Thermodynamics Quiz: Humidification Dehumidification
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Humidification DehumidificationQuestion 1 of 20

An air conditioning system processes 2000 kg/h of dry air. The air enters at 35°C with humidity ratio 0.022 kg/kg and exits at 20°C with humidity ratio 0.009 kg/kg. What is the rate of moisture removal from the air stream?

18 kg/h
42 kg/h
34 kg/h
26 kg/h
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Thermodynamics Quiz

Thermodynamics Quiz: Humidification Dehumidification

Practice Humidification Dehumidification in Thermodynamics with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Humidification Dehumidification, giving you a quick way to practice the rules, question types, and explanations that matter most for Thermodynamics.

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Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

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Question 1

An air conditioning system processes 2000 kg/h of dry air. The air enters at 35°C with humidity ratio 0.022 kg/kg and exits at 20°C with humidity ratio 0.009 kg/kg. What is the rate of moisture removal from the air stream?

  1. 18 kg/h
  2. 42 kg/h
  3. 34 kg/h
  4. 26 kg/h (correct answer)
Explanation: Moisture removal rate = mass flow of dry air × change in humidity ratio = 2000 kg/h × (0.022 - 0.009) kg/kg = 2000 × 0.013 = 26 kg/h. Choice A uses only the exit humidity ratio. Choice C adds the humidity ratios instead of finding the difference. Choice D uses an incorrect mass flow rate calculation including the initial moisture content.

Question 2

A humidifier adds water at 15°C to an air stream at 25°C and 30% relative humidity. If the process is adiabatic and the water temperature is below the air wet-bulb temperature, what is the primary heat transfer mechanism?

  1. Sensible heat transfer from air to water until thermal equilibrium
  2. Latent heat transfer during water evaporation into the air stream
  3. Combined sensible and latent heat transfer with air cooling (correct answer)
  4. Radiation heat transfer between water droplets and surrounding air
  5. Convective heat transfer maintaining constant air temperature
Explanation: When analyzing adiabatic humidification processes, you need to consider what happens when water below the wet-bulb temperature contacts warm, unsaturated air. The wet-bulb temperature represents the theoretical minimum temperature air can reach through evaporative cooling, so water at 15°C below the air's wet-bulb temperature will experience both sensible and latent heat effects. In this scenario, two simultaneous processes occur. First, sensible heat transfers from the warmer air (25°C) to the cooler water (15°C), causing the air temperature to drop. Second, because the air is only at 30% relative humidity, some water evaporates into the air stream, which requires latent heat. This latent heat comes from the air itself, causing additional cooling. The combination of these effects results in air that exits cooler but more humid than when it entered. Answer A is incomplete because it only considers sensible heat transfer and ignores the evaporation occurring simultaneously. Answer B focuses solely on latent heat transfer while overlooking the significant temperature difference between air and water that drives sensible heat transfer. Answer D incorrectly identifies radiation as the primary mechanism, when convection and mass transfer dominate in direct air-water contact scenarios. Answer C correctly captures both heat transfer modes occurring simultaneously, with the air cooling as it provides energy for both warming the water and enabling evaporation. Remember: In adiabatic humidification problems, always check if both temperature differences (driving sensible heat transfer) and humidity differences (driving latent heat transfer) exist—both usually contribute to the overall process.

Question 3

During a dehumidification process, moist air at 25°C and 80% relative humidity is cooled to its dew point temperature. What is the primary mechanism by which moisture is removed from the air?

