incur friction losses on the airflow, which must be calculated by the engineer in order to properly size the fan. The amount of friction loss is a function of the velocity of air and the size of the duct. Another important tool that is required is the Standard Friction Loss in Standard Duct graphs, which can be found in the ASHRAE Fundamentals Handbook or the Mechanical Engineering Reference Manual.
(2b) Duct Fitting Losses: Each fitting also will have a friction loss associated with its construction. In order to find these fiction losses, the engineer will need the ASHRAE Fundamentals Handbook or the Mechanical Engineering Reference Manual. Duct fittings losses are dependent on the type of fitting and the velocity of the air through the fitting. The type of fitting will have a corresponding “K-factor” or “C-Coefficient”, which can be found in the ASHRAE of Fundamentals book and some typical fitting losses are also shown in the Mechanical Engineering Reference Manual. The “K-factor” or “C-Coefficient” is the multiplied by the velocity pressure in order to get the pressure loss due to the duct fitting. Remember that the velocity is found by first converting the rectangular or oval duct to equivalent diameter, then calculating the area.
(2c) Miscellaneous Equipment Friction Losses: In a duct system, there are also miscellaneous equipment losses due to different types of equipment, like filters, fans, diffusers, registers and grilles. The friction losses are given by the equipment manufacturer for different velocities and flow rates.
The fan curve is graph depicting the various points that the fan can operate. It indicates the amount of CFM the fan will provide at a given total static pressure, which is dependent on the connected ducted system. Fans should be selected to operate at the stable region. The stable region is the area on the fan curve where there is a single flow rate [CFM] value for ever pressure value. In the unstable region, a pressure value can have multiple CFM values, which will cause the fan system to surge. The stable region also has very little change in CFM for large changes in total pressure.
The second curve that works in conjunction with the fan curve is the system resistance curve. This curve is summation of all the friction losses in the ducting system at varying CFM's. Typically, the friction losses are summed up at the design CFM values, then this design point is connected to the 0,0 point by an upward sloping square polynomial curve, as shown below. If for example, the ducting system has a closed damper or dirty filter, this will cause the curve to shift to the left. If a damper is opened or the dirty filter is cleaned then the curve will shift to the right.
Combining the system curve with the selected fan curve, determines the operating point of the fan system, indicated in the figure below in green. Following the vertical line down determines the CFM and the horizontal line from that point indicates the operating total pressure. During system operation as dampers close, the system curve shifts toward the right in red. This movement decreases the amount of CFM delivered by the fan. The opposite occurs as dampers open in the system, the amount of CFM delivered by the fan increases.
It has been shown that the amount of CFM blown by a fan can be changed by shifting the system resistance curve. However, the volumetric flow rate can also be changed by changing the speed of the fan, which shifts the fan curve.
Fan Affinity Laws
Often times a fan’s speed or impeller diameter will be changed. If the fan is a centrifugal fan, then the change in performance of the fan can be predicted quickly through the affinity laws.
First if the impeller diameter is held constant and the speed of the fan is changed, then flow rate varies directly with the speed, available pressure varies with the square of the speed and the power use varies with the cube of the speed.
Second if the speed is held constant and the impeller diameter of the fan is changed, then flow rate varies directly with the diameter, available pressure varies with the diameter of the speed and the power use varies with the cube of the diameter.
FANS IN PARALLEL
A parallel arrangement of fans is characterized by the same pressure increase across each fan and the total flow is the sum of flows through each individual fan. In the figure below, the total flow is shown as x1 + x2 + x3, where xn is the flow through fan “n”. The resulting total pressure is equal to each individual fan pressure, since they are all the same
FANS IN SERIES
Fans in series are characterized by the same flow across each fan and the total pressure increase is the sum of the pressure increase through each individual fan. In the figure on the following page, the total flow is shown simply as y, which is consistent throughout each fan. The resulting total pressure is equal to sum of each fan’s individual pressure increase, y1 + y2 + y3, where yn is the resulting fan pressure increase at fan “n”.
