{"id":29,"date":"2022-04-27T12:12:16","date_gmt":"2022-04-27T16:12:16","guid":{"rendered":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/the-concept-of-heat-and-heat-transfer\/"},"modified":"2022-08-10T16:31:20","modified_gmt":"2022-08-10T20:31:20","slug":"the-concept-of-heat-and-heat-transfer","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/plumbing3f\/chapter\/the-concept-of-heat-and-heat-transfer\/","title":{"raw":"Learning Task 2","rendered":"Learning Task 2"},"content":{"raw":"

Heat Loss (\u201cLoad\u201d) Calculations<\/h1>\nA btu (British Thermal Unit) is the quantity<\/em> of heat required to raise the temperature of one pound of water by one degree Fahrenheit. Therefore, it would require adding 2 btus to 2 pounds of water in order to raise its temperature by 1\u00b0F, 3 btus for 3 pounds of water through 1\u00b0F and so on. The quantity of heat in btus needed for any process that involves the sensible heat that changes the temperature of an amount of water can be calculated by using this formula:\n

BTUs = mass \u00d7 <\/strong>\u0394<\/strong>T \u00d7 S.H.<\/strong><\/p>\nMass will be the weight of water in pounds, \u0394T will be the desired change in temperature in \u00b0F, and S.H. (specific heat) of water is 1 (see the previous paragraph).\n

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Example 1<\/p>\n\n<\/header>\n

\n\nCalculate the required btus to take 40 Imp. gallons of water from 50\u00b0F to 140\u00b0F, such as would be occurring when initially filling and heating the water in a standard electric water heater.\n\nMass will be 40 Imperial gallons \u00d7 10 pounds\/Imperial gallon = 400 pounds of water\n\n\u0394T will be (140\u00b0F \u2212 50\u00b0F) = 90\u00b0F\n\nS.H. (specific heat) of water = 1\n\nTherefore, the heat quantity required for that job would be:\n\n400 \u00d7 90 \u00d7 1 = 36,000 btus.\n\nIf the expectation is that this job needed to be done in an hour, then we would have to add 36,000 btuh to the water. If the job were to be compressed to within \u00bd hour, then we would have to double the heat quantity to meet that time frame, so we would have to apply 36,000 \u00d7 2 = 72,000 btuh.\n\n<\/div>\n<\/div>\n
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Example 2<\/p>\n\n<\/header>\n

\n\nCalculate the heat load on a boiler that contains 20 Imperial gallons of water, if the water temperature has to be increased from 170\u00b0F to 190\u00b0F.\n\nMass = 20 Imp. Gallons \u00d7 10 lbs\/Imp gal = 200 lbs\n\n\u0394T = (190 \u2212 170) = 20\u00b0F\n\nS.H. of water = 1\n\n200 \u00d7 20 \u00d7 1 = 4,000 BTUs\n\n<\/div>\n<\/div>\nBoilers used in residential hydronic (hot water) heating systems typically operate on a temperature differential (\u0394T) of 20\u00b0F, unlike a storage-type water heater that normally operates on a \u0394T of approximately 90 to 100\u00b0F. Traditional non-condensing boilers must keep the temperature of the water returning back into the boiler at a minimum of 140\u00b0F so as to keep condensation from forming on the external surfaces of the heat exchanger. Condensation is caused when the large amounts of water vapour produced by the burning of hydrocarbon gases, such as natural gas and propane, come in contact with a heat exchanger that, although considered hot, has a temperature below atmospheric dewpoint (approximately 129\u00b0F). Condensation would create many problems including corrosion, sooting, cooling of the flame, plugged heat exchangers and premature failure.\n\nIn the heating industry, the heat quantities required are predominantly expressed within a 1-hour time frame. Thus, our heat loads are always referenced as \u201cBTUH\u201d. We will look at the losses from a building and the heat we intend to put back into it in order to offset those losses, and we\u2019ll specify those requirements in terms of BTUH.\n

Water vs Air as a Heating Medium<\/h1>\nIt has long been known that the use of water for heating buildings has many advantages over the use of air for the same purposes. This is covered within the Level 2 learning guide and should be revisited to get the most knowledge from this learning package. Aside from other factors discussed, the single most-utilized advantage is water\u2019s ability to hold more heat than can air. One US gallon of water (the industry standard ) would contain approximately 167 btus if it were to be heated from 170\u00b0F to 190\u00b0F. (8.33 pounds \u00d7 20\u00b0F\u0394T \u00d7 1). By comparison, the volume of air required to hold the same amount of heat would be staggering. The specific heat of one cubic foot of air is 0.018 btu. In other words, it would take 0.018 btus to raise the temperature of one cubic foot of air by one degree Fahrenheit. If Q represents the amount of air in ft\u00b3 needed to hold 167 btus, and we were trying to increase temperature by 20\u00b0F, then we would apply the formula:\n

