Refrigeration Test #5
42-28) What is the airflow in CFM through a 10 inch by 20 inch duct with a total pressure of 0.15 inches wc and a static pressure of 0.1 inches wc?
0.15 - 0.1 = 0.05 inches wc velocity pressure 4005 x √0.05 = 896 CFM (rounded)
42-27) What is the velocity of the air in a duct with a total pressure of 0.15 inches wc and a static pressure of 0.1 inches wc?
0.15 - 0.10 = 0.05" wc velocity pressure
42-14) What would the mixed air temperature be for a system with 20% outdoor air if the outdoor temperature is 10°F and the return air temperature is 75°F?
0.2 x 10°F =2°F 0.8 x 75°F = 60°F 2°F + 60°F = 62°F mixed air temperature.
42-29) What is the typical airflow per ton of residential air conditioning equipment?
400 CFM per ton
42-25) What is the airflow through a register with a free area of 72 square inches if the air velocity is 300 feet per minute?
72/144 = 0.5 ft2 0.5ft2 x 300 ft/min = 150 CFM
A major controlling factor in the location of the condensate return piping from a heating system is/are ______. a) the requirement for a gravity return b) the requirement for short laterals run c) the requirement to minimize conflict with headroom and other services d) all of the above
A
Determine the values of "A", "B", and "C" in the table accompanying the chart shown in the illustration. See illustration RA-0021. a) A = 71.2°F, B = 62.5°F, C = 35.0 BTU/lb b) A = 90.0°F, B = 62.5°F, C = 27.0 BTU/lb c) A = 40.0°F, B = 71.2°F, C = 35.0 BTU/lb d) A = 52.1°F, B = 65.2°F, C = 37.0 BTU/lb
A
For the proper control of the air temperature in an air conditioning system using chilled water circulation, which of the listed conditions should remain constant regardless of load changes? a) Chilled water system supply temperature. b) Chilled water system return temperature. c) Compressor discharge temperature. d) Compressor suction pressure.
A
Under standard atmospheric conditions, which of the following is true concerning the characteristics of air? a) Dry air is heavier than moist air. b) Dry air is lighter than moist air. c) Cold air can hold more moisture than warm air. d) Heating a sample of air will increase its relative humidity.
A
Which type of space heating systems is shown in the illustration? See illustration GS-0150. a) Steam heating b) Circulated hot water heating c) Electric element heater d) Forced hot air heating
A
42-4) How is the air volume affected by temperature?
Air volume increases as the air temperature increases.
A cargo hold has been determined to have a relative humidity of 80% and a dry bulb temperature of 85°F. When the hold is closed and the dry bulb temperature decreases, the relative humidity in the space will ______. a) decrease b) increase c) decrease to zero d) remain unchanged
B
The relative humidity is 30% and the dry bulb temperature is 90°F. Using the information shown in the illustration, determine the difference between the dew point and wet bulb temperature. See illustration RA-0021. a) 6.5°F b) 12.5°F c) 54.8°F d) 67.3°F
B
Under standard atmospheric conditions, air at a higher temperature can ______. a) give up moisture faster as condensation b) hold more moisture than air at a lower temperature c) become supersaturated at low relative humidity d) absorb less free atmospheric moisture
B
What is the maximum steam pressure allowed by Coast Guard Regulations (46 CFR) to be provided in the steam heating system of the living spaces? a) 30 psig b) 45 psig c) 60 psig d) 75 psig
B
When air at a given temperature, contains the maximum amount of moisture for that temperature, the air is said to be ______. a) superheated b) saturated c) condensing d) evaporating
B
Which of the following instruments can be used to measure the rate of air flow? a) Thermometer b) Anemometer c) Psychrometer d) Areometer
B
42-2) List four ways the human body expels heat?
Conduction Convection Radiation Evaporation
42-24) What units are used to measure air volume?