In the HVAC and Refrigeration field, cooling and heating coils are used to exchange heat to/from air to a heat exchange fluid. A heat exchange fluid is flown through the coil and as air is passed over the coil, the air is either heated or cooled. Coils consist of a metal box framing, which holds a series of copper tubes in staggered rows and columns.
The amount of heat that is transferred is related to the amount of surface area that the air is in contact with. In order to increase surface area, the size of the tubes may be decreased and more tubes can be provided, the number of rows increased or the amount of fins per inch are increased. Aluminum or copper fins are provided on each tube to increase the amount of surface area. Coils are rated by the height of the fins and the number of fins per inch.
Cooling and Heating Coil Fluids
There are several different types of heat exchange fluids used in cooling/heating coils.
Refrigerant: Hot refrigerant gas or cool refrigerant liquid can be used in a coil to provide either heating or cooling. In a heating-coil, cool air is passed over a coil containing hot gas. Heat is exchanged to the cool air, which warms the air. The heat lost by the refrigerant gas causes it to condense to a liquid. In a cooling-coil, warm air is passed over a coil containing cool refrigerant liquid. Heat is exchanged to the cool refrigerant liquid, causing it to evaporate. The warm air loses heat, thereby decreasing the air temperature.
Water: Chilled water or hot water can be used in a coil to provide either heating or cooling. The air temperature is either raised or lowered as heat is transferred to raise or lower the temperature of the chilled or hot water.
Steam: Steam can be provided to a coil to provide heating. Steam enters the coil and as the air passes over the coil its air temperature increases. As the steam loses heat, it condenses to its liquid form.
Cooling and Heating Coil Terms
It is important to be able to understand the following terms, (1a) Apparatus Dew Point or (1b) Effective Surface Temperature and the (2) Contact Factor
Apparatus Dew Point or Effective Surface Temperature is the temperature at which all air would be cooled to if the cooling coil was 100% effective. The ADP must be located on the saturation curve, refer to the psychrometric chart below. The ADP, leaving coil conditions and the entering coil conditions are located on the same line.
How close the leaving coil condition is to the apparatus dew point is a function of the contact factor.
The bypass factor describes the percentage of air that is not cooled to the ADP. The air that is bypassed remains unchanged from the entering coil conditions. The bypass factor is a function of the airflow, number of rows, surface temperature, number of fins per inch, height of fins and many other construction attributes of coils. The origin of the bypass factor is not important, but the use of the bypass factor in calculations is important. The bypass factor can be found through the use of (a) enthalpy, (b) dry bulb temperature or (c) humidity ratio. The contact factoris the inverse of the bypass factor. It describes the amount of air that is contact with the coil and that is cooled to the ADP.
In the HVAC and Refrigeration field, humidification and dehumidification systems are used to transfer moisture to/from the air. These types of systems are sized based on the amount of moisture, measured in pounds of water per hour that is added or removed to the air.
Humidifiers are used to add moisture to air typically in order to achieve the best conditions for human occupancy. In dry areas, low humidity causes moisture to evaporate from people’s skin, creating the feeling that it is much colder than the dry bulb temperature indicates. Other times humidifiers are used to maintain best humidity levels for equipment or produce.
There are two main types of humidifiers, (1) Steam and (2) Evaporative humidifiers.
(1) Steam Humidifiers, also known as isothermal humidifiers, add moisture to air without the change in dry bulb temperature, hence isothermal humidifier. Steam is created through an external means like a gas fired boiler or electric boiler. Then the steam is typically directly injected into the air stream. It is common to assume that the temperature of the air will rise since steam is 212 F. However, it is important to think of steam as water vapor and as it is added to air, it will correspond to an upward movement on the psychrometric chart [Pt 1 to Pt 2].
(2) Evaporative Humidifiers, also known as adiabatic humidifiers, add moisture to air without a change in enthalpy, hence adiabatic humidifier. Evaporative humidifiers do not require an external energy source like Steam Humidifiers. Evaporative humidifiers work by blowing dry air over water or through water droplets. The energy to vaporize the water comes from the dry air. As the air releases heat to vaporize the water, the air also cools. On the psychrometric chart, adiabatic humidification is shown as an upward-left movement, along a constant enthalpy line. It is constant enthalpy because the enthalpy lost to sensible cooling is gained by latent heating [humidification].
