Unit 4: Science and Materials in Construction and the Built Environment. Room Temperatures

Transcription

1 3.1 Room Temperatures Room Temperatures The temperature of the surrounding surfaces can affect the thermal comfort of people as much as the temperature of the surrounding air. This is because the rate at which heat is radiated from a person is affected by the radiant properties of the surroundings. For example, when sitting near the cold surface of a window the heat radiated from the body increases and can cause discomfort. A satisfactory design temperature for achieving thermal comfort needs to take account of both air temperatures and radiant effects. Different types of temperatures are described below Inside air temperature t ai The inside air temperature is the average temperature of the bulk air inside a room. It is usually measured by an ordinary dry-bulb thermometer which is suspended in the centre of the space and shielded from radiation. One of the most precise types of wet-dry bulb psychrometer was invented in the late 19th century by Adolph Richard Aßmann ( ); in English-language references the device is usually spelled "Assmann psychrometer." In this device, each thermometer is suspended within a vertical tube of polished metal, and that tube is in turn suspended within a second metal tube of slightly larger diameter; these double tubes serve to isolate the thermometers from radiant heating. Air is drawn through the tubes with a fan that is driven by a clockwork mechanism to ensure a consistent speed (some modern versions use an electric fan with electronic speed control). According to Middleton, 1966, "an essential point is that air is drawn between the concentric tubes, as well as through the inner one." Chapter 3 Page 1

2 3.1.2 Mean Radiant temperature t r The mean radiant temperature is the average effect of radiation from surrounding surfaces. At the centre of the room this temperature can be taken as being equal to the mean surface temperature as calculate by Where t 1, t 2, are the surface temperatures of the areas A 1, A 2,, etc. The mean radiant temperature should be kept near the air temperature but not more than about 3 o C below it, otherwise conditions are sensed as stuffy Inside environment temperature t ei The environmental temperature is a combination of air temperature and radiant temperature. The exact value depends upon convection and radiation effects. For average conditions t ei can be derived from the following formula. Environmental temperature is recommended for the calculation of heat losses and energy requirements Dry resultant temperature t res Dry resultant temperature is a combination of air temperature, radiant temperature and air movement. When the air movement is low (0.1 m/s), t res can be derived from the following formula Room-centre comfort temperature t c A comfort temperature is a measure of temperature which gives an acceptable agreement with thermal comfort. When air movement is low, the dry resultant temperature at the centre of a room is a commonly-used comfort temperature. The globe thermometer is a regular thermometer fixed inside a blackened globe of specified diameter (150 mm is one standard). This globe Chapter 3 Page 2

3 temperature can be used to calculate other temperatures and when air movement is small it approximates to the comfort temperature. The volume of fresh air (make up air) required for proper ventilation is determined of the size and use of the space. The table below indicates recommended air change rates (air changes per hour) in some common types of rooms and buildings: Table 3.1 Air ventilation rates Building / Room All spaces in general Air Change Rates - n - (1/hr) min 4 Building / Room Attic spaces for cooling Air Change Rates - n - (1/hr) Auditoriums 8-15 Banks 4-10 Barber Shops 6-10 Bars Beauty Shops 6-10 Boiler rooms Bowling Alleys Cafeterias Churches 8-15 Clubhouses Cocktail Lounges Computer Rooms Court Houses 4-10 Dental Centers 8-12 Department Stores 6-10 Dining Halls Dining rooms hotels 5 Dress Shops 6-10 Drug Shops 6-10 Engine rooms 4-6 Chapter 3 Page 3

4 Factory buildings, ordinary 2-4 Factory buildings, fumes and moisture Fire Stations 4-10 Foundries Galvanizing plants Garages repair Garages storage 4-6 Homes, night cooling Jewelry shops 6-10 Kitchens Laundries Libraries, public 4 Lunch Rooms Luncheonettes Nightclubs Malls 6-10 Medical Centers 8-12 Medical Clinics 8-12 Medical Offices 8-12 Mills, paper Mills, textile general buildings 4 Mills, textile dye houses Municipal Buildings 4-10 Museums Offices, public 3 Offices, private 4 Police Stations 4-10 Post Offices 4-10 Precision Manufacturing Pump rooms 5 Restaurants 8-12 Retail 6-10 School Classrooms 4-12 Shoe Shops 6-10 Shopping Centers 6-10 Shops, machine 5 Shops, paint Shops, woodworking 5 Substation, electric 5-10 Supermarkets 4-10 Town Halls 4-10 Taverns Theaters 8-15 Warehouses 2 Turbine rooms, electric Waiting rooms, public Chapter 3 Page 4

5 Note that in many cases local regulations and codes will govern the ventilation requirements. The fresh air supply to a room can be calculated as where q = n V (1) q = fresh air supply (ft 3 /h, m 3 /h) n = air change rate (1/h or h -1 ) V = volume of room (ft 3, m 3 ) Example - Fresh Air Supply to a Public Library The fresh air supply to a public library with volume 1000 m 3 can be calculated as Q = (4 1/h) (1000 m 3 ) = 4000 m 3 /h For more information on room temperatures click this link. Chapter 3 Page 5

6 17.9 o C 17.9 o C Unit 4: Science and Materials in Construction and the Built Environment Example 1 This diagram shows the cross-section area of a private office. Each section is divided into known shapes and the radiant temperature is listed in each section. (See diagrams) The dimensions of the room and the other objects are found in a table after the drawings. If the inside air temperature is calculated as 25.5 o C, calculate; a) The mean radiant temperature b) The inside environmental temperature and c) The dry resultant temperature Floor view from ceiling x x x x 35 o C 2.2 m 31.2 o C 3 m 26 o C 24.1 o C 21.8 o C 28.4 o C 27.8 o C 28 o C 22.2 o C Wall 1 view from opposite wall 34.8 o C 27.6 o C 25.8 o C 23.6 o C 21 o C 18 o C Chapter 3 Page 6

7 17.8 o C 18.8 o C 17.9 o C Unit 4: Science and Materials in Construction and the Built Environment Wall 2 view from opposite wall 25.8 o C 34.8 o C 27.6 o C 23.6 o C 21 o C 27.2 o C 21 o C Window wall view from opposite wall Wall unit view from opposite wall 18 o C 27 o C 36 o C 32.5 o C 19 o C 30 o C 33 o C Ceiling view from wall 34 o C 27.8 o C 26 o C 24.1 o C 21.8 o C 28 o C Chapter 3 Page 7

8 Dimensions Object Length Width Height Room 8 m 3.75 m 3.5 m Desk 1.5 m 0.5 m 1.25 m Wall Unit 3 m 0.75 m 2.75 m Picture 1.5 m 1.5 m 0.02 m Window 1.8 m 1.5 m 0.3 m Question 1 Which temperature calculation would you consider if the air movement is low? Question 2 Are these temperature results acceptable for human comfort temperature? Give reasons for your answer. Question 3 Calculate the fresh air supply of this private office. Chapter 3 Page 8