2022年powerengineeringlecturesbg3hotwaterheatingsystems動力工程英文教材教程教案講義

1、Power Engineering Lectures - BG3 Hot Water Heating Systems 動力工程英文教材教程教案講義 Hot Water Heating Systems HVAC 6018 1 Learning Outcome When you complete this module you will be able to: Describe the various designs of hot water heating systems. Learning Objectives Here is what you will be able to do when

2、you complete each objective: 1. Sketch and describe the standard piping and circulations layouts of hot water heating systems. 2. Compare the advantages and disadvantages of hot water and steam heating systems. 3. Describe radiant panel and snow melting hot water systems. INTRODUCTION A hot water he

3、ating system also known as a hydronic heating system is similar in many respects to a steam heating system. It uses a boiler to transfer heat freed by the combustion of a fuel in a furnace to the water. In the space that is heated, the same heat transfer devices such as radiators, convectors, and un

4、it heaters are used and these heating units are connected by a piping system to the boiler. The hot water heating system differs, however, from a steam heating system in that the temperature of the water in the boiler is not raised to its boiling point and, thus no steam is produced. Also the entire

5、 system - boiler, piping and heating units - is completely filled with water, except for some air space in the expansion tank. The water is heated in the boiler and is then circulated through the supply piping to the heating units where it gives up part of its heat. The cooler water is returned to t

6、he boiler through the return piping and is heated again. GRAVITY CIRCULATION SYSTEMS In early hot water heating systems, many of which are still operating satisfactorily, the circulation of the water was produced by gravity acting on the difference in density of the hot water in the supply main and

7、the cooler water in the return main; in other words, by natural convection. Although the gravity hot water system has the advantages of simple operation and low maintenance, its use has sharply declined because of the following disadvantages: 1. Large pipe sizes are necessary in order to keep flow r

8、esistance