Evaporative humidifiers operate on the same principle as air washers, evaporative coolers and cooling towers. These principles will be discussed in the Cooling Tower section. D
De-Humidifiers are used to remove moisture to air typically in order to achieve the best conditions for human occupancy. In humid areas, high humidity causes the feeling that it is much hotter than the dry bulb temperature indicates. Other times de-humidifiers are used to maintain best humidity levels for equipment or produce. De-humidifiers are especially important in preventing mold and mildew from forming.
There are two main types of de-humidifiers, (1) Condensing and (2) Desiccant de-humidifiers.
(1) Condensing de-humidifiers or cooling humidifiers work by decreasing the temperature of the incoming air so that it is unable to hold moisture, which causes condensation. A cooling coil acts a dehumidifier. In the Psychrometric chart below, hot, humid air enters the coil and leaves as cool air. The amount of water vapor removed from the air is shown in red. In some cases the air is reheated in order to lower the relative humidity and increase the dry bulb temperature.
(2) Desiccant de-humidifiers or chemical dehumidifiers use desiccants to adsorb water from air. As the air loses its water vapor, the heat from condensing the water vapor is gained by the air stream, which causes the air to increase its dry bulb temperature. A desiccant de-humidifier is shown as a downward-right movement, along the constant enthalpy line (adiabatic).
Energy Recovery Devices
An energy recovery device is an air to air heat exchanging device. In the HVAC and Refrigeration field, energy recovery devices are used to exchange energy from outgoing exhaust air to incoming outside air. During the winter months the outside air is pre-heated prior to entering the air handler and during the summer the outside air is pre-cooled.
Energy recovery devices are governed by the following equations.
The effectiveness of an energy recovery device is defined as the ratio of the actual heat transferred to the maximum amount of heat that can be transferred. The effectiveness can be rated in terms of sensible heat transfer, latent heat transfer or total heat transfer.
The actual amount of energy transferred is found by multiplying each individual airstreams mass flow rates by the change in conditions, whether it is a change in temperature, change in humidity or change in total enthalpy.
The maximum amount of energy transferred is met if the entering condition of the 1st air stream exits the energy recovery device at the same conditions as the entering condition of the 2nd air stream. However, if one airstream has more air flow than the other, then the smallest airstream should be used.
There are various types of energy recovery devices listed below:
Rotary Sensible Wheel
A rotary sensible wheel is typically a metal wheel that rotates and exchanges heat from one air stream to another. The wheel is connected to a gear and motor, which rotates the wheel. As a section of the wheel picks up heat from air stream, the wheel then rotates to the other air stream to move the heat to the cooler air stream.
Rotary Enthalpy Wheel
A rotary enthalpy wheel is similar to a rotary sensible wheel, in that it has the same type of construction and parts. But in addition, a rotary enthalpy wheel has a desiccant material that is used to absorb moisture. A section of the wheel absorbs moisture from the more humid air stream, then rotates and transfers the moisture to the more dry air mixture.
Wrap-Around Heat Pipe
A wrap around heat pipe is used typically in warm humid climates in spaces with a high amount of outside air requirements. In these types of environments, warm, humid outside air is conditioned to a low temperature in order to condense the water out of the air. A wrap-around heat pipe is used to pre-cool the incoming warm humid outside air by transferring heat to the exiting cool supply air. This has the effect of providing sensible re-heat to the supply air, which also decreases the need for additional re-heat.
The heat pipe contains a pressurized refrigerant, which proceeds through the vapor compression cycle with the design temperatures. In the first phase, warm air passes over the cool liquid refrigerant. This effectively pre-cools the outside air before it enters the main cooling coil. During this first phase, the liquid refrigerants gains heat, causing it to vaporize and move to the other side of the coil. In the second phase, on the other side of the coil, the cool air passes over the warm vapor, which re-heats the air. In addition, the warm vapor is condensed to a liquid, allowing the process to start over again.