Cubic feet per minute CFM
A lower thermostatic temperature setting will generally tend to cause an air conditioned space to have a ______. a) lower relative humidity b) lower air circulation c) higher air circulation d) higher relative humidity
D
When air contains some moisture, but is not saturated, the dew point temperature is ______. a) between the wet and dry bulb temperatures b) equal to the total heat of air c) higher than the wet bulb temperature d) lower than the dry bulb temperature
D
42-7) In general, which is heavier humid air or dry air?
Humid air lighter than dry air because water vapor is less dense than air. Dry air is lighter than humid air. Air at 80°F and 83% RH has a volume of about 14 ft3/lb. Drier air let's say at 80°F and 30% RH would be drier and have a volume of about 13.75ft3/lb. Density is lb/ ft3. MORE HUMID (Density @ 80°F and 83% RH) 1 ÷ 14 ft3/lb = 0.0714 lb/ft3 DRIER (Density @ 80°F and 30% RH) 1 ÷ 13.75 ft3/lb = 0.0727 lb/ft3
42-12) What is a psychrometric chart?
The psychrometric chart is a graphical representation of air properties.
42-3) Why does the air's relative humidity affect comfort?
The relative humidity of the air affects how rapidly perspiration can evaporate form the skin and cool the body. High humidity levels slow evaporation and reduce body cooling.
42-5) What is the relationship between air volume and air weight (density)?
The volume and weight are affected oppositely. Increases in temperature increase the volume and decrease the weight, while decreases in temperature decrease the volume and increase the weight
42-8) What is a wet bulb temperature reading?
The wet bulb temperature is measured by placing a wick soaked with water around the thermometer bulb and moving it rapidly through the air. Water evaporates from the wick, cooling the bulb and lowering the temperature.
42-9) How is relative humidity measured?
Wet bulb and dry bulb temperature readings are obtained with a sling psychrometer and then compared to a wet bulb depression chart, a relative humidity slide rule, or a psychrometric chart.
If the air temperature increases while atmospheric pressure remains constant, the air will ______. a) have a greater capacity to absorb moisture b) absorb less free atmospheric moisture c) become supersaturated at high relative humidity d) give up moisture as condensation
A
42-15) Identify the scales represented by the lines on Figure 42-49.
A -% Relative humidity B - dry bulb temperature C - dew point temperature D -specific humidity in grains E - enthalpy in BTU/lb F - Volume G - wet bulb temperature
42-6) What are grains and what air property is measured in grains?
A grain is a small measurement of weight. 7000 grains = 1 pound. Grains are used to measure the amount of water in the air. This is called the specific humidity.
Air conditioning duct-work begins at the large mixing chamber called the ______. a) exhaust chamber b) plenum chamber c) intake chamber d) vapor chamber
B
The convector shown in the illustration, should have a minimal circulation air space from the bulkhead of approximately ______. See illustration GS-0150. a) 1/2 inch b) 2 inches c) 5 inches d) 10 inches
A
The converter pneumatic control bypass valve, shown in the illustration, receives it low pressure signal from the ______. See illustration GS-0151. a) hot water return manifold b) converter c) hot water circulating pump discharge line d) hot water supply line
A
The dew point is reached when the wet bulb temperature is ______. a) equal to the dry bulb temperature b) twice the dry bulb temperature c) 100°F less than the dry bulb temperature d) 50°F above the dry bulb temperature
A
The preconditioned air temperature is 90°F with a relative humidity of 66%. After the air has been conditioned, its temperature has been reduced to 60°F and is saturated. Using the psychrometric chart shown in the illustration, calculate the number of grains of moisture removed from one pound of the air. See illustration RA-0021. a) 62.00 grains b) 78.00 grains c) 140.0 grains d) 218.0 grains
A
The sensible heat of air is dependent upon the ______. a) dry bulb temperature b) wet bulb temperature c) saturation temperature d) water vapor superheat
A
The temperature at which water vapor in the atmosphere begins to condense is called the ______. a) dew point temperature b) condensation temperature c) psychometric temperature d) absolute humidity temperature
