mechanical exam review #2

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.

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.

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.

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.

Why are the casings of air-handling equipment insulated?

Casings are insulated to prevent thermal losses, prevent sweating, and control noise.

Compare the air quality of conventional overhead air distribution with floor distribution.

Conventional overhead systems rely on mixing of cold air in the space, which spreads odors and impurities throughout the occupied area. Floor distribution can be designed to lift odors and impurities up to the system returns without spreading through the occupied space.

What is the function of a diffuser in an HVAC distribution system?

Diffusers mix supply air into the room without undue drafts of localized hot or cold spots.

Name the three commonly used methods for varying fan output. Which is most efficient?

Discharge dampers, inlet guide vane dampers and variable speed drives are the three options for varying fan output. Discharge dampers are least efficient because they offer artificial resistance to airflow. Variable speed is most efficient and has become the standard solution.

Name several conditions that could cause uncomfortable dumping of cold air from a diffuser.

Discontinuities in the ceiling surface, low airflow in VAV applications, and over sizing of diffusers can cause dumping.

Why is a return fan or a relief fan good practice in design of large air handling systems?

Economizer cycle uses outside air for cooling when weather permits. Air must be expelled from the building to make room for the outside air. If return fans or relief fans are not installed to remove the air, building spaces will over pressurize during economizer cycle operation. This can result in inability to introduce sufficient outside air for cooling, doors standing open, and noise through air leaks in the building envelope.

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.

How might floor air distribution affect thermal loads compared with overhead air distribution?

Floor air distribution can be designed to lift heat up to system returns rather than allowing the heat to spread and raise air temperature in occupied space.

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.

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.

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

Briefly compare the relative benefits of commonly used diffuser types.

Louvered face diffusers are economical and work well over a wide range of airflow. Linear slot diffusers also work well over a wide range of airflow, typical of VAV systems. Perforated face diffusers are less obtrusive, but prone to dumping at low airflow. Troffers, supply openings integrated with light fixtures are least obtrusive, but require good workmanship to avoid excessive leakage.

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.

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.

What is the significance of NC levels listed in catalogues? Will NC levels be higher or lower as the airflow increases for a given diffuser? Why?

NC levels are an indication of noise level for diffusers for listed airflows. NC is higher for higher airflow due to turbulence of high velocity.

Briefly discuss pros and cons of natural ventilation for your current residence.

Open windows interfere with controls of air conditioning systems and can result in freezing pipes in the building if windows are left open in cold weather. Open windows allow dust and allergens unfiltered into occupied space. On the other hand, opening windows can reduce energy used for air conditioning and ventilation. Occupants are tolerant of wider temperature and humidity ranges when the windows are open. Sense of control and connection with outdoors is also an advantage.

Briefly discuss pros and cons of natural ventilation for a high rise office building.

Open windows interfere with controls of air conditioning systems and can result in freezing pipes in the building if windows are left open in cold weather. Open windows allow dust and allergens unfiltered into occupied space. Operable windows can work against the control of building airflow and pressure relationships. Tall buildings are subject to stack effect, which draws cold air low in the building during winter and warm air into the building during summer. Operable windows will increase the infiltration loads due to stack effect. In mild climates, this may not be a problem, and operable windows might be considered beneficial to occupants' sense of well being.

Briefly discuss pros and cons of natural ventilation for a low rise faculty office wing.

Open windows interfere with controls of air conditioning systems, which can be problematic if multiple offices are controlled from a single thermostat. Operable windows can also result in freezing pipes in the building if windows are left open in cold weather. Open windows allow dust and allergens unfiltered into occupied space. On the other hand, natural ventilation can be an effective strategy for energy conservation from natural cooling and ventilation. In addition, occupants are tolerant of wider temperature and humidity ranges when the windows are open. Sense of control and connection with outdoors make natural ventilation a desirable feature for most building users.

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.

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.

Your classroom building has rooftop air-handling units, and the ventilation rate is high due to dense occupancy in the classrooms. A significant amount of exhaust must be relieved. What exhaust energy recovery systems would you consider? Why?

Rotary heat exchangers with total energy capability (desiccant coating) would be the preferred option. They would reduce latent loads as well as sensible loads from the ventilation air. Since the exhaust is basically room air, there's no concern for cross contamination. Lastly, the exhaust can be designed to leave the building at the roof level close enough to the intakes to allow an economical installation. In many applications the rooftop unit is designed with the exhaust and the heat reclaim integrated in the same package.

Your laboratory building has outside air intakes and air handling units in a basement mechanical room. The fume exhaust fans are on the roof. What type of exhaust energy recover system would be appropriate? Why?

Run around coils would be appropriate for two reasons. First, run around coils allow the outside air and the exhaust air streams to be remote from one another. Second, there is no possibility of transferring pollutants from the exhaust to the intake, as might be the case with a rotary device.

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.

What applications might be appropriate for considering floor distribution? Why?

Underfloor systems can have life-cycle cost advantage where the floor plenum is also used for distribution of electrical and data wiring in buildings which require frequent remodeling. Underfloor air is also appropriate for auditoria and tiered classrooms, where low air noise, and good air quality are important. High spaces such as lobbies and atria are also good applications for floor supply due to the advantages achieved from temperature stratification

What is the most significant advantage of rotary exhaust energy recovery devices with desiccant coating compared with other energy recovery devices?

Unlike other devices, a desiccant coat rotary heat recovery device can transfer latent heat as well as sensible heat. This feature greatly reduces cooling loads in summer and can retain humidity in humidified buildings in winter.

What are the benefits of using light fixture slots for air return from the room to the ceiling?

Using light fixture slots avoids the need for separate return air devices - less expensive, less obtrusive.

What potential problems do variable air volume systems pose in the selection of diffusers?

VAV systems require that diffusers operate over a wide range of airflow. At low airflow, dumping or poor distribution can result. At high airflow, systems can be noisy and drafty.

What special provisions should be made to prevent heating supply air from stratifying at the ceiling?

Velocity of discharge must be high enough to overcome buoyancy and force the air to the floor. If an air supply device is used both for cooling and for heating, cooling will generally require more air. A compromise might be necessary, selecting a device slightly smaller (higher discharge velocity) than ideal for cooling and slightly large (lower discharge velocity) than ideal for heating.