In this example, energy is transferred in the same air stream from the entering outside air to the existing supply air. The heat pipe can also be used to transfer energy between two different air streams. For example, it can be used between the outdoor/supply air and the return/exhaust airstreams.
The last energy recovery device that is explained in this section is the run-around coil. The run around loops consists of two heat exchange coils connected by piping, a fluid and a pump. A heat transfer fluid, typically water or a glycol-water mixture is pumped between the two coils. The fluid transfers heat from one air stream to the other air stream.
An economizer is another means of energy saving. In its simplest form it consists of two sets of dampers, one set controlling the amount of return air that is directed either to exhaust or back to the air handler and a second set controlling the amount of outside air routed to the air handler.
Cooling Season: When the outside air (OAIR) has a lower enthalpy than the return air (RAIR), then the OAIR is directed to the coils and the RAIR is routed to the exhaust. By routing the lower enthalpy air (OAIR), the coil requires less energy to provide cooling. If the enthalpy of the RAIR is lower than the OAIR, then the RAIR is routed to the coil and only the minimum amount of OAIR is routed to the coil. OAIR is still required in order to maintain the proper amounts of fresh air to the occupants.
The main focus of the pump section is to help the engineer develop an understanding of the major skills and concepts needed for the PE exam. These concepts include (1) determining total head, (2) determining net positive suction head, (3) reading pump curves, (4) using affinity laws. However, in order to focus on these skills and concepts, the engineer will require a brief introduction on pumps, types of pumps and how they work.
There are three main types of pumps, centrifugal, rotary and reciprocating pumps. Rotary and reciprocating pumps are positive displacement pumps. This document will not cover positive displacement pumps in detail because they are not typically used in the HVAC and Refrigeration field. Centrifugal pumps are the most common type of pumps used in HVAC and Refrigeration. The following information is tailored to centrifugal pumps and should not be applied freely to positive displacement pumps.
Centrifugal pumps operate on the principle of "centrifugal force", which is the conversion of rotational kinetic energy imparted by rotating impellers onto the fluid to produce a flow rate (kinetic energy) at a certain pressure (pressure energy). Fluid enters the pump at the center or eye of the impeller, there the rotating impellers push the fluid to the outer edges, imparting a flow rate and pressure. See Figures 1 and 2 for a diagram of the fluid flow.
There are various types of centrifugal pumps, but there are two main families of centrifugal pumps are (1) end suction pumps [refer to Figure 1] and (2) in-line pumps [refer to Figure 2]. These two families differ on the path the water takes from the inlet to the outlet. In the end-suction pumps, the fluid enters the pump at the impeller and exits the pump at a 90 degree angle from the inlet. The in-line pumps have parallel inlets and outlets.
Within each family are horizontal versus vertical pumps, which are characterized by the orientation of the pump shaft as either horizontal or vertical. In addition, pumps can be further classified by the number of stages that the fluid proceeds through. Finally the last classification is how the pump is connected to the motor. Pumps can be long-coupled where the pump is connected to the motor by a flexible coupling or they can be close-coupled where the connection between the pump and motor is through a rigid coupling.
Total Pump Head
In order to properly select a type of pump, the engineer must know which type is most applicable to the situation. Sizing a pump depends on two criteria, (1) the flow rate and the (2) total dynamic head. The flow rate is determined by the needs of the HVAC and Refrigeration system. The pump may be a chilled water pump serving several air handlers, so the flow rate (GPM) can be found by adding up the design flow rates to the air handlers and any diversity required. The (2) second criteria is the total dynamic head. Determining total head is a must-have skill for the aspiring professional engineer.
Pump Selection:(1)GPM and (2)TDH [total dynamic head]
Total head or total dynamic head is the total equivalent height of water that a fluid must be pumped against.
Head is a unit of pressure and has the units of feet of head, which is the total pressure exerted by a certain amount of feet of a water column.