Q \u00d7 0.018 \u00d7 20 = 167<\/p>\nWorking the formula through, we would calculate Q to be approximately 464 ft\u00b3.\n\nWhat this means is that, in order to deliver a 20\u00b0F increase in temperature to a hot water heating system, we would only have to send out 1 US gallon of water into it, whereas we would have to deliver 464 ft\u00b3 of air to a forced air heating system in order to get the same result. A single \u00be\u2033 diameter pipe could easily and noiselessly achieve this. A warm air duct, having a great internal area and contributing far more noise to the building (along with other associated down-sides) could also deliver this same amount of heat. Ducts, however, are much harder to conceal within a structure without compromising the structure\u2019s integrity. Also, \u201czoning\u201d, which is the strategy of applying heat to only certain areas of a building as needed, is much easier to accomplish with water rather than with air. With zoning, the correct amount of heat is delivered to each area based upon that area\u2019s identified heat losses. Although ducted air systems can be \u201czoned\u201d, it is far more involved and expensive than when using water. Again, it has long been known that hydronic systems are preferred over air systems for heating purposes for just those reasons. The one downside to a hydronic system when compared to an air system was always the relative complexity of controls and heat transfer units needed when trying to provide cooling to a building using water. The same ductwork that heats a house in the winter can cool the house in the summer simply by the use of an outdoor condensing unit coupled to a cooling coil mounted within the supply air duct (plenum) connected to a forced warm air furnace.\n

Heat Transfer<\/h1>\nAs mentioned earlier, the human body transfers heat in 5 ways, which are radiation, convection, perspiration, conduction and respiration. Seeing as a building doesn\u2019t have lungs or sweat glands, a building\u2019s heat is lost via radiation, conduction and convection. Ever since man discovered fire, it has been known that some materials allow heat to move through them better than others. These materials are known as thermal conductors or simply conductors. Conductors such as glass, metal and concrete have their molecules bound together fairly tightly and although the robust qualities of both metal and concrete make them well suited as construction materials, those same qualities are undesirable when trying to maintain a comfortable indoor environment. Other materials, such as fiberglass and polystyrene (\u201cstyrofoam\u201d), don\u2019t have such tight molecular bonds, and as such won\u2019t conduct heat through them as readily. These materials are known as insulators, and are used in cavities between structural members in buildings to reduce the movement of heat. Fiberglass batts, polystyrene board and, more recently, sprayed-in foam are the most common materials used to insulate buildings.\n\n[caption id=\"attachment_28\" align=\"aligncenter\" width=\"400\"]\"\" Figure 1 Fiberglass batt insulation[\/caption]\n\nAir is also considered an insulator, as long as it can\u2019t move freely in or out of a confined space, such as between two sealed panes of glass. For many decades now, window manufacturers have utilized the practise of creating a \u201cdead-air space\u201d between sealed glass panes in order to increase the insulative value of windows. Reflective coatings and the use of argon and similar inert gases increase the insulation values even further.\n\n[caption id=\"attachment_28\" align=\"aligncenter\" width=\"400\"]\"Low-E Figure 2 Modern double-pane sealed \u201clow-E\u201d window[\/caption]\n

DTD (Design Temperature Difference or Differential)<\/h1>\nThe \u201cR\u201d, \u201cK\u201d and \u201cU\u201d values for materials are expressions of thermal resistance or conductivity for materials or assemblies when there is a 1\u00b0F differential, or \u201c\u0394T\u201d, between the two sides or faces of that material or assembly. As mentioned earlier, if the \u0394T is doubled, then the rate of heat transfer will also double. If the \u0394T is halved, then so will be the rate of transfer and so on. The \u0394T that is used in heat loss calculations for buildings will the temperature difference between what is desired indoors, known as \u201cIndoor Design Temperature\u201d or \u201cIDT\u201d, and the outdoor design temperature or \u201cODT\u201d.\n\nPart 9 of the BC Building Code (\u201cBCBC\u201d) identifies the indoor design temperature to be used in three areas of buildings.\n\nSpecifically, 9.33.3.1 (1) contains:\n