A
To prevent the unnecessary loading of an air conditioning system while maintaining the designed dry bulb temperature and relative humidity in an air conditioning system, you should ______. a) admit only enough fresh outside air to provide proper ventilation b) reduce the air reheating system load c) lower the compressor head pressure d) operate the purge recovery unit continuously
A
As the amount of moisture in the air increases, the difference between the dry bulb and wet bulb temperatures will ______. a) increase b) decrease c) remain unchanged d) be greatest at dew point
B
Expansion tanks when used in a ships hot water heating system, may be of the open or closed type. In a closed type system, what would be the normal temperature range of the water? a) 180°F to 212°F b) 220°F to 240°F c) 260°F to 280°F d) 320°F to 360°F
B
If a stateroom has a 6" by 12" opening through which air travels at 100 feet per minute, what is the volume of air being supplied to the stateroom? a) 36 cu ft/min b) 50 cu ft/min c) 72 cu ft/min d) 100 cu ft/min
B
If high relative humidity is maintained in a cargo hold, there is a significant possibility that ______. I. there will be an accumulation of static electricity II. mold will grow and contaminate the cargo a) I only b) II only c) Both I and II d) Neither I nor II
B
In reference to air conditioning, when air attains the maximum amount of moisture it can hold at a specific temperature, it is said to be ______. a) superheated b) saturated c) condensed d) convected
B
Relative humidity can be determined by using a psychrometric chart and a/an ______. a) hydrometer b) sling psychrometer c) aneroid barometer d) compound barometer
B
Sample #5 in the table shown in the illustration indicates a relative humidity of 90%. Using the accompanying psychrometric chart, determine which of the samples contains the greatest amount of moisture per pound of dry air. See illustration RA-0021. a) Sample 1 b) Sample 2 c) Sample 3 d) None of the above are correct, as it is contradictory to measure moisture on a weight per pound of 'dry' air basis.
B
The process of removing moisture from air is known as ______. a) humidification b) dehumidification c) vaporization d) evaporation
B
The ratio of the weight of moisture contained in a given volume of air, to the weight of moisture that the same sample would hold if saturated, is called the ______. a) absolute humidity b) relative humidity c) specific humidity d) total humidity
B
Which of the following guidelines is considered to reflect good design practices for shipboard steam heating systems? a) Provide orifice-type bypasses for all traps and automatic valves. b) Provide a dirt pocket and strainer ahead of the steam trap on a unit heater return. c) Provide all units with a dirt trap and gate valve in the supply and a check valve on the return. d) Wherever possible install vertical runs for condensate piping
B
Any air mixture whose dew point remains constant will also have an unchanging ______. a) dry bulb temperature b) wet bulb temperature c) specific humidity d) specific volume
C
Hot water system heat exchangers are selected for the heating load required plus about 50% excess capacity to allow for ______. a) start-up b) increased thermal efficiency c) increase capacity during times of peak demand d) heat loss due to insufficient insulation
C
If air at 95°F dry bulb temperature and 50% relative humidity is conditioned to 75°F dry bulb temperature and 50% relative humidity, it is an example of ______. a) cooling only b) cooling and humidifying c) cooling and dehumidifying d) adiabatic cooling
C
In a chilled water air conditioning unit using a reciprocating compressor, the refrigerating effect of the primary refrigerant can be increased by ______. a) increasing refrigerant pressure in the coil b) increasing chilled water flow through the cooler c) subcooling the refrigerant in the condenser d) superheating the refrigerant in the compressor
C
In a two stage centrifugal air conditioning system, the liquid refrigerant passes through the condenser directly to the ______. a) evaporator b) chiller c) economizer d) expansion valve
C
