Thermal Comfort PDF

School of Architecture & the Built Environment **MECHANICAL SERVICES BE6812** **[Thermal Comfort]** **Objective**s : To provide students with an understanding of the various thermal conditions necessary for human comfort, effects of solar heat gain and the principles of psychrometry. **2.1 [Gene...

School of Architecture & the Built Environment **MECHANICAL SERVICES BE6812** **[Thermal Comfort]** **Objective**s : To provide students with an understanding of the various thermal conditions necessary for human comfort, effects of solar heat gain and the principles of psychrometry. **2.1 [General]** UN SDG Goal 3: Good Health and Well-being - Ensuring thermal comfort contributes to overall health and well-being by preventing heat-related illnesses and improving mental health and productivity. The human body constantly produces heat energy from the food energy it consumes. This heat needs to be dissipated at an appropriate rate to keep the body at constant temperature. The body normally loses heat by radiation, convection and evaporation. It can, within limits, effectively and instinctively adjust to environmental conditions that vary from the optimum. Essentially, the body functions to lose heat to the surrounding cooler air and cooler surfaces. If this cooling action is either too slight or excessive, then the human body is likely to experience thermal discomfort. The term \'comfort\' is not easily defined as it varies from person to person. It is related to how an individual maintains a thermal balance between himself and the environment and can be complicated by personal prejudices, age, clothing and the level of activity. In any particular thermal environment it is difficult to get more than 50% of the people affected to agree that the conditions are comfortable **2.2 [Factors Affecting Thermal Comfort]** The principle factors can be classified into two groups, i.e. personal and physical. **(a) Personal Factors** Clothing ----------------------- --------------- ------------------------------------------------------------------ Activity Heat Emission The greater the activity of the body the more heat it gives off. Sleeping 70 W Office work 140 W Factory assembly work 265 W Heavy lifting work 440 W ***Table 1*** **(b) Physical Factors** +-----------------------------------+-----------------------------------+ | Air Temperature ( °C ) | This is the average temperature | | | of the bulk air inside a room | +===================================+===================================+ | Relative Humidity ( % ) | Humidity is caused by moisture in | | | the air. | | | | | | High RH = feel oppressive. | | | | | | Low RH = can cause dryness of | | | skin and throats. | +-----------------------------------+-----------------------------------+ | Air Movement ( m/s ) | | +-----------------------------------+-----------------------------------+ | Surface Temperature ( °C ) | The heat that is radiated from a | | | person is affected by the radiant | | | properties (i.e. temperature) of | | | the surrounding surfaces. | +-----------------------------------+-----------------------------------+ ***Table 2*** **2.3 [Criteria for Ideal Thermal Comfort]** A minimum of 1.0 air change per hour should be maintained. The air temperature of the room should range from 24 to 26 °C with a relative humidity between 50% and 70%. The air movement should not exceed 0.2 m/s or below 0.05 m/s. Other subjective characteristics of thermal comfort are :- - Woman, older people and young children usually require higher air temperature; - People may not feel uncomfortable if only exposed to a short duration of extreme condition; - One may not feel uncomfortable under the direct hot sun on the beaches, but would feel uncomfortable under the same conditions on the streets. **2.4 [Effects of Solar Radiation]** The heat gained in a building by radiation from the sun depends upon the following factors :- - The geographical latitude of the site; - - The season of the year; - The local cloud conditions; - The angles between the rays of the sun and the building surfaces; - The type of the window glass; - The rate of heat from the sun which falls on a surface varies throughout the day and the year. Singapore, being very near to the equator, is exposed to a very high intensity of solar radiation throughout the year. This result in a high proportion of heat being transferred to the interior space of the building (through the enclosing roof, walls and glass windows) and can create uncomfortable thermal conditions. **2.5 [Principles of Psychrometry]** **(a) Basic Terms** \(i) Dry-bulb temperature (DBT) The temperature *( °C )* of air as registered by an ordinary thermometer. \(ii) Relative Humidity (RH) Quantity of water contained in air expressed *as a percentage* of the maximum which could be contained in air at that temperature. A RH of 100% represents \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. A RH of 0% represents perfectly dry air. \(iii) Wet-bulb temperature (WBT) The temperature *( °C )* of air registered by a thermometer with its bulb covered by a wetted wick. For air that is not saturated, WBT is lower than DBT. \(iv) Dew-point temperature Temperature *( °C )* at which a sample of air with given moisture content becomes saturated. *(Condensation occurs whenever warm moist air meets a surface which is at or below the dew-point temperature of that air. This can be either surface condensation or interstitial condensation. )* *(v) Moisture Content* *The actual weight of water in the air, expressed in* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. **(b) The Psychrometric Chart** The psychrometric chart is probably the best way of showing what happens to air and water as the properties are changed. The psychrometric chart is a graphic representation of the thermodynamics and thermal properties of moist air. The dry-bulb temperature, wet-bulb temperature, and the relative humidity are so related that if any two properties are known, all other properties may then be determined from the chart. On the chart (see Fig. 1), the vertical lines having numbers along the bottom are the **dry-bulb temperatures** and are the air temperatures measured with an ordinary thermometer. All points falling along a given vertical line will be at the same dry-bulb temperature. The **wet-bulb temperatures (measured with wet bulb thermometers)** are designated by the diagonal lines that slope from the upper left to the lower right of the psychrometric chart. At 100% RH, the dry-bulb and wet-bulb temperatures of air coincide. At any other condition less than saturation, the wet-bulb temperature is always less than the dry-bulb temperature. The curves that radiate from the lower left of the chart to the upper right are the **relative humidity (RH)** lines. The uppermost curved line is the 100% RH or saturation line. The RH of air can be obtained from the *intersection of the dry-bulb and wet-bulb temperature lines*. **Moisture Content** are the horizontal lines from the right to the left of the chart. ***Fig. 1*** **Finding Relative Humidity (RH) & Moisture Content (MC)** If the sample of air is at 31 ºC (dry bulb) and 22 ºC (wet bulb) From Fig 2 : the relative humidity will be \_\_\_\_\_\_ and moisture content is \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_. ***Fig. 2*** The **dew point** is obtained by projecting a horizontal line from the intersection of the wb and db line to the saturation line (100% RH). ***Fig. 3 - To locate the dew point of a sample of air.*** ***Fig. 4*** In Fig. 4 above (using the same air conditions as in Fig.2), the dewpoint temperature of the sample of air will be \_\_\_\_\_\_\_. *( Note : At dewpoint, the dry bulb temperature is equal to the wet bulb temperature. )* **(c) Other examples of using the Psychrometric Chart** i. **Cooling of the air (or removing heat)** Air at 30 ºC and 50% RH is cooled to 25 ºC. What will happen to the : (a) RH \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \(b) Moisture content - \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ ii. **Heating the air (or introducing heat)** Air at 30 ºC and 50% RH is heated to 35 ºC. What will happen to the : (a) RH \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \(b) Moisture content - \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ **(iii) De-humidifying the air (or removing moisture)** The moisture content of air at 30 ºC and 50% RH is reduced to 0.010 kg/kg (dry air). What will happen to the : (a) RH \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \(b) DBT - \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ \(c) WBT \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ **(iv) Condensation** Air at 30 ºC and 70% RH is cooled to 22 ºC saturated. What is the amount of moisture removed? *Thermal Comfort/YHK*