M&E Systems Exam 1 Review
one ton of air is equivalent to:
12,000 btuh
COP for an a/c system that uses the vapor compression refrigeration cycle ranges from
2.5-7
A heating plant for a large medical campus requires 22,000 MBH (MBH=1,000,000 btuh) of heating capacity. What pipe size & velocity in feet per second (fps) for the main would be needed if the plant is a 50 psig medium-pressure steam system?
8'' pipe at 130 fps
Which factor below does NOT affect the solar heat gains to a space? Time of day Month of the year Air ventilation Use of shading devices
Air Ventilation
This system uses an air-conditioning unit to cool primary air, and terminal units in the zones supplied with hot or cold water to achieve the final temperature
Air-and-water system
This system uses an air-conditioning unit to cool primary air, and does not provide any supplementary cooling units in the zones. Heating may be provided by the primary air stream or by terminal units in the zones
All-air system
This system distributes hot or cold water or steam to terminal units in the zones for heating or cooling, and relies on air ventilation combined with those terminal units or separate arrangements that may include damper-controlled openings in the building envelope or exhaust fans.
All-water system
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.
Which type of heat transfer occurs when thermal energy flows through a solid material?
Conduction
This system always delivers the same airflow but the air may be hotter or colder as required to maintain or change the temperature in the space.
Constant volume
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.
A vapor compression refrigeration cycle system uses 30kW of electric power to produce 50 tons of cooling. What is the coefficient of performance of this system?
Convert kW to BTU 1kW=3413 BTU/hr 30x3413 = 1-2390 write the conversion unit to convert ton of refrigeration to Btu/hr 1TR=12000BTU/hr 50x12000= 600000BTu/hr calculate the coefficnet of performance(COP) of the vapor compression ref. system COP=RE/P COP=600000/102390 =5.86
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.
Select the answer which best describes this system. Air is supplied to a conditioned space by a system capable of providing a warm airflow and a cool airflow which is then mixed by a box at the point of entry to the space.
Dual-duct
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.
How can a single-duct system be divided into zones with different thermal requirements?
Either reheat method or VAV method could be used
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.
(T/F)The flue gas, which is a by-product of the combustion occurring in boilers and furnaces, is non-toxic and may or may not be exhausted to the outdoors.
False
(T/F)When calculating conduction loads for winter conditions through walls and roofs, you need to use the total equivalent temperature difference (TETD) value instead of the actual temperature difference between indoor and outdoor conditions.
False
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.
This piping configuration uses one set of supply and return pipes for cold water and another set for hot water. This configuration allows more flexibility, higher efficiency and lower operating costs, but incurs higher initial installation costs.
Four Pipe
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.
What measures the winter seasonal performance of heat pumps?
Heating Seasonal Performance Factor
Which of the following measures the winter seasonal performance of heat pumps? Select one: a. Solar heat gain factor b. Seasonal Energy Efficiency Ratio c. Heating Seasonal Performance Factor d. Coefficient of Performance e. None of these
Heating Seasonal Performance Factor
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
Which of the following is NOT an advantage of an air-and-water system:
Induction and fan-coil units require less maintenance than terminal reheat units and VAV boxes
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.
Select the answer which best describes this system. Air is supplied to a conditioned space by a system which produces both warm and cool air and mixes the airflows using dampers at the air handling unit.
Multizone
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.
Which of the following is/are disadvantages of a zoned variable air volume system?
Possibly poor ventilation if a zone does not require much air for cooling
To calculate the sensible heat gain due to ventilation for a room designed to be 75°F and the outdoor temperature is 87°F, which equation below would you use?
Q = (1.1) X (CFM) X (TD)
Find the winter heat loss due to conductance and solar gain through a 42" x 78" double glazed, insulated glass, regular sheet window (R-value = 2.1) that faces west with the following conditions: The window has a dark colored blind window covering. It is a clear day in January at 4:00pm, outdoor temperature is 34°F and the indoor temperature is 74°F, and the location of the facility is 40° latitude.
Qloss = 308.49 btuh Qsolargain = 2272.73btuh Qnet = 1964.24 btuh
Find the U-value of a wall section with the following construction (winter conditions): 4" face brick 1-1/2" air space 1" unfaced polyurethane 1/2" plywood 5-1/2" vermiculite 1/2" gypsum board Use the R-values provided in table 2.3 from the textbook. Also, remember to include air films.
R-Value total= R1+R2+... Rn U-factor= 1/Rtotal U = 0.047
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.
This occurs when the dry bulb, wet bulb, and dew point temperatures are the same
Saturation
Which of the following is not a true statement about an all-water system?
Since ventilation is controlled separately from heating and cooling, acceptable indoor air quality is easily maintain
Select the answer which best describes this system. Air is supplied to an undivided conditioned space by a system capable of providing a warm or cool airflow as required.
Single-duct single-zone constant volume
Which of the following approaches has the lowest energy consumption:
Single-duct zoned variable air volume
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.
In an air-and-water system, most of the heating and cooling is provided by water. The air system only deals with ventilation needs, and hence there will be less airflow and less dilution of contaminants. To compensate for this
The central system filters should be of higher efficiency than those typically used in all-air systems
An air-side economizer is used when:
The enthalpy of outdoor air is lower than that of the return air. The outdoor air is then used to provide free cooling, by bringing in more outdoor air than is required to meet needs for fresh air ventilation.