  1. Chemical absorption of water vapor by cooling coils
  2. Mechanical separation of water droplets using centrifugal force
  3. Condensation of water vapor when saturation is reached (correct answer)
  4. Evaporation of excess moisture due to temperature reduction
  5. Sublimation of water vapor directly into ice crystals
Explanation: When you encounter dehumidification problems in thermodynamics, focus on the relationship between temperature, saturation, and phase change. Air can only hold a specific amount of water vapor at any given temperature, and this capacity decreases as temperature drops. During this dehumidification process, the moist air starts at 25°C with 80% relative humidity, meaning it contains 80% of the maximum water vapor it could hold at that temperature. When you cool this air to its dew point temperature, you're bringing it to the exact temperature where it becomes 100% saturated. Any further cooling forces the excess water vapor to condense into liquid droplets, which can then be collected and removed. This condensation process is the fundamental mechanism of dehumidification. Option A incorrectly suggests chemical absorption, but cooling coils provide only physical cooling and condensation surfaces—no chemical reactions occur. Option B describes mechanical separation, which might remove already-condensed droplets but isn't the primary moisture removal mechanism during the cooling process itself. Option D completely misunderstands the physics—evaporation increases with temperature, so cooling reduces evaporation rather than causing it. The correct answer is C because condensation occurs naturally when air reaches saturation and cannot hold additional water vapor. Remember this key principle: dehumidification through cooling always relies on reducing air's moisture-holding capacity until condensation occurs. Watch for problems involving dew point—this temperature represents the threshold where condensation begins, making it central to understanding humidity control systems.

Question 4

An adiabatic humidification process uses a water spray to add moisture to air. If the initial dry-bulb temperature is 30°C and the wet-bulb temperature is 20°C, what happens to the dry-bulb temperature as humidity increases?

  1. The dry-bulb temperature increases toward the initial wet-bulb temperature
  2. The dry-bulb temperature decreases toward the initial wet-bulb temperature (correct answer)
  3. The dry-bulb temperature remains constant at 30°C throughout
  4. The dry-bulb temperature increases proportionally to moisture addition
  5. The dry-bulb temperature fluctuates randomly around the mean value
Explanation: When you encounter adiabatic humidification problems, focus on the fundamental principle: energy is conserved, but heat and moisture are exchanged between the air and water spray. In adiabatic humidification, water evaporates into the air stream, absorbing latent heat energy from the air itself. This energy transfer causes the air's sensible heat (dry-bulb temperature) to decrease while its moisture content increases. The process continues until the air reaches saturation at the wet-bulb temperature, which represents the lowest temperature achievable through evaporative cooling at constant enthalpy. Starting with dry-bulb temperature of 30°C and wet-bulb temperature of 20°C, the air has significant capacity for moisture. As water spray evaporates, it draws energy from the air, progressively lowering the dry-bulb temperature toward the initial wet-bulb temperature of 20°C. Answer A incorrectly suggests temperature increases - this would violate conservation of energy since the evaporation process must cool the air. Answer C assumes constant temperature, ignoring the cooling effect of evaporation entirely. Answer D suggests proportional increase, which again contradicts the fundamental physics of evaporative cooling. Answer B correctly identifies that the dry-bulb temperature decreases toward the initial wet-bulb temperature as the cooling effect of evaporation dominates. Study tip: Remember that in adiabatic processes involving water addition, the wet-bulb temperature acts as the "target" - the air will cool toward this value as moisture increases. Always think "evaporation cools" when analyzing humidification processes.

Question 5

A cooling coil dehumidifies air from 35°C and 70% relative humidity to 15°C. If condensate forms at 18°C during the process, what can be concluded about the coil surface temperature?

  1. The coil surface temperature varies linearly from 35°C to 15°C
  2. The coil surface temperature is constant at exactly 18°C throughout
  3. The coil surface temperature is below 18°C in the condensation region (correct answer)
  4. The coil surface temperature equals the final air temperature of 15°C
  5. The coil surface temperature oscillates around the dew point temperature
Explanation: When you encounter dehumidification problems, focus on the relationship between air temperature, dew point, and surface temperatures needed for condensation to occur. For condensation to form on a cooling coil surface, the surface temperature must be below the dew point of the air contacting it. Since condensate begins forming at 18°C, this tells us that 18°C is the dew point temperature of the incoming air (35°C, 70% RH). Once condensation starts, it will continue as long as the coil surface remains below this dew point temperature. The correct answer is C because condensation physics requires the coil surface temperature to be below 18°C in the region where dehumidification occurs. If the surface temperature were at or above 18°C, no condensation would form. Choice A is wrong because coil surface temperatures don't follow air temperature linearly - they're determined by the cooling medium temperature and heat transfer conditions. Choice B incorrectly assumes the surface temperature equals the condensation initiation temperature, but condensation requires the surface to be below the dew point, not at it. Choice D is wrong because the coil surface temperature is controlled by the cooling medium (like chilled water), which is typically several degrees colder than the final air temperature to maintain the necessary temperature difference for heat transfer. Remember: for any condensation process, the surface temperature must be below the dew point temperature of the air. The temperature at which condensation first appears indicates the dew point, not the actual surface temperature.