In an atmosphere of 100% relative humidity, the wet bulb thermometer would register a temperature ______. a) above the dry bulb temperature b) below the dry bulb temperature c) the same as the dry bulb temperature d) above the dry bulb temperature, but below the saturation temperature
C
In order to achieve greater dehumidification with an air conditioning system, you should ______. I. reduce the cooling coil temperature II. increase the reheater temperature a) I only b) II only c) Both I and II d) Neither I nor II
C
In the illustrated heating system, what is the maximum height the heater should be mounted above the deck? See illustration GS-0150. a) 5" b) 6" c) 12" d) 24"
C
One benefit of proper air circulation in a refrigerated cargo space is ______. a) more temperature differential b) increased moisture content c) reduced slime and mold d) increased density of the air
C
The effective temperature of air is dependent upon ______. I. relative humidity II. air velocity a) I only b) II only c) Both I and II d) Neither I nor II
C
The latent heat of water vapor in air is dependent upon the ______. a) dry bulb temperature b) wet bulb temperature c) dew point d) dry point
C
The relative humidity is 50% and the dry bulb temperature is 90°F. Using the information shown in the illustration, determine the difference between the dew point and wet bulb temperature. See illustration RA-0021. a) 2°F b) 4°F c) 6°F d) 12°F
C
Using the information given, solve for "H", "I", and "J" in the table accompanying the psychrometric chart shown in the illustration. See illustration RA-0021. a) H = 33.0 BTU/lb, I = 15.5 cubic ft/lb, J = .0127 lbs/lb b) H = 33.0 BTU/lb, I = 13.97 cubic ft/lb, J = 78.0 grains/lb c) H = 34.1 BTU/lb, I = 13.97 cubic ft/lb, J = 88.0 grains/lb d) H = 34.1 BTU/lb, I = 14.97 cubic ft./lb, J = 98.0 grains/lb
C
Using the psychrometric chart shown in the illustration, determine the quantity of heat removed from one pound of air as it is cooled from 85°F at 80% relative humidity to 57°F at 70% relative humidity? See illustration RA-0021. a) 12.2 BTU b) 17.2 BTU c) 22.5 BTU d) 64.9 BTU
C
When using a sling psychrometer to determine relative humidity, the indicated difference between the dry bulb and the wet bulb reading is the ______. a) relative humidity b) dew point c) wet bulb 'depression' d) partial saturation temperature
C
Which of the processes listed would be the most satisfactory method to use to lower the humidity of the air being circulated by an air conditioning system? a) Cooling the air to a temperature just above dew point. b) Heating the air to a point at which moisture will boil off, then recooling it. c) Cooling the air to a point below dew point, then reheating it. d) Heating the air and then cooling it to a point below dew point.
C
A lower thermostatic temperature setting will provide a desired degree of comfort in a room where ______. a) low relative humidity is maintained b) triple banded squirrel cage fans are used c) air circulation is a maximum d) high relative humidity is maintained
D
A machinery space supply fan is delivering 5000 cubic feet of air per minute. The temperature of the air is 71°F, with a relative humidity of 38%. How many pounds of air are delivered to the space in a 24 hour period? See illustration RA-0021. a) 22,222.22 lbs/day b) 67,500.00 lbs/day c) 432,000.00 lbs/day d) 533,333.33 lbs/day
D
A reheater in an air conditioning system functions to ______. a) control inlet air temperature b) control inlet air volume c) maintain relative humidity at 15% d) restore conditioned air temperature to a comfortable level
D
A reheater, as used in an air conditioning system, is designed to control the ______. a) chilled water temperature b) dew point temperature c) primary air temperature d) dry bulb room temperature
D
A room humidistat initiates the lowering of the humidity of the conditioned supply air to a space, while the actual process is accomplished by ______. a) raising the cooling coil temperature and lowering the reheater temperature b) raising both the cooling coil temperature and the reheater temperature c) lowering both the cooling coil temperature and the reheater temperature d) lowering the cooling coil temperature and raising the reheater temperature
D