Total head can be broken up into the following components, (1) Static head or Elevation Difference between the inlet and the outlet of a piping system (2) Friction loss. In a closed system, both static (elevation) head and friction loss are present. However, in a closed system there is no elevation difference, the beginning and the end of the piping system are the same, therefore there is no elevation difference. Refer to the following figures, which describe the different pressure losses in a open and closed system./
The typical example of an open system in the HVAC and Refrigeration field is the condenser water system serving a cooling tower. The pump moves the condenser water from the cooling tower basin through piping, then the chiller and back to the top of the cooling tower. The pump must provide a total dynamic head to account for the (1) Static [Elevation] head and (2) the Friction Head through the piping, chiller, fittings, other equipment and appurtenances.
(1) The static head is the difference between the inlet and the outlet. The elevation difference between the inlet and the pump, on the suction side of the pump is called the suction static head and the elevation difference between the outlet and pump, on the discharge side is called the discharge static head. The difference between discharge and suction static head is the static/elevation head that the pump must pump against.
(2) Friction head. Friction head consists of pressure losses due to equipment like chillers, cooling towers, filters, strainers, heat exchangers, air handlers, etc. The amount of friction head from these pieces of equipment are provided by the manufacturer and are typically provided in a table format with total friction head or pressure loss for the equipment versus the flow rate. Friction head also consists of pressure losses due to the piping and the various fittings like elbows, tees, valves, etc. Calculating friction had due to piping will be discussed later in this section.
The typical example of a closed system in the HVAC and Refrigeration field is the chilled water system serving the air handlers and chillers. The pump moves chilled water to and from the chiller and through the air handlers. The pump must provide a total dynamic head to account for only the Friction Head through the piping, chiller, fittings, other equipment and appurtenances. There is no static/elevation head because the system is closed.
Friction Loss: Friction loss is found through the use of either the Darcy Weisbach equation or the Hazen-Williams equation. The Darcy Weisbach equation is slightly more involved and will be explained below, starting with the equation.h
During the exam, in order to quickly complete a friction loss question using the Darcy Weisbach, the aspiring professional engineer must have the necessary tools readily available to find the values necessary to complete the equation. These include the following, 1) Inner Diameter tables of common pipe materials and sizes, 2) Flow unit conversions, 3) Inner Area table of common pipe materials and sizes, 4) Kinematic viscosity tables of common fluids at various temperatures and 5) the Moody Diagram.
1) Inner Diameter Table of Common Pipe Materials
Collect inner diameter [ft] tables of schedule 40/80 steel [Pipe sizes to 30"], type K, L, and M copper tubing [Pipe sizes to 6"] and schedule 40/80 PVC [Pipe sizes to 30"]. Provide inner diameters in feet for ease in using the Darcy Weisbach Equation.
2) GPM to FT^3/sec Conversion Factor
Multiply GPM by 1/448.83 to get (FT^3)/sec.
3) Inner Area Table of Common Pipe Materials
Collect inner area [ft^2] tables of schedule 40/80 steel [Pipe sizes to 30"], type K, L, and M copper tubing [Pipe sizes to 6"] and schedule 40/80 PVC [Pipe sizes to 30"]. Provide inner areas in feet^2 for ease in finding the velocities through the pipes.
4) Kinematic Viscosity Tables [used to get Reynolds number which leads to the friction factor]
5) Pipe Roughness
Collect pipe roughness factors for common pipe materials, steel, PVC, copper, etc.
6) Moody Diagram: The Moody diagram uses the Reynold's number and the relative roughness factor to determine the friction factor. The relative roughness factor is found by first finding the roughness value corresponding to the pipe material. Then dividing the roughness factor by the inner diameter of the pipe. Ensure that the roughness factor and the diameter are in the same units. The Reynold's number is found by multiplying the velocity of the fluid through the pipe by the diameter of the pipe and dividing by the kinematic viscosity of the fluid. Once these two values are found (a) Relative Roughness and (b) Reynold's Number, then the friction factor can be found by finding the intersection of the vertical Reynold's number line shown in black and the Relative Roughness factor curves shown in red.
