charminglai的部落格

Calculating sensible and latent heat from persons, lights, electric equipment, machines, evaporation from water surfaces, polluting fluids and miscellaneous loads.
The indoor climate is influenced by

• sensible and latent heat from persons, lights, machines and electrical equipment and industrial processes
• pollution and gases from persons, building materials, inventory and industrial processes

The most important sources may be summarized as

1. Sensible and latent heat from persons
2. Sensible heat from lights
3. Sensible heat from electric equipment
4. Sensible heat from machines
5. Latent heat from evaporation from water surfaces
6. Evaporation from polluting fluids
7. Miscellaneous loads

1. Sensible and latent heat from persons

Sensible heat from persons are transferred through conduction, convection and radiation. Latent heat from persons are transferred through water vapor.

The sensible heat influence on the air temperature and latent heat influence the moisture content of air.

The heat transferred from persons depends on their activity, clothing, air temperature and the number of persons in the building.

2. Sensible heat from lights

Heat transferred to the room from the lights can be calculated as

H_l = P_inst K₁ K₂ (1)

where

H_l = heat transferred from the lights (W)

P_inst = installed effect (W)

K₁ = simultaneous coefficient

K₂ = correction coefficient if lights are ventilated. (= 1 for no ventilation, = 0.3-0.6 if ventilated)

The table below can be used to estimate heat load from lights. (The manufacturers datasheets should be checked for details)

Normal illumination of rooms:

3. Sensible heat from electric equipment

Heat transferred from electrical equipment can be calculated as

H_eq = P_eq K₁ K₂ (2)

where

H_eq = heat transferred from electrical equipment (W)

P_eq = electrical power consumption (W)

K₁ = load coefficient

K₂ = running time coefficient

4. Sensible heat from machines

When machines runs heat can be transferred to the room from the motor and/or the machine.

If the motor is in the room and the machine is outside the heat transferred can be calculated as

H_m = P_m / h_m - P_m (3)

where

H_m = heat transferred from the machine to the room (W)

P_m = electrical motor power consumption (W)

h_m = motor efficiency

If the motor is belt driven and the motor and belt is in the room and the machine is outside the heat transferred can be calculated as

H_m = P_m / h_m - P_m h_b (3b)

where

h_b = belt efficiency

If the motor and the machine is in the room the heat transferred can be calculated as

H_m = P_m / h_m (3c)

In this situation the total power is transferred as heat to the room.

Note! If the machine is a pump or a fan, most of the power is transferred as energy to the medium and may be transported out of the room.

If the motor is outside and the machine is in the room the heat transferred can be calculated as

H_m = P_m (3d)

If the motor is belt driven and the motor and belt is outside and the machine is in the room the heat transferred can be calculated as

H_m = P_m h_b (3e)

5. Latent heat from evaporation from water surfaces

Evaporation from open vessels or similar can be calculated as

q_m = A (x₁ - x₂ ) a_e (4)

where

q_m = evaporated water (kg/s)

A = surface area (m²)

x₁ = water content in saturated air at water surface temperature (kg/kg)

x₂ = water content in the air (kg/kg)

a_e = evaporation constant (kg/m²s)

The evaporation constant can be estimated

a_e = (25 + 19v)/3600 (5)

where

v = air speed close to the water surface (m/s)

The temperature in the water surface will be lower than the temperature below the surface.

It can be calculated as

t₁ = t₂ - (t₂ - t₃) / 8 (6)

where

t₁ = temperature in water surface (^oC)

t₂ = temperature below the surface (^oC)

t₃ = wet bulb temperature in the air (^oC)

The heat for evaporation can be calculated as

H_e = q_m / (x₁ - x₂) (h₁ - h₂) (7)

where

h₁ = enthalpy in saturated air (J/kg)

h₂ = enthalpy in air (J/kg)

6. Evaporation from polluting fluids

The flow of a polluting fluid can be calculated as

q_f = 22.4 q_e / M T / 273 (8)

where

q_f = flow of the fluid (m³/s)

q_e = evaporated fluid

M = molecule mass of the fluid at 0 ^oC and 101.3 Pa (kg/mole)

T = temperature (K)

7. Miscellaneous loads

Carbon dioxide - CO₂

Carbon dioxide (CO₂) concentration in "clean" air is 575 mg/m³.

Huge concentrations can cause headaches and the concentration should be below 9000 mg/m³.

Carbon dioxide are produced by persons during the combustion. The concentration of carbon dioxide in the air can be measured and used as an indicator of air quality.

Smell