The is the Mechanical Engineering PM HVAC/Refrigeration Session. Questions are ordered from Q 041 through Q 080. Problems are in either S.I. or U.S. Customary Units.
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Question to Topic Map
|Q 041–Q 043||Thermodynamics|
|Q 044–Q 049||Psychrometrics|
|Q 050–Q 054||Heat Transfer|
|Q 055–Q 057||Fluid Mechanics|
|Q 058–Q 060||Compressible Flow|
|Q 061–Q 062||Energy Balances|
|Q 063–Q 070||Equipment and Components|
|Q 071–Q 077||Systems (heating , cooling, etc.)|
|Q 078–Q 080||Supporting Knowledges|
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An automotive air-conditioning system based on R-134a operates between 500 psia and 12 psia.
If 500 psia saturated liquid R-134a is throttled to 12 psia, the change [Btu / lbm x °R] in entropy (sFinal - sInitial) will be most nearly.
The amount of energy required to vaporize 2 pound mass of water originally at 50 °F and STP is most nearly (in Btu):
Air is originally at 90 °F and 14.7 psia. It is compressed isentropically to 100 psia. The new temperature and enthalpy ( °F , Btu/lbm) is most nearly.
Recirculated air at 6,000 cfm, pulled from a room conditioned to 75 °F and 50% relative humidity, is mixed with 1,000 cfm of outdoor air at 90 °F db and 75 °F wb prior to passing through a cooling coil and a fan for distribution back into the room.
The coil has a total capacity of 400,000 Btu/hr and a sensible cooling capacity of 275,000 Btu/hr. The fan power at the shaft is 4.2 bhp. The temperature of the air leaving the coil (in °F) is most nearly.
In the psychometric process diagram shown, what process is best described in going from stage 3 to stage 4:
- [A] sensible and latent heating
- [B] dehumidification
- [C] sensible and latent cooling
- [D] evaporative cooling
An experimental refrigeration system using R-134a operates at 50 psia on the low pressure side and 100 psia on the high pressure side. The 300 ton system delivers refrigerant vapor with 40 °F superheat to the compressor.
If the refrigerator on the high pressure side is cooled to saturated liquid before expansion, the required refrigerant mass is most nearly.
Sally, a junior level thermodynamics student, determines the following parameters for air that is at 65 °F dry bulb and 30% relative humidity:
- The wet bulb temperature ( °F ) = 49 °F
- humidity ratio ( ω ) = 28 grains of moisture / lbm dry air
- enthalpy (h, Btu/lbm) = 20 Btu / lbm dry air
- dew point temperature ( °F ) = 34 °F
- specific volume (ft^3 per lbm) = 13.3 ft^3 / lbm
- vapor pressure (psia) = .093 psia
- degree of saturation = .3011
What can we say about her analysis?
- [A] She got dew point temperature incorrect
- [B] She got enthalpy incorrect
- [C] She got the wrong answer for vapor pressure
- [D] She determined all parameters correctly
6000 cfm of 40 °F dry bulb, 35 °F wet bulb air are mixed with 65,000 cfm of 75 °F dry bulb, 50% relative humidity air. The final dry bulb temperature in °F is most nearly.
The infiltration (in ft^3 / hr) around a 4 ft wide by 5 ft high double hung wood window in a residence facing south if the due-south wind velocity is 25 mph is most nearly.
A microprocessor is to be mounted on a printed circuit board inside a personal computer. The microprocessor measures 1 inches x 1 inches x 1/8 inch high and emits 30 W of heat through its top and side surfaces. The base is completely insulated.
Surface emissivity is 0.65, the heat transfer coefficient is approximately 2 Btu/(hr x ft^2 x °F), and and surroundings are at 70 °F, and no forced cooling is provided. Radiation heat transfer accounts for 90% of the total heat transfer. The surface temperature in °F is most nearly.
A motor transfers heat to the atmosphere through an 10 inch x 10 inch vertical flat plate. Assume laminar natural convection is the dominant transfer mechanism. Radiation is negligible. Atmospheric temperature is 65 °F, and the surface temperature measures 155 °F. The transferred heat flow in Btu/hr is most nearly.
An electric heater is needed on the second floor of a three story building. The room has no windows. The 1200 ft^2 external wall is made up of 2 inch of polystyrene rigid insulation sandwiched between 8 inch brick.