Step 1: Find relative roughness factor, step 2: find intersection of reynold's number and relative roughness factor. step 3: read corresponding friction factor.
Net Positive Suction Head
The professional engineer must be able to properly determine net positive suction head in order to avoid cavitation. Cavitation occurs when the suction pressure (head) is less than the vapor pressure of the water. If the suction pressure is lower than the vapor pressure, then small vapor bubbles form and when these bubbles reach the pump where the pressure is increases, the bubbles implode causing damage to the impellers and other parts of the pump. This is what is known as cavitation.
Suction head is defined as the pressure at the inlet of the pump and net positive suction head is the difference between the suction head at the inlet and the vapor pressure of the water at the inlet of the pump.
Net Positive Suction Head Available=Suction Head_(inlet of pump)-Vapor Pressure_water
Net positive suction head is the total amount of head or pressure at the inlet of the pump. This value is found by determining all the pressures acting upon the fluid whether positive or negative. The following figure best describes all the pressures that can be acting upon a pump.
(1) P_abs: This pressure refers to the absolute pressure acting on the fluid. If the tank is pressurized, then the value is pre-determined. If the tank is open to the atmosphere, then the pressure is equal to 1 atmosphere [atm] or 14.7 psia or 33.9 ft of water.
(2) P_elev: This pressure identifies the elevation difference between the top surface of the liquid and the pump centerline. This value can be positive or negative and is measured in ft of head.
(3) P_fric: The friction pressure or head is the amount of pressure lost due to friction in the piping, fittings, equipment, valves, etc. leading from the fluid source to the pump.
(4) P_vel: The velocity head pressure is the pressure due to the flowing liquid.
(5) P_suction: Finally, all of the pressures leading to the pump are summed and the resulting value is the suction pressure at the pump.
The vapor pressure of the water is found by simply looking up water (or pumping fluid) tables and finding the vapor pressure at the operating temperature. In the HVAC and Refrigeration field, water is the most common fluid used in pumping systems and a table of corresponding vapor pressure and temperatures are shown below.
From the table above, it can be seen that as the temperature of the water increases, the pressure at which vaporization will occur also increases. Thus cavitation becomes even more critical at higher temperatures.
Pump curves are created by the manufacturers of the pumps through a series of tests and describe the operating points for a specific impeller diameter and pump type. The curve plots the corresponding flow rates at varying pressures
Pump Affinity Laws
It is often necessary to determine how a pump will operate under differing operating conditions. The operating conditions of a pump that can most readily be changed are the impeller diameter and the rotational speed of the pump. In order to predict how the pump will behave prior to changing the speed or the impeller diameter.
The first affinity law is that the flow rate (Q) is directly proportional to the size of the diameter of the pump impeller (D) and/or the rotational speed (N) of the pump.
The second affinity law is that the total head (H) is directly proportional to the square of the size of the diameter of the pump impeller (D) and/or the square of the rotational speed (N) of the pump.
The third affinity law is that the power (P) is directly proportional to the cube of the size of the diameter of the pump impeller (D) and/or the cube of the rotational speed (N) of the pump.
Insulation is provided in HVAC and Refrigeration systems on pipes, ducts, walls and roofs. The primary purpose of the insulation is to limit heat transfer. For example, in chilled water pipes, insulation is provided to limit heat transfer to the chilled water and to keep the water cold. In hot air ducts, insulation is provided to limit heat loss to the surrounding areas.
Insulation is characterized by its ability to conduct heat transfer and is rated by either a k-value, U-factor or an R-value. K-values are often used when rating pipe, duct or equipment insulation where R-values and U-factors are typically used to describe roof and wall insulation. Please refer to the Heat Transfer section for more detail on insulation for roofs and walls. This section primarily deals with insulation for pipes and ducts, specifically being able to determine the insulation requirements for a pipe or duct, in order to (1) Control Surface Temperature.