Question 6

During winter operation, outdoor air at -5°C and 80% relative humidity is heated to 20°C before humidification. If no moisture is added during heating, what is the relative humidity of the air after heating?

  1. 13.7% (correct answer)
  2. 18.2%
  3. 25.6%
  4. 32.0%
  5. 45.3%
Explanation: When you encounter problems involving heating air without adding moisture, you're dealing with a constant absolute humidity process where only temperature changes. The key insight is that relative humidity depends on both the actual moisture content and the air's capacity to hold moisture at that temperature. To solve this, you need the saturation vapor pressures at both temperatures. At -5°C, the saturation vapor pressure is 0.4019 kPa, and at 20°C, it's 2.339 kPa. Since the initial air is at 80% relative humidity, the actual vapor pressure is 0.80×0.4019=0.32150.80 \times 0.4019 = 0.3215 kPa. During heating, this vapor pressure remains constant because no moisture is added or removed. However, the air's capacity to hold moisture increases dramatically with temperature. At 20°C, the relative humidity becomes 0.32152.339=0.137\frac{0.3215}{2.339} = 0.137 or 13.7%, confirming answer A. Answer B (18.2%) likely comes from using incorrect saturation pressure values or rounding errors in the calculation. Answer C (25.6%) might result from incorrectly assuming some relationship between the temperature ratio and humidity ratio. Answer D (32.0%) could stem from miscalculating the initial vapor pressure or confusing absolute and relative humidity concepts. Remember this pattern: when air is heated without humidification, relative humidity always decreases because warm air can hold much more moisture than cold air. The greater the temperature increase, the more dramatic the relative humidity drop—this is why winter heating systems require humidification to maintain comfort.

Question 7

A chemical dehumidification system uses silica gel to remove moisture from air at 30°C and 60% relative humidity. Compared to cooling dehumidification, this process typically results in:

  1. Lower final air temperature due to evaporative cooling effects
  2. Higher final air temperature due to heat of adsorption release (correct answer)
  3. Identical final air temperature with only humidity reduction
  4. Variable final temperature depending on ambient conditions only
  5. Lower final humidity ratio due to chemical decomposition
Explanation: When you encounter questions about chemical versus mechanical dehumidification processes, focus on the fundamental energy changes occurring during moisture removal. Chemical dehumidification involves adsorption, which is always an exothermic process. In chemical dehumidification using silica gel, water vapor from the humid air adsorbs onto the silica gel surface. This adsorption process releases the heat of adsorption - essentially the binding energy released when water molecules attach to the adsorbent material. This released heat warms the air stream, making option B correct. The final air temperature will be higher than the initial 30°C due to this energy release. Option A incorrectly suggests evaporative cooling, but no evaporation occurs in chemical dehumidification - instead, water vapor is being removed from the gas phase and bound to the solid adsorbent. Option C assumes temperature remains constant, ignoring the thermodynamic reality that adsorption releases energy that must go somewhere - and it goes into heating the air. Option D suggests temperature change depends only on ambient conditions, but this ignores the inherent exothermic nature of the adsorption process itself. This contrasts sharply with cooling dehumidification, where air temperature drops to condense moisture, then may be reheated. Chemical dehumidification simultaneously removes moisture and adds heat to the air stream. Remember: adsorption processes are exothermic by nature. When you see chemical dehumidification questions, expect temperature rise due to heat of adsorption release - this is a fundamental thermodynamic principle that applies regardless of the specific adsorbent material used.

Question 8

An air conditioning system first cools and dehumidifies air from 32°C and 70% RH to 12°C and 95% RH, then reheats it to 22°C. What is the primary purpose of the reheating step?

