Support Knowledge accounts for approximately 4 questions on the Thermal & Fluids Mechanical PE exam. The support knowledge section includes any principle that did not fit in the principle categories in the previous sections, but might be used in the application section. The topics covered in support knowledge include Pipe System Analysis, Joints, Psychrometrics, and Codes and Standards.
Pipe system analysis could be used in the Hydraulic and Fluid Applications – Distribution Systems section. This part covers the supports and stresses in a piping system. Similarly, different types of joints could also be included in this section. Psychrometrics are building blocks to air properties in cooling/heating air and also in air heat exchangers. Finally, codes and standards that govern the minimum safety and efficiency requirements for practicing engineers are mentioned here.
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This topic is primarily covered in the Machine Design and Materials exam, but for the Thermal & Fluids exam, you should know the basics.
Stresses in pipes are the critical part of this support knowledge principal. Stresses are the internal forces per unit area within the pipe material, when an external force is applied. The external forces can be from weight, pressure of the fluid inside the pipe or the thermal expansion of the pipe.
Strain is the result of the stress. Strain is the deformation of the pipe in the direction of the force, an example is a pipe being stretched, where the strain would be observed in the longitudinal direction (length of pipe).
The modulus of elasticity is a number that describes the pipe’s resistance to being deformed. A large modulus of elasticity means that it will take more force to deform the pipe. There are two types of modulus of elasticity. The Young’s Modulus E describes the tensile elasticity or the elasticity along the length of the pipe, when the pipe is stretched. The shear modulus describes the shear deformation elasticity, which is the ability of a material to slide in opposite, parallel directions, imagine a pipe twisting.
These values are dependent on the material. Be sure to locate a table of these values for various materials in your Mechanical Engineering Reference Manual.
The moment of inertia, I, can be found with the following equation. This value describes the pipe’s ability to resist angular acceleration (spinning).
Pipes are joined through the use of joints. There are multiple types of joining methods and these include:
These various joining methods are used based on the pipe material, the fluid within the pipe, size of pipe and the pressure within the pipe. For example, copper pipe is typically soldered together and high pressure pipe uses welding. Bolted and threaded joints have a limit on the pressure it can withstand. The welding of large pipes may sometimes be financially infeasible and thus another joining method may be used.
A welded pipe joint is formed by melting the pipe material between two pipes and filling the gap with a filler material. As the two sides of the pipe and material cool together, the two pipes become a continuous piece. The joint is often just as strong as the actual pipe material. Welding can be completed with both metal and plastic pipes.
The Psychrometric Chart describes the various properties (Dry Bulb Temperature, Wet Bulb Temperature, Enthalpy, Humidity Ratio, Relative Humidity and Specific Volume) of moist air. Moist air is defined as an air-water mixture. Moist air on a psychrometric chart ranges from Dry Air to Saturated Air.
Dry air is defined as having no water vapor and is located on the X-axis of the psychrometric chart. Saturated air is defined as an air-water mixture at equilibrium between the liquid and vapor phase. Saturated air is moist air in balance with its liquid and vapor phases. At saturation, air cannot hold any more moisture. It is the extreme opposite of dry air. This saturated air is defined by the exponential curve, called the saturation curve, which is clearly shown on Figure 3.
The psychrometric chart does not account for variations in pressure. Pressure is not shown on any axis, because it is constant. For the PE exam and for this guide, it is assumed that the psychrometric chart is based on atmospheric pressure (14.696 psia or 1 atm or 29.921 in. Hg), unless noted otherwise. The psychrometric chart for the PE exam also only shows a range of the temperatures typically encountered by a typical Thermal & Fluids engineer.
The following (3) Psychrometric Chart Topics will now be discussed in detail:
Air Properties on the Psychrometric Chart: What does each property tell of the air-water mixture? How much energy does an air mixture have? How do two different air conditions compare? Is the air hot/cold, wet/dry?