Controlling Surface Temperature: One important skill that the professional engineer must attain is to be able to determine the insulation required to keep the surface temperature of a pipe, duct, wall, roof or other piece of equipment within a set range. A common problem encountered in the HVAC and Refrigeration field is determining the required insulation for a chilled water pipe in order to stop condensation from forming on the surface.
The governing equation for this problem is that the heat transfer from the chilled water pipe through the insulation and to the outer surface is equal to the heat transfer from the outer surface to the ambient air.
Cooling towers are mechanical pieces of equipment that function on the principle of evaporative cooling. Evaporative cooling is the process by which a liquid is cooled to a lower temperature by evaporating a small portion of the liquid into an airstream. Relatively dry air moves through a falling liquid and as the air moves it picks up water vapor from the liquid, thereby increasing the air’s moisture content. In order for the liquid to evaporate, the liquid needs a heat source to meet the latent heat of vaporization. This heat source is the sensible heat loss from the remaining liquid.
A cooling tower consists of two fluid flows, the air flow and the water flow. The water flow starts from the top of the cooling tower. Warm water is pumped to a series of nozzles. The nozzles’ purpose is to break up the water into tiny droplets to increase the surface area of the water that is in contact with the air stream. The droplets then fall through a fill material, which also serves to break up the droplets further to increase the surface area of the water. As the water moves downward it steadily decreases in temperature as heat is lost due to evaporation. Finally the water collects at the basin and is sucked out distributed to its required location.
The air flow starts at the bottom of the tower, where cold dry air is brought into the cooling tower where it comes into contact with the water droplets. As the air moves upward through the tower it picks up water vapor and slightly increases in temperature. Prior to exiting the cooling tower, the air must travel through the drift eliminators which is a series of baffles. The purpose of the drift eliminators is to catch any suspended water droplets in the air stream and return them to the fill.
CHARACTERIZING COOLING TOWERS
The following section provides information on the different types of cooling towers used in the HVAC and Refrigeration field. This information is merely provided to give the engineer additional background on cooling towers.
Mechanical vs. Natural Draft Cooling Towers:
There are two main categories of cooling towers: (1) Mechanical draft and (2) Natural draft cooling towers. Natural draft cooling towers move air based on the difference in buoyancy of the airstream inside and outside of the cooling tower. Mechanical draft cooling towers move air through the cooling tower by means of a mechanical fan. In the HVAC and Refrigeration field, mechanical draft cooling towers are the primary type of cooling tower.
Induced vs. Forced Draft Cooling Towers:
Induced and forced draft cooling towers are both mechanical draft type fans and differ by the location of their fan. Forced draft fans blow air into the cooling tower and are located at the airstream entrance into the cooling tower. Induced draft cooling towers on the other hand, have the fans located at the exit of the airstream for the cooling tower and suck air into the cooling tower.
Counter-flow vs. Cross-flow Cooling Towers:
Counter-flow and cross-flow cooling towers are characterized by the relationship between the air flow and water flow. In a counter flow tower the air and water flow are at 90 degrees to each other. The water is falling downwards and the air is moving across from either left to right or right to left. In a cross-flow tower, the air and water flows have directly opposing directions. The water is falling downwards and the air is moving upwards.
The following figure is a schematic of a forced mechanical draft, counter flow cooling tower. The fans are located at the air inlets, near the bottom of the cooling tower. Also the air flow counters the water as the water drops downward through the fill material.
The following figure is a schematic of a forced mechanical draft, cross flow cooling tower. Since this cooling tower is forced draft, the fans are again located at the inlet of the cooling tower near the bottom. The air flows counter or perpendicular to the water as the water falls downward through the fill.
The following figure is a schematic of an induced mechanical draft, counter flow cooling tower. The fan is located at the exit of the cooling tower and air is sucked or induced through the cooling tower. This cooling tower is also a counter flow type, where air flows upward through the fill and counters the downward moving water droplets.
The following figure is a schematic of an induced mechanical draft, cross flow cooling tower. Again the fan is located at the exit of the cooling tower. This cooling tower is a cross flow cooling tower, where air flows perpendicular through the fill as it crosses the falling water droplets.