Reheat systems have many advantages - they permit simple zone control for areas with different loads, can provide heating or cooling to different areas at the same time, maintains close control over humidity and offers flexibility when zones are selected at design stage. What is the main disadvantage?
The system uses energy to cool air and then uses energy again to heat it back up. This is wasteful of energy and uneconomical
A hydronic heating system uses a boiler to heat water that is then transported through supply lines to spaces within a facility
True
Air-cooled equipment for heat rejection handle much smaller tons of capacity when compared to the tons of capacity for water-cooled heat rejection equipment
True
An over sized cooling system will not only cost more, but can result in short cycling of the system
True
Direct expansion systems are typically sized for accumulated peak loads & are louder and more visible than chilled water systems
True
In an air-and-water system during transitional weather the primary air is supplied to the space at room temperature and the water supplied to the terminal units in the space is chilled.
True
In an air-and-water system, during cold weather the primary air supplied to the space is cold, and the water supplied to the terminal units in the space is hot.
True
Since heat pumps can be used for heating or cooling purposes, typically one will see heat pumps with a SEER and a HSPF value
True
This piping configuration uses one pipe to supply water and one to return it to the boiler for re-heating od the chiller for re-cooling, depending on the season. This configuration is frequently used, since it is the least expensive to install, but it performs poorly when there are wide load variations and incurs higher operating costs.
Two Pipe
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.
This system keeps supply air always at the same temperature, and delivers more or less air as required to maintain or change the temperature in the space.
Variable air volume
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.
Heat gain from lights is calculated by multiplying ____________ by 3.41:
Wattage
What does the compressor do in the vapor compression refrigeration system? a. The compressor takes the low-pressure refrigerant vapor from the evaporator and compresses it to a higher pressure Correct b. The compressor allows warm air to be cooled as a refrigerant absorbs the heat and boils into a refrigerant vapor c. The compressor transfers heat from the hot refrigerant vapor to the surrounding ambient air to return the vapor to liquid d. The compressor is used to create large pressure drops to lower the pressure and corresponding temperature of the liquid refrigerant e. None of these
a
this phenomenon occurs when an air stream of conditioned air moves through a space
coanda effect
Which of the following would represent a common coefficient of performance value for an a/c system that uses the vapor compression refrigeration cycle? a. 8 b. 9 c. 0.5 d. 1 e. None of these
e
(T/F)Infiltration through a building's envelope is much higher in hot weather than cold weather
false
(T/F)The vapor compression refrigeration cycle uses an absorbent such as lithium bromide
false
a heat pump loop can provide cooling by using a reversing valve to convert the indoor coil to an evaporator and the outdoor coil to a condensor
false
sensible heat is measured using a thermometer with a wet sock attached to the bulb
false
of the following items that create hvac loads, which one produced only sensible loads during the summer cooling season
infiltration
what is the function of a diffuser in an HVAC distribution system?
mixes supply air without localized hot/cold spots
A heating system that uses heated water has a total heating load of 300,000 btuh. How much heated water flow in gallons per minute (GPM) is required to satisfy the total load if the system is designed for: a) A 25°F temperature change b) A 35°F temperature change
system delivered BTUH for water =500xGPMx system water temp change Origin of 500: 1 gal of water = 8.33 lb BTU measure in hours 1 Hour = 60 min pdxmin = 8.33 x 60= 500 load = 300,00 BTUh change in temp = 25 F GPM = load / (500xchangeintemp) GPM = 300,000/500x25 gpm= 300,000/500x35
what does the compressor do in the vapor compression refrigeration system?
the compressor takes the low-pressure refrigerant vapor from the evaporator & compresses it to a higher pressure
(T/F)A hydronic heating system uses a boiler to heat water that is then transported through supply lines to spaces within a facility
true
(T/F)An oversized cooling system will not only cost more, but can result in short cycling of the system
true
(T/F)Direct expansion systems are typically sized for accumulated peak loads and are louder and more visible than chilled water systems
true
(T/F)Some characteristics of water tube boilers are that they can efficiently respond to quick load variations, which occur in large institutional buildings such as large assembly halls and hospitals. Typically water tube boilers come in vary large sizes for use in central plans for large projects such as medical complexes and college universities.
true
(T/F)Ventilation loads can be calculated using CFM per person or room changes per hour
true
(t/f)Some of the requirements for human comfort are proper temperature, humidity, air flow, filtration, and ventilation
true
(t/f)The energy given off by lights creates a heat gain for a space
true
A two pipe system includes a supply branch, allowing each unit in the sytem to be individually controlled
true
Comfort for building occupants can be impacted by high humidity.
true
check valves isolate flow and prevent backflow in piping systems
true
registers are composed of dampers and grilles
true
steam generating heating systems typically cost more for installation and require more maintence when compared to hot water generating heating systems
true
the coefficient of performance for systems that use the vapor compression refrigeration cycle typically range from 2.5-7
true
the static regain method of sizing ductwork is the most accurate method for designing a well balanced duct system
true
vane axial fans are commonly used for air handling and return air
true
water pressure can be expressed in feet or head or in pounds per square inch
true
Which of the following HVAC split systems has ductwork found above the furnace and cooling coil? Select one: a. Up-flow b. Horizontal c. Single Package d. Down-flow e. None of these
up-flow
