MODULE 6 Handling Equipment and Systems Air
The examples are shown in an accordion-style display. Some questions are accompanied by equations and/or tables for quick reference. Solve the problem on your own and then click on the question to view the solution. This will help you make sense of the work as you read through the problem-solving process.
1.
What is the difference between an air-handling unit and a fan-coil?
Small air-handling units are called fan-coils, generally less than 3000 cfm in capacity.
2.
Why are the casings of air-handling equipment insulated?
Casings are insulated to prevent thermal losses, prevent sweating, and control noise.
3.
Why can heating coils be designed for higher face velocity than cooling coils can?
Heating coils operate dry (no condensation); so there is no need to keep velocities low for prevention of water carryover.
4.
What materials of construction are appropriate for indoor air-handling unit casings? Outdoor?
Indoor air handling units are generally constructed of painted or galvanized steel. Outdoor units are generally painted and galvanized.
5.
What is the advantage of bag-type filters over flat filters?
Bag filters allow more surface area within a given cross section of air handling unit.
6.
What type of air filters would be appropriate for hazardous particulate materials?
HEPA filters are used for removal of hazardous materials. Filter housings are constructed to allow replacement without exposing old filters to the environment.
7.
What are the advantages of blow-through air-handling units in comparison with draw-through units?
Blow through units have the advantage of promoting mixing of air upstream of coils, minimizing risk of freezing. Blow-through units allow lower discharge temperatures, which can reduce the amount of air needed for space cooling.
8.
What is the chief consideration in designing air-handling unit sections for mixing outside air and return air?
Mixing sections must be designed to prevent stratification of cold outside air in winter, which could cause freezing of coils.
9.
Why is plastic film ductwork especially appropriate for greenhouses?
Plastic is impervious to water and will not corrode. These qualities are ideal in high moisture applications. In addition, film plastic is transparent, which avoids shadows on plants.
10.
What is the advantage of belt-drive fans compared with direct-drive fans?
Belt drives can be adjusted to alter performance. Direct drives cannot be adjusted.
11.
What is the limitation of low-velocity ductwork in large distribution system applications?
Low-velocity ductwork requires larger cross sectional area than medium or high velocity ductwork. Low-velocity ductwork would be too large, expensive and bulky to install for large distribution applications.
12.
When might aluminum or stainless steel be preferred over galvanized steel duct construction?
Aluminum or stainless steel would be preferred for resistance to moisture or chemical fumes.
13.
When might a plug fan be considered instead of a conventional centrifugal fan?
Plug fans are more compact and used when space saving and simplicity of installation is critical.
14.
When is fabric ductwork an appropriate alternative to sheet metal ductwork?
Fabric ductwork is good in high humidity spaces to achieve good diffusion at low velocity even with high air volumes. This feature is beneficial in avoiding drafts. Fabric ductwork can be an inexpensive alternative to sheet metal where exposed, round ductwork with color is desired.