If increasing the cooling water flow to a refrigeration condenser fails to lower the condenser pressure, the probable cause may be due to ______. a) a high level of freon in the reciever b) a low level of freon in the reciever c) partially blocked thermal expansion valve d) excessive amount of non-condensable gases trapped in the condenser
D
If the people in an air conditioned room complain of being too cool, the trouble might be that the ______. a) preheater has failed to cutout b) air velocity is too low c) relative humidity and dry bulb is high d) dry bulb temperature is too low
D
In an air conditioning system, moisture is removed from the air by ______. a) filters b) separators c) ducted traps d) dehumidifiers
D
In order to achieve greater dehumidification, you would adjust the air conditioning system by ______. I. reducing the chill water flow through the cooling coil II. increasing the preheater temperature a) I only b) II only c) Both I and II d) Neither I nor II
D
Increasing the moisture content of conditioned air is known as ______. a) moisturizing b) dehumidification c) dampening d) humidification
D
The amount of moisture in a given sample of air, when compared with the amount of moisture the air could hold if totally saturated at the existing temperature of the sample, is called ______. a) absolute humidity b) specific humidity c) effective humidity d) relative humidity
D
The converter pneumatic control bypass valve, shown in the illustration, receives its high pressure signal from the ______. See illustration GS-0151. a) hot water return manifold b) converter c) hot water circulating pump discharge line d) hot water supply line
D
The dew point of air is reached when the wet bulb temperature is ______. a) twice the dry bulb temperature b) 10°F above the dry bulb temperature c) 5°F above the dry bulb temperature d) equal to the dry bulb temperature
D
The introduction of outside air to the air conditioning system is 90°F with a relative humidity of 60%. The air has been conditioned to 56°F with a relative humidity of 80%. Using the psychrometric chart, shown in the illustration, determine the quantity of moisture removed from one pound of the conditioned air. See illustration RA-0021. a) 44 grains b) 54 grains c) 64 grains d) 74 grains
D
The quarters steam heating system warms the outside air from 30°F at 80% relative humidity to 105°F. Additional humidification is not provided and the air feels very dry. Using the psychrometric chart, determine the amount of heat added to each pound of air and its approximate relative humidity after being heated. See illustration RA-0021. a) 28.7 BTU, 26% relative humidity b) 14.8 BTU, 6% relative humidity c) 10.3 BTU, 16% relative humidity d) 18.4 BTU, 6% relative Humidity
D
The relative humidity is 90% and the dry bulb temperature is 50°F. Using the information shown in the illustration, determine the difference between the dew point and wet bulb temperature. See illustration RA-0021. a) 19.50°F b) 12.50°F c) 4.90°F d) 1.50°F
D
Using the psychrometric chart, which of the air samples listed below has the identical specific volume to an air sample having a dry bulb temperature of 70°F, and a moisture content of 11 grains per pound of dry air? See illustration RA-0021. a) Dry bulb 72°F, Dew point 33.5°F. b) Dry bulb 63.0°F, Relative humidity 60%. c) Dry bulb 69.0°F, Heat content 23.25 BTU/lb. d) Dry bulb 63.4°F, Wet bulb 59.5°F
D
When air is at its dew point and there is no change in either the temperature or pressure, the air ______. a) has a low absolute humidity b) has the lowest relative humidity c) cannot give up its moisture d) will gain no additional moisture
D
When air is at its dew point, it is at its ______. a) lowest absolute humidity b) lowest relative humidity c) highest absolute humidity d) highest relative humidity
D
Which of the following factors should be considered when calculating the air conditioning requirements for a vessels accomodation spaces? a) Solar gain b) Quantity of fresh air supply c) Number of occupants in the space d) All of the above
D
Which of the following guidelines is considered to reflect good design practices of shipboard steam heating systems? a) Provide constant steam service for equipment operating throughout the year. b) Provide either a common or individual reducing station for each group of units requiring the same operating pressure. c) Run all piping so it may be drained by gravity. d) All of the above.