The outdoor winder design temperature is 15 °F, and the room is to be maintained at 70 °F. Using a 100% safety factor, the required heating capacity of the electric heater in kW is most nearly.
A 0.7 inch x 0.7 inch x 15 inch rod, with k = 95 Btu / hr∙ft∙°F, has one end held at a constant 500 °F. The still air temperature is 90 °F. The temperature (in °F) 3 inches from the base if the average film coefficient is 1.65 is most nearly.
You are in charge of the design of a tankless water heater, which must heat a municipal supply of water as it flows through the device. The heating tube (ID = 2.00 inch, OD = 2.125 inch) shall raise the temperature of water flowing in the pipe at 30 fps from 50 °F to 110 °F. The tube wall is held constant at 200 °F by an electric heating element that runs on a 240 volt supply. The Nusselt Prandtl number is 4.52 and the Reynolds number is 3.38 x 10^5.
The length of heating tube (in ft) needed to meet these requirements is most nearly.
A large ceiling fan supplies 2300 cfm of air through a 200 ft long rectangular duct. The duct dimensions are 18 inches x 24 inches. The duct has a friction factor of 0.032 and a roughness of .0006 ft. The total static pressure drop due to friction in inches of water is most nearly.
A 5 inch diameter steel sphere achieves a terminal velocity of 105 inches/sec when dropping through a tall column of liquid. The density of the liquid is 72 lbm/ft^3. The density of steel is 200 lbm/ft^3. The final drag coefficient of the sphere is most nearly.
A 60 lbm car top carrier measures 36 inch long by 24 inch wide by 12 inch high.
If a 3000 lbm car with a frontal area of 7.5 ft2 and a Cd of .24 is driven at 75 mph into a 10 mph headwind, the added net motor power ( in hp) that is required when the carrier is used is most nearly.
A blunt bullet shot from a 9mm handgun passes through STP air at 3000 fps. A ballistics consultant estimates the stagnation values of temperature, pressure, and enthalpy at the bullet face as the following:
temperature: 1263 °R pressure: 155 psia enthalpy: 210 Btu / lbm
Which parameters did the consultant calculate correctly?
New windows are to be installed in a building specified with frames having an R-value of 2.0 hr ∙ ft^2 ∙ °F / Btu. Outdoor temperatures as low as -30 °F are experienced in the winter, and the indoor space is to be heated to 70 °F.
Assuming indoor and outdoor window film coefficients of 1.2 Btu/hr ∙ ft^2 ∙ °F and 8.0 Btu/hr ∙ ft^2 ∙ °F respectively, the max relative humidity (%) that can be maintained without experiencing condensation on the inside of the frames is most nearly
The constant volume of specific heat of an ideal gas varies according to the equation aT^2, where a = 3.1 x 10^-6 BTU / lbm ∙ °R^3.
If the gas is heated from 200 °F to 260 °F at constant volume, the change in entropy ( in Btu / lbm ∙ °R) is most nearly.
A 5 hp motor is used on a high performance industrial blender to stir a tank containing 30 lbm of water for 20 min. Assuming the process occurs at constant volume, the maximum possible rise in temperature (in °F) is most nearly.
A centrifugal pump is driven at 2300 rpm by a 12 hp rated motor and delivers 200 gpm of 85 °F water against 50 ft water head at some intermediate flow rate setting.
The initial pump efficiency of 75% does not vary appreciably, the max flow the pump can deliver (in gpm) is most nearly.
A manufacturer's catalog indicates that power of 41 BHP is required for a centrifugal fan to deliver 50,000 cfm of standard air against 4 inches of water. The mechanical efficiency of the fan (%) is most nearly.
A natural draft cooling tower in the middle of town has a 275 ton heat rejection load that operates at an efficiency of 78% when the outdoor wet bulb temperature is 65 °F and the entering water temperature is 105 °F.
The temperature of the water leaving the tower in °F under these conditions is most nearly.
To reduce the load on a chiller plant, a newly installed air water recirculating 57 °F water is used to evaporatively precool 50,000 cfm outdoor air. The outdoor air is introduced at 87 °F and 57 °F wb. If the saturation efficiency of the process is 77 percent, the cooling requirement reduction in tons is most nearly.