Movement on the Psychrometric Chart: What causes each type of movement on the chart (Right to Left, Up and Down, Diagonal). What is sensible heating/cooling? What is humidification/dehumidification? What is the sensible heat ratio?
Typical Psychrometric Questions: Mixtures of Moist Air, Humidification/ Dehumidification, Heating/ Cooling Questions, Condensation, etc.
Properties of Moist Air
Psychrometric Chart – Properties: The psychrometric chart as previously discussed, graphically shows the following thermodynamic properties of Moist Air,
Dry Bulb Temperature
Wet Bulb Temperature
If any of the two above properties are known of an air mixture, then the five other corresponding properties can be found.
The following sections go into detail on each of the thermodynamic properties. It is the intent of these sections for the reader to gain an understanding of the concepts and to grasp the meaning of each property.
See technical study guide for more detail on Dry Bulb Temperature, Wet Bulb Temperature, Relative Humidity, Humidity Ratio, Enthalpy, Specific Volume, and Dew Point.
Movement on Psychrometric Chart
Sensible heating and cooling is defined as the removal or addition of heat to an air mixture, with no effect on the moisture content. Its sole effect is on the increase or decrease of the dry bulb temperature.
On a Psychrometric Chart, sensible heating and cooling is shown as a horizontal line. It is horizontal because the amount of water vapor in the air is not changed, thus the Humidity Ratio remains the same. A horizontal movement increases or decreases the dry bulb temperature. As the dry bulb temperature increases with sensible heating, the air's capacity to hold water also increases. The opposite is true with sensible cooling. In addition, sensible heating is shown on the graph below to decrease relative humidity, while sensible cooling increases relative humidity.
Sensible Heating is characterized by the following: (1) Increase in dry bulb temperature, (2) Decrease in relative humidity, (3) No change in humidity ratio, (4) Increase in enthalpy, (5) Increase in specific volume [hotter air takes up more volume], (6) No change in dew point, (7) Increase in wet bulb.
Sensible Cooling is characterized by the following: (1) Decrease in dry bulb temperature, (2) Increase in relative humidity, (3) No change in humidity ratio, (4) Decrease in enthalpy, (5) Decrease in specific volume, (6) No change in dew point, (7) Decrease in wet bulb.
The test requires you to be familiar with Codes and Standards regularly used in the field of Thermal and Fluids. However, remember that the total Support Knowledge section is 4 questions and this will be a relatively small portion.
ASTM (American Society of Testing and Materials) is a voluntary standards organization that has over 12,000 ASTM standards. For the purposes of the exam, it is not important to know all the standards or even to have access to these standards. However, it is important to know what standards are available and to have an overview of the standards that are specific to the Thermal & Fluids field.
UL is an independent safety science company. It tests equipment, materials and products to confirm if they meet the UL safety standards. You will often find the following seal on a product, which indicates that the product has been tested and certified to meet a certain standard.
For the exam, you should be aware of this listing and what it means for products.
The National Fire Protection Agency or NFPA has more than 300 codes and standards related to fire hazards. These codes are primarily used by Fire Protection Engineers, but in the Thermal & Fluids field, you may enter an area where you will need to use these codes or engage a Fire Protection Engineer.
These codes are primarily focused on Fire Protection and safety issues in and around buildings.
For example, here is a list of some of the commonly used NFPA codes by Fire Protection Engineers.
ASHRAE stands for American Society of Heating, Refrigeration and Air Conditioning Engineers. This organization and its standards play a more important role in the HVAC & Refrigeration exam. For the Thermal & Fluids exam, you simply need to know of this organization and some of its common standards.
For example, here is a list of some of the commonly used ASHRAE codes. Do not purchase these standards for the exam.
ASHRAE 15 Safety Code for Mechanical Refrigeration
ASHRAE 55 Thermal Environmental Conditions for Human Occupancy
ASHRAE 62.1 Ventilation for Acceptable Indoor Air Quality
ASHRAE 90.1 Energy Standard for Buildings except Low-Rise Residential Buildings