  1. To increase the final humidity ratio above the original value
  2. To reduce the final relative humidity to a comfortable level (correct answer)
  3. To eliminate all condensate formed during the cooling process
  4. To restore the original dry-bulb temperature for energy efficiency
  5. To prevent frost formation on the downstream ductwork
Explanation: When you encounter air conditioning psychrometrics problems, focus on understanding what happens at each process step and why engineers design systems this way. The cooling and dehumidification process takes air from comfortable conditions (32°C, 70% RH) down to 12°C at 95% RH. This high relative humidity occurs because cooling air to its dew point causes water vapor to condense out, leaving the remaining air nearly saturated. However, 95% RH feels uncomfortably humid to occupants, even at a cool temperature. The reheating step from 12°C to 22°C serves a crucial purpose: it reduces relative humidity without changing the moisture content. When you heat air while keeping its humidity ratio constant, the relative humidity drops significantly. This creates the ideal final condition - comfortable temperature with comfortable humidity levels. Looking at the wrong answers: (A) is incorrect because reheating doesn't add moisture; it only changes temperature while humidity ratio stays constant. (C) misses the point - condensate removal already happened during the cooling phase, not during reheating. (D) focuses on temperature restoration for efficiency, but the primary driver is humidity control, not energy recovery. This cooling-then-reheating approach might seem energy wasteful, but it's often the most practical way to achieve precise humidity control, especially in climates where outdoor air is very humid. Study tip: Remember that relative humidity depends on both moisture content and temperature. Heating air with constant moisture content always reduces RH - this principle appears frequently in HVAC system analysis problems.

Question 9

Air at 20°C and 40% relative humidity is passed over a wetted surface at 15°C in an evaporative cooling process. What determines whether net humidification or dehumidification occurs?

  1. The difference between air and surface temperatures exclusively
  2. The initial relative humidity of the incoming air stream
  3. The comparison between surface temperature and air dew point temperature (correct answer)
  4. The velocity of air flow over the wetted surface area
  5. The total surface area available for heat and mass transfer
Explanation: When you encounter evaporative cooling problems, the key is understanding that moisture transfer depends on the driving force between vapor pressures, not just temperatures or humidity levels alone. The correct answer is C because net humidification or dehumidification depends on comparing the surface temperature to the air's dew point temperature. If the wetted surface temperature (15°C) is above the air's dew point, water will evaporate from the surface into the air stream, causing humidification. If the surface temperature is below the dew point, water vapor will condense from the air onto the surface, causing dehumidification. The dew point represents the temperature at which the air becomes saturated - it's the critical threshold that determines the direction of moisture transfer. Option A is wrong because temperature difference alone doesn't determine moisture transfer direction. Even though the surface is cooler than the air, humidification can still occur if the surface temperature exceeds the dew point. Option B incorrectly suggests that initial relative humidity alone determines the process direction. While 40% RH tells you the air isn't saturated, it doesn't indicate whether the air will gain or lose moisture without knowing the dew point. Option D is incorrect because air velocity affects the rate of heat and mass transfer but not the fundamental direction - it influences how quickly the process occurs, not whether humidification or dehumidification happens. Remember: in any psychrometric process involving moisture transfer, always compare relevant temperatures to the dew point temperature. The dew point acts as the "switching point" that determines transfer direction.

Question 10

In a cooling and dehumidification process, air enters a coil at state 1 (28°C, 65% RH) and exits at state 2 (12°C, 90% RH). If condensate removal occurs between states 1 and 2, what can be concluded about the coil surface temperature distribution?

  1. The coil surface temperature is uniform at 12°C throughout
  2. The coil surface temperature varies linearly from 28°C to 12°C
  3. Some portions of the coil surface are below the entering air dew point (correct answer)
  4. The coil surface temperature equals the saturation temperature at exit conditions
  5. The coil surface temperature oscillates around the average of inlet and outlet temperatures
Explanation: When analyzing cooling and dehumidification processes, you need to understand the relationship between surface temperatures and moisture condensation. For water vapor to condense out of air, the air must come into contact with surfaces below its dew point temperature. The key insight here is that condensate removal indicates water vapor is condensing somewhere in the process. At state 1 (28°C, 65% RH), you can determine the dew point temperature - this is the temperature at which the entering air becomes saturated. Since condensation occurs between states 1 and 2, portions of the coil surface must be below this entering air dew point temperature. This confirms answer C is correct. Let's examine why the other options fail: Option A suggests uniform 12°C throughout, but this ignores that cooling coils typically have temperature gradients, with the coldest sections near the refrigerant inlet. Option B proposes a linear temperature variation matching the air temperature change, but coil surface temperatures depend on refrigerant conditions, not air temperatures. Option D claims the surface equals the saturation temperature at exit conditions, but this would only occur under very specific heat transfer conditions that aren't typical in real coils. The critical concept is that condensation requires surface temperatures below the dew point. In practical cooling coils, surface temperatures vary based on refrigerant flow and heat exchanger design, with the coldest sections enabling dehumidification while warmer sections primarily provide sensible cooling. Study tip: Always check the dew point of entering air conditions when condensate removal is mentioned - successful dehumidification requires coil surfaces below this temperature.