A ventilation system is being considered for a new 20,000 ft^3 health club consisting of 50% weight rooms and 50% aerobic rooms by area. At a minimum, it should provide treated outdoor air of acceptable quality at a rate (in cfm) of most nearly.
A six row cooling coil with eight fins per lineal inch has a bypass factor of 0.19 and apparatus dew point of 45 °F. If air entering the coil is 92 °F db and 67 °F wb, the dry bulb temperature (in °F) of the air leaving the coil is most nearly.
Atmospheric air at 640 °F is compressed in a centrifugal compressor to 10 atmospheres with a 85 percent isentropic process efficiency. The final work done in Btu for 2 lbm of air compressed is most nearly.
A 9 inch x 12 inch (bore and stroke) 2 cylinder, 4 stroke engine operates with a mean effective pressure of 105 psig at 300 rpm. If the actual developed torque is 500 ft∙lbf, the friction horsepower is most nearly.
A home use fire sprinkler system uses 1 inch nominal diameter schedule-40 steel pipe. The sprinklers are located 20 ft apart on each line. The minimum pressure at any sprinkler is 20 psig. All sprinklers have standard .5 inch orifices with a discharge coefficient of 0.75.
During the last fire, the last three sprinklers on a branch line are opened simultaneously. Ignoring velocity pressure, the third branch sprinkler from the end will discharge water (in gpm) most nearly.
A small retail space measures 15 ft x 15 ft and has a 9 ft ceiling. The cooling load was calculated by the HVAC consultant to be 5,500 Btu/hr. A single ceiling diffuser will be installed in the center of the room ceiling.
To achieve good occupant comfort and maximize the air distribution performance index (ADPI), a the diffuser throw (ft) should be nearly.
An air handler serves a mixed office space consisting of a reception area, interior office and a perimeter office. The necessary air quantities for the spaces are:
According to ASHRAE, the correct fraction of outside air in cfm to comply with multiple ventilation standards for different spaces is most nearly.
A 1/10th scale model fan tested at 200 rpm moves 60 cfm at standard conditions against 1/4 w.g. Assume a constant efficiency if using affinity laws. The horsepower that will be required if a full size fan operates outside at 4000 feet altitude at one third the model's speed is most nearly.
A new duct is being installed in an office building that flows 4200 scfm. A branch of this air of 1000 cfm is directed to another building. In order to re-pressurize the duct, it is changed from 18 inches to 20.5 inches over an equivalent length of 40 ft after the takeoff. The static regain in inches of water is most nearly.
A new event center is being built in Washington DC and the owner wants to estimate the heating cost from October 15 to May 15. The design loss at 0 °F is 1.1 x 10^6 Btu / hr. The price of Bunker C oil is $2.45 per gallon. There are 4982 degree days form October to the May period. The interior design temperature is 65 °F and the efficiency of the heater is 85%. The heating cost for this period is most nearly.
An industrial refrigerator that utilizes ammonia operates on the Carnot cycle between 90 °F and 5 °F. The HVAC consultant in charge of the analysis determines the following theoretical parameters:
- COP = 5.47
- Horsepower per ton = .86 hp
- Energy efficiency ratio = 12
Which parameters were calculated correctly?
A home owner plans to have a contractor install a new furnace with a rated annual fuel utilization efficiency (AFUE) of 77%. It is estimated that the furnace will consume 2000 therms of natural gas per year.
The contractor has offered to install a different high-efficiency condensing furnace with an AFUE of 95% for an additional $800. Assuming a constant cost of natural gas of $1.85 per therm, the simple payback period in years of this additional investment is most nearly.
A real estate tycoon has installed an air-cooled chiller on the side of his Malibu Mansion, as shown. The neighboring property owner has complained that the unit is too noisy, particularly at night. The manufacturer's data states that the unit has a sound power rating of 77.0 dB.
The local code requires that the sound pressure level at the property line be no greater than 52 dB. The sound pressure level at the property line is most nearly.
An experimental Pelton wheel impulse turbine developing 60 bhp net is driven by a water stream from an 4 inch schedule 40 penstock. Total head before nozzle loss is 200 ft. If the turbine runner is rotating at 500 rpm and its efficiency is 85%, the pressure head at the nozzle entrance (cv = 0.94) in ft is most nearly.