D
Which of the following methods is normally used to control the circulated air temperature of an air conditioning system using chilled water circulation? a) A fan speed controller regulates the amount of air flowing across the coils. b) Control dampers varying the number of passes the air makes across the cooling coils. c) A regulating valve changes the inlet temperature of the water in the cooling coils. d) A regulating valve controls the quantity of chilled water flowing in the cooling coils.
D
Which of the following methods is normally used to control the individual room temperature with an air conditioning system using a chilled water circulation system? a) A room thermostat activates an electric heater at each individual chilled air outlet when necessary. b) Varying the number of passes the air makes across the cooling coils. c) Varying the temperature of the water to the cooling coils. d) A regulating valve controls the quantity of chilled water flowing through the air cooling coils.
D
42-16) Use the psychrometric chart in Figure 41-1 to look up the missing data in this table.
Dry Bulb Wet Bulb Relative Humidity Dew Point Specific Humidity Volume Enthalpy 72°F 60°F 50% 52.5°F 59 grains 13.6 ft3 26.5 BTU/lb 75°F 61 45% 52°F 58 grains 13.7 ft3 27 BTU/lb 85 74°F 60% 70°F 110 grains 14 ft3 37.6 BTU/lb 87.5°F 62°F 23% 45 44 grains 14 ft3 28 BTU/lb 85 70 48% 63°F 87 grains 14 ft3 34 BTU/lb Note: The given values are in blue, the answers are in black. Some reasonable amount of leeway should be given the students when looking up these values since they will sometimes be interpolating between lines on the chart
42-13) List the air properties that are plotted on a psychrometric chart and give a brief description of each.
Dry bulb temperature - the normal air temperature in degrees Fahrenheit Wet bulb temperature - the temperature of a thermometer with a wet bulb in degrees Fahrenheit. The wet bulb temperature reading takes the moisture in the air into account. Enthalpy - Measures the total heat content of the air in BTUs per pound of air. Dew point temperature - Measures the temperature where water will start to condense out of the air. Specific Humidity - Measures the weight of water vapor in the air in grains Relative humidity - Expresses the percentage of water vapor in the air compared to the amount of water vapor the air can hold. Specific volume - Measures the space in cubic feet occupied by each pound of air.
42-17) What pressure units are used for measuring duct pressures?
Inches of water column (inches WC)
42-22) What units are used to measure air velocity?
Inches of water column (wc)
42-19) What instruments are used for measuring duct pressure?
Inclined manometer, digital manometer, magnehelic gauge
42-21) Explain how to take a velocity pressure reading on a duct system.
Insert one pressure probe facing the airflow to read total pressure and connect this probe to the high pressure side of the inclined manometer or magnehelic gauge. Insert a second probe perpendicular to the airflow to read static pressure. Connect this probe to the low pressure side of the inclined manometer or magnehelic. The difference in total pressure and static pressure is velocity pressure
42-20) Explain how to take a static pressure reading on a duct system.
Insert pressure probe at a right angle to the airflow and read the pressure using an inclined manometer, digital manometer, or magnehelic gauge.
42-26) How does a flowhood measure airflow?
It measures the velocity though the known area of the flowhood and displays the airflow in CFM.
42-23) What tools are used to measure air velocity?
Magnehelic, inclined manometer, digital manometer
42-11) What is the difference between specific humidity and relative humidity?
Specific humidity is the actual weight of the water in each pound of dry air, measured in grains. Relative humidity compares the amount of water in the air to the amount of water the air can hold at a given temperature.
42-18) What two pressures are created by air moving through the duct system?
Static pressure pushing out in all directions and velocity pressure pushing forward.
42-1) What three environmental conditions have the most effect on comfort
Temperature, humidity, and air movement.
42-10) What is the dew point temperature?
The dew point temperature is the temperature where water vapor can begin condensing out of the air.