Question 11

A bypass humidification system splits an air stream: 70% goes through an adiabatic humidifier while 30% bypasses it. If the humidifier achieves 85% saturation efficiency, and the initial air is at 26°C dry-bulb and 18°C wet-bulb, what determines the final mixed air relative humidity?

  1. The initial dry-bulb temperature and bypass percentage only
  2. The saturation efficiency and wet-bulb temperature only
  3. The mass flow split ratio and humidifier exit conditions (correct answer)
  4. The initial relative humidity and total air flow rate
  5. The wet-bulb temperature and atmospheric pressure conditions
Explanation: When analyzing bypass humidification systems, you need to understand that the final mixed air properties result from combining two separate air streams with different moisture contents. This is fundamentally a mass balance problem involving psychrometric properties. The correct answer is C because the final mixed air relative humidity depends on exactly these two factors: how much air goes through each path (mass flow split ratio) and what conditions the humidified air reaches (humidifier exit conditions). The humidifier exit conditions are determined by the saturation efficiency and inlet conditions, while the bypass air remains unchanged. These two streams then mix according to their mass flow rates to produce the final mixture properties. Option A is incomplete because it ignores the humidifier's performance and the actual moisture addition that occurs. The bypass percentage alone doesn't tell you what happens to the air that does get humidified. Option B misses the critical mixing process - even if you know the humidified air conditions, you still need the flow split to determine the final mixture. Option D focuses on the wrong parameters entirely; total flow rate doesn't affect the intensive properties of the mixed air, and initial relative humidity is less directly relevant than the actual exit conditions from each path. Remember that bypass systems are essentially mixing problems. Always identify what happens to each stream separately, then apply mass balance principles to find the mixed result. The key is tracking both the moisture content changes and the flow distribution.

Question 12

During winter, outdoor air at -10°C and 70% relative humidity undergoes heating to 22°C, then steam humidification to 45% relative humidity. If the steam is injected at 100°C, what is the most significant energy requirement in this process?

  1. Latent heat required for steam generation from liquid water source
  2. Sensible heat required for initial air heating from -10°C to 22°C (correct answer)
  3. Sensible heat required for steam superheat above saturation temperature
  4. Latent heat released during steam condensation in the air stream
  5. Energy required for air circulation and distribution system operation
Explanation: When analyzing HVAC processes involving both sensible and latent heat transfers, you need to calculate the energy requirements for each step and compare their magnitudes. The air heating from -10°C to 22°C requires sensible heat: q=m˙air×cp×ΔT=m˙air×1.006×32=32.2m˙airq = \dot{m}_{air} \times c_p \times \Delta T = \dot{m}_{air} \times 1.006 \times 32 = 32.2\dot{m}_{air} kJ/kg. This is a substantial energy requirement because you're heating a large mass of air through a significant temperature difference. The humidification process adds water vapor, but the energy analysis shows this requires much less energy than the initial heating step. Even though steam generation involves latent heat, the mass of water added for humidification is typically only 1-3% of the air mass flow rate. Answer A incorrectly assumes steam generation dominates. While the latent heat of vaporization (≈2260 kJ/kg) is large per unit mass of water, the small mass of water needed makes this contribution relatively minor. Answer C misidentifies steam superheat as significant - the steam is injected at 100°C (saturation temperature), so minimal superheat energy is involved. Answer D describes latent heat release during condensation, but this actually reduces the net energy input rather than representing an energy requirement. The correct answer is B because heating the large mass flow of air through 32°C requires far more energy than the humidification step, despite the high specific energy of steam generation. Study tip: In HVAC energy calculations, always compare the mass flow rates involved - air heating typically dominates because air mass flow greatly exceeds the water mass flow for humidification.

Question 13

In a steam humidification process, dry saturated steam at 100°C is injected into an air stream at 25°C and 40% relative humidity. Compared to adiabatic humidification, this process will result in:

  1. Lower final humidity ratio due to steam condensation effects
  2. Higher final dry-bulb temperature due to sensible heat addition (correct answer)
  3. Identical psychrometric path following constant enthalpy lines
  4. Lower final relative humidity due to temperature increase
  5. Constant final dry-bulb temperature with only humidity changes
Explanation: When analyzing humidification processes, you need to distinguish between adiabatic and steam injection methods based on their energy effects. Adiabatic humidification (like evaporative cooling) adds moisture without external heat input, following constant enthalpy lines on the psychrometric chart. Steam injection, however, introduces both moisture AND sensible heat energy. In steam humidification, you're injecting saturated steam at 100°C into air at 25°C. This hot steam carries significant thermal energy beyond what's needed for phase change. The latent heat of vaporization was already supplied when the steam was generated, and now this high-temperature vapor transfers additional sensible heat to the air stream. This dual effect of moisture addition plus heat transfer increases both the humidity ratio and the dry-bulb temperature, making option B correct. Option A is wrong because steam injection actually increases the humidity ratio - no condensation occurs when hot steam mixes with cooler air. Option C incorrectly assumes the process follows constant enthalpy lines, which only applies to adiabatic humidification where no external heat is added. Option D contains faulty reasoning - while temperature does increase, the simultaneous addition of moisture typically results in higher relative humidity, not lower. The key insight is recognizing that steam injection processes involve two simultaneous effects: latent heat (moisture addition) and sensible heat (temperature rise). Always check whether a humidification process adds external energy - this determines whether you follow constant enthalpy lines or expect temperature changes.

Question 14

A desiccant dehumidification wheel operates with regeneration air at 120°C. If the process air enters at 25°C and 60% RH, what happens to the air temperature as it passes through the wheel?

  1. Temperature decreases due to evaporative cooling from moisture removal
  2. Temperature increases due to heat transfer from the regeneration process (correct answer)
  3. Temperature remains constant since only moisture transfer occurs
  4. Temperature fluctuates cyclically with wheel rotation speed
  5. Temperature decreases initially then increases to original value
Explanation: When you encounter desiccant dehumidification systems, focus on the heat transfer mechanisms at play. These systems use materials that absorb moisture, and this absorption process releases heat through adsorption. As process air passes through the desiccant wheel, two key thermal effects occur simultaneously. First, the desiccant material absorbs water vapor from the air through adsorption, which is an exothermic process that releases heat of adsorption directly into the airstream. Second, the wheel itself has been heated by the 120°C regeneration air on the opposite side, creating a temperature gradient that transfers sensible heat to the cooler process air. Both mechanisms work together to increase the air temperature, making B correct. Option A incorrectly assumes evaporative cooling occurs, but this is backwards - the desiccant is removing moisture from the air, not adding it. There's no evaporation happening in the process air stream. Option C misunderstands that desiccant wheels involve significant heat transfer, not just mass transfer. The adsorption process is inherently thermal, and the wheel acts as a heat exchanger. Option D suggests temperature fluctuations with rotation speed, but the continuous rotation creates steady-state conditions where air temperature rises consistently as it passes through. Remember this key principle: desiccant dehumidification always involves heating the process air due to heat of adsorption plus sensible heat transfer from the regeneration side. If you see a desiccant system question, expect the air to exit warmer and drier than it entered.

Question 15

In a two-stage dehumidification process, air is first cooled from 28°C to 8°C (removing condensate), then heated back to 20°C. If the initial humidity ratio is 0.015 kg/kg, what determines the final humidity ratio?

  1. The initial air temperature and relative humidity conditions
  2. The final reheating temperature and atmospheric pressure
  3. The lowest temperature reached during the cooling stage (correct answer)
  4. The total energy input during both cooling and heating stages
  5. The relative humidity maintained during the intermediate stage
Explanation: When you encounter dehumidification problems, focus on understanding that humidity ratio changes only when water is physically removed from or added to the air. This process follows the principles of psychrometrics, where cooling air below its dew point causes condensation. The final humidity ratio is determined by the lowest temperature reached during cooling (8°C) because this is where condensation occurs. When air is cooled to 8°C, its humidity ratio drops to the saturation value at that temperature - approximately 0.0055 kg/kg. During the subsequent reheating to 20°C, no moisture is added or removed, so the humidity ratio remains constant at 0.0055 kg/kg. Answer A is incorrect because while initial conditions determine the starting point, they don't control the final humidity ratio after condensation has occurred. Answer B is wrong because the reheating temperature only affects the final dry-bulb temperature and relative humidity, not the absolute moisture content. The atmospheric pressure has minimal effect on this process. Answer D is incorrect because energy input affects temperatures but doesn't determine moisture removal - only the cooling to the dew point accomplishes dehumidification. Remember this key principle: in cooling-based dehumidification, the lowest temperature reached is the controlling factor because it determines how much moisture condenses out. The humidity ratio after this point remains constant during any subsequent heating process, since heating alone cannot change the absolute moisture content of the air.

Question 16

A humidification system uses atomized water spray at 10°C injected into air at 30°C and 20% relative humidity. If the water droplets completely evaporate before reaching thermal equilibrium, what primarily determines the final air state?

  1. The initial temperature difference between water and air only
  2. The mass flow ratio of water to dry air in the process (correct answer)
  3. The droplet size distribution of the atomized water spray
  4. The residence time available for heat and mass transfer
  5. The initial relative humidity of the air stream exclusively
Explanation: When you encounter adiabatic saturation or humidification problems, you're dealing with a process where energy and mass conservation principles determine the final state. This particular scenario involves water evaporating into air, which is a classic psychrometric process. The key insight is that when water droplets completely evaporate, the process conserves both mass and enthalpy. The final equilibrium temperature and humidity depend on how much water is added relative to the amount of dry air present. This mass flow ratio determines how the total enthalpy of the system gets redistributed between sensible heat (temperature) and latent heat (moisture content). More water relative to air means more cooling and higher final humidity; less water means less cooling and lower final humidity. Option A is incorrect because while the initial temperature difference affects the driving force for heat transfer, it doesn't determine the final equilibrium state—that's governed by energy balance. Option C misses the mark because droplet size affects evaporation rate and heat transfer kinetics, but once complete evaporation occurs, the final state is independent of how we got there. Option D focuses on transfer rates rather than equilibrium—residence time affects whether the process reaches completion, but given that complete evaporation occurs, it doesn't determine the final state. For psychrometric problems, remember that the mass ratio of water to dry air is your key parameter. It appears in the energy balance equation and directly controls where your final state lands on the psychrometric chart.

Question 17

A humidification process adds water vapor to air at constant temperature. If the initial relative humidity is 30% and the final relative humidity is 70%, what happens to the humidity ratio during this process?

  1. The humidity ratio increases by a factor of 2.33 (correct answer)
  2. The humidity ratio decreases by a factor of 0.43
  3. The humidity ratio remains constant throughout the process
  4. The humidity ratio increases by a factor of 1.67
  5. The humidity ratio changes proportionally to temperature variations
Explanation: When you encounter humidification problems, focus on the relationship between relative humidity and humidity ratio at constant temperature. Relative humidity is the ratio of actual water vapor to the maximum possible water vapor at that temperature, while humidity ratio is the actual mass of water vapor per unit mass of dry air. During constant-temperature humidification, the saturation humidity ratio (maximum possible) stays the same because it depends only on temperature. Since relative humidity ϕ=WWs\phi = \frac{W}{W_s} where WW is the humidity ratio and WsW_s is the saturation humidity ratio, we can write: W2W1=ϕ2ϕ1=0.700.30=2.33\frac{W_2}{W_1} = \frac{\phi_2}{\phi_1} = \frac{0.70}{0.30} = 2.33 The humidity ratio increases by exactly the same factor as the relative humidity because the denominator (saturation humidity ratio) remains constant. Answer A correctly identifies this factor of 2.33. Answer B (0.43) incorrectly suggests the humidity ratio decreases, which contradicts adding water vapor to the air. Answer C claims the humidity ratio stays constant, which would mean no water was actually added despite the relative humidity increase. Answer D (1.67) appears to come from incorrectly calculating 70−3030+1=2.33−0.67=1.67\frac{70-30}{30} + 1 = 2.33 - 0.67 = 1.67, mixing percentage point differences with ratios. Remember: in constant-temperature humidification or dehumidification processes, the humidity ratio changes proportionally with relative humidity since saturation conditions don't change. Always set up the ratio ϕ2ϕ1=W2W1\frac{\phi_2}{\phi_1} = \frac{W_2}{W_1} for these problems.