Cooling Tower Performance
The professional engineer must be able to properly design and size and select a cooling tower to fit the HVAC and Refrigeration application. Cooling towers are characterized by two terms the approach and the range. The range of the cooling tower is the difference between the entering and exiting temperatures of the cooling tower water.
The approach or approach to wet bulb is the temperature difference between the water out and the wet bulb temperature of the air.
The approach is important because it describes the level of performance of the cooling tower. The smaller the approach the better the cooling tower is at providing cooling. The wet bulb temperature of the entering air is the lowest the temperature of the exiting water can reach. If a cooling tower has a 0 degree approach then the cooling tower is using all of the available heat exchange from the air to cool the water. Typical approaches are in the range of ~10 °F.
Approach also leads to another important term in determining the performance of cooling towers, called effectiveness. Effectiveness is a term used to describe how effective the cooling tower is at cooling the water or how close the actual temperature difference between the water temperatures in and out is to the maximum temperature difference. The maximum temperature difference that a cooling tower can produce is the difference between the water temperature in and the air wet bulb temperature.
The range is important because when used in conjunction with the water flow rate, the capacity of the cooling tower can be found. The capacity and the amount of cooling provided by the cooling tower are found by multiplying the flow rate of the cooling water by the difference in temperature at the inlet and outlet of the cooling tower, using the following equation, Q = mc∆T and for a simplified equation to use during the test, follow the derivation below.
Cooling Tower Water Loss and Make-up
In a cooling tower, water is lost due to multiple sources such as evaporation, drift and blow-down. The first term, evaporation, is calculated through the following equation, where the assumption is made that the total heat loss is due to the heat loss through evaporation.
The second water loss is due to drift. Drift is the amount of water that is carried out through the airstream. Drift eliminators provided prior to the discharge are best described as a maze of baffles that the air must travel through before exiting to atmosphere. The drift eliminator trap the water droplets that get picked up by the exiting air and send the droplets back to the fill material. Typical water loss due to drift is less than 0.2%.
The third major source of water loss is due to blow-down. Blow-down is required because as water is evaporated it leaves behind the total dissolved solids (TDS), which increases the concentration of the TDS in the water. In order to bring the concentration of the TDS back to normal conditions so that it may be used safely with the equipment, the high concentrated TDS water is drained regularly and this is what is referred to as blow-down. The water is then replaced with fresh water and this is referred to as make-up water.
Furnaces are mechanical pieces of equipment used for space heating. Furnaces consist of a burner with a combustion air intake, fuel intake and an igniter. The hot combustion flames are routed through a heat exchanger, where heat is exchanged to the cold air as it is blown across the heat exchanger coils. Warm air is then blown to the space and the combustion gases/products exit the furnace through an exhaust vent pipe.
The fuel that is most commonly used is natural gas. Furnaces can be used in both residential and commercial situations.
TYPES OF FURNACES
The two main types of furnaces are condensing and non-condensing furnaces. The traditional non-condensing furnace operates in the initial description of a furnace. These furnaces can have efficiencies in the range of 80% to 84% AFUE. A condensing furnace takes the combustion products that were initially routed to the exhaust vent and passes them through another heat exchanger. This extracts more heat to the air and cools the combustion products to a temperature where water begins to condense out of the air. Because of the water, this second heat exchanger is made of a corrosive resistive material. A condensing furnace can have efficiencies in the range of 90% to 98% AFUE.
The annual fuel utilization efficiency (AFUE) is the term used by manufacturers to rate the annual efficiency of their furnaces. It describes the ratio of the amount of useful heat out of the furnace compared to the amount of fuel input to the furnace. This efficiency rating is regulated by the Department of Energy (DOE) and is used to take into account the constant on/off operation and seasonal effects on the furnace. The DOE requires that all furnaces have efficiencies greater than 78% AFUE.
Steady state efficiencies are also provided by the manufacturer and indicate the best efficiency of the furnace when operated at peak conditions.