Assumed Water Use Per Person for Psychrometric Calculations

Estimate latent moisture load and humidity impact based on occupancy activity levels.

What is Assumed Water Use Per Person for Psychrometric Calculations?

In the field of HVAC (Heating, Ventilation, and Air Conditioning) engineering, assumed water use per person for psychrometric calculations refers to the physiological process where human beings release moisture into the surrounding air through respiration and perspiration. This is not about drinking water, but rather the internal “water use” or moisture output that contributes to the latent cooling load of a building.

Engineers must accurately account for this factor to size dehumidifiers, air conditioners, and ventilation systems. Failing to include the assumed water use per person for psychrometric calculations leads to high indoor humidity, occupant discomfort, and potential mold growth. This tool is used by mechanical engineers, architects, and facility managers to ensure healthy indoor air quality (IAQ).

Common misconceptions include assuming that moisture output is constant. In reality, it varies significantly based on metabolic rate, ambient temperature, and clothing insulation.

Formula and Mathematical Explanation

The calculation for total moisture addition involves multiplying the per-person rate by the number of occupants and the duration of stay. Furthermore, the latent heat impact is derived using the latent heat of vaporization for water.

Step 1: Calculate Hourly Moisture Rate
M_hourly = N × R
Where N is the number of people and R is the moisture rate per person (kg/h).

Step 2: Calculate Total Latent Heat Gain (Watts)
Q_L = M_hourly × h_fg / 3.6
Where h_fg is approximately 2450 kJ/kg (latent heat of vaporization of water at room temperature).

Variable	Meaning	Unit	Typical Range
N	Occupant Count	People	1 – 10,000+
R	Moisture Rate	kg/h/person	0.035 – 0.400
QL	Latent Heat Gain	kW or BTU/h	Depends on N
hfg	Latent Heat of Vaporization	kJ/kg	~2450 @ 20°C

Practical Examples

Example 1: Standard Office Environment

A typical office has 50 employees performing seated desk work. The assumed water use per person for psychrometric calculations for seated activity is roughly 0.055 kg/h.

• Inputs: 50 people, 0.055 kg/h moisture rate.
• Calculation: 50 × 0.055 = 2.75 kg of water added to the air every hour.
• Interpretation: This adds approximately 1.87 kW of latent heat load that the HVAC system must remove to maintain 50% relative humidity.

Example 2: Commercial Gym

A gym floor has 20 people engaged in heavy exercise. The moisture rate increases to 0.320 kg/h per person.

• Inputs: 20 people, 0.320 kg/h moisture rate.
• Calculation: 20 × 0.320 = 6.4 kg/h.
• Interpretation: Despite fewer people than the office, the moisture load is more than double, requiring specialized high-capacity dehumidification.

How to Use This Assumed Water Use Per Person for Psychrometric Calculations Tool

1. Enter Occupants: Input the maximum expected occupancy for the space.
2. Select Activity: Choose the metabolic rate that best describes the room’s function (e.g., “Office Work” for commercial spaces, “Heavy Work” for industrial or athletic sites).
3. Set Duration: Define the time period (in hours) you are analyzing (e.g., an 8-hour shift).
4. Analyze Results: View the total kilograms of water and the required latent heat removal (kW).
5. Review Chart: The dynamic SVG chart illustrates the cumulative moisture build-up over the specified timeframe.

Key Factors Affecting Results

• Metabolic Rate (MET): The single most influential factor; vigorous activity can increase moisture output by 10x compared to resting.
• Ambient Temperature: Higher dry-bulb temperatures increase perspiration rates as the body works harder to cool down.
• Relative Humidity: If ambient humidity is already high, the body’s ability to evaporate sweat decreases, though moisture is still released.
• Clothing Insulation: Heavy clothing traps moisture but eventually releases it into the air as vapor.
• Air Velocity: Faster air movement over the skin increases the rate of evaporation and moisture contribution to the psychrometric balance.
• Diet and Health: Individual hydration levels and health status slightly alter the assumed water use per person for psychrometric calculations.

Frequently Asked Questions (FAQ)

Why is latent heat gain different from sensible heat gain?

Sensible heat changes the temperature, while latent heat involves the phase change of water (liquid to vapor), which affects humidity without necessarily changing the dry-bulb temperature.

How does humidity affect air conditioner sizing?

Air conditioners must remove both heat and moisture. If the moisture load is high, you need a unit with a lower sensible heat ratio (SHR).

Is the assumed water use per person for psychrometric calculations different for children?

Yes, children generally have lower metabolic rates and smaller surface areas, leading to lower absolute moisture generation per individual.

Can I use these values for residential design?

Yes, though residential occupancy is usually much lower than commercial, making the moisture load from cooking and bathing more significant than the occupants themselves.

What happens if I ignore these calculations?

Indoor air will become “clammy,” surfaces may experience condensation, and CO2 levels often correlate with moisture build-up if ventilation is poor.

What is the standard ASHRAE value for office moisture?

ASHRAE typically assumes a latent heat gain of about 55-60 Watts per person for moderately active office work.

Does altitude affect these psychrometric values?

Yes, air density and the partial pressure of water vapor change with altitude, though the per-person moisture output remains largely biologically driven.

How do I convert kg to lbs for these results?

Multiply the kilogram result by 2.20462 to get the moisture load in pounds.

Related Tools and Internal Resources

• HVAC Load Calculator – Complete sensible and latent load analysis.
• Latent Heat Analysis – Deep dive into phase change energy requirements.
• Dew Point Guide – Understand when moisture will condense on surfaces.
• Metabolic Rate Data – Detailed tables for various human activities.
• Indoor Air Quality Standards – Guidelines for humidity and CO2.
• Building Moisture Control – Prevention strategies for mold and dampness.