Question 18

A laboratory requires air at 23°C and 45% relative humidity. The available outdoor air is at 23°C and 75% relative humidity. What combination of processes is needed?

  1. Cooling followed by heating to achieve the required conditions
  2. Dehumidification followed by temperature adjustment as needed (correct answer)
  3. Direct heating to reduce the relative humidity to target level
  4. Humidification followed by cooling to achieve final conditions
  5. Mixing with dryer air to achieve the intermediate humidity level
Explanation: When you encounter psychrometric problems involving air conditioning, focus on what's actually changing: the air needs to go from higher to lower relative humidity at the same temperature. Since both the outdoor air and required indoor air are at 23°C, but the relative humidity must decrease from 75% to 45%, you need to remove moisture from the air. The most straightforward approach is dehumidification followed by any necessary temperature adjustment. Even though the final temperature target is the same, the dehumidification process might slightly affect temperature, so minor heating or cooling could be needed afterward. Option A (cooling followed by heating) is incorrect because while cooling can remove moisture through condensation, it's an indirect and energy-intensive method when the target temperature equals the starting temperature. This approach would unnecessarily cool the air below 23°C just to condense moisture, then reheat it back to 23°C. Option C (direct heating) reflects a common misconception. Heating air decreases relative humidity by increasing the air's capacity to hold moisture, but it also raises the temperature above your 23°C target. You'd then need cooling to return to the correct temperature, making this inefficient. Option D (humidification followed by cooling) moves in the wrong direction entirely—you're adding moisture when you need to remove it. Remember this pattern: when relative humidity must decrease at constant temperature, think dehumidification first. When you see psychrometric problems, always identify what's changing (temperature, moisture content, or both) before selecting your process sequence.

Question 19

During a dehumidification process, humid air at 30°C and 70% relative humidity is cooled to its dew point temperature, then reheated to 25°C. What is the approximate relative humidity of the final state?

  1. 45%
  2. 55% (correct answer)
  3. 65%
  4. 75%
Explanation: First, find the initial humidity ratio from the 30°C, 70% RH state (ω ≈ 0.0186 kg/kg). The dew point corresponds to saturation at this humidity ratio (≈ 24.2°C). After reheating to 25°C with constant humidity ratio, the relative humidity = ω_actual/ω_sat(25°C) = 0.0186/0.0201 ≈ 55%. Choice A assumes excessive moisture removal. Choice C incorrectly applies the original relative humidity. Choice D neglects the cooling and condensation process.

Question 20

Two air streams are mixed adiabatically: Stream 1 has 1000 kg/h dry air at 15°C, 90% RH; Stream 2 has 2000 kg/h dry air at 35°C, 40% RH. If the mixed stream exits at 28°C, what is the approximate relative humidity of the mixed stream?

  1. 52%
  2. 58% (correct answer)
  3. 64%
  4. 70%
Explanation: Using mass-weighted averaging: ω₁ ≈ 0.0096 kg/kg, ω₂ ≈ 0.0140 kg/kg. Mixed humidity ratio = (1000 × 0.0096 + 2000 × 0.0140)/(1000 + 2000) = 0.0125 kg/kg. At 28°C, ωₛₐₜ ≈ 0.0242 kg/kg, so RH = 0.0125/0.0242 ≈ 58%. Choice A uses incorrect mixing ratios. Choice C assumes simple arithmetic average of inlet relative humidities. Choice D neglects the temperature effect on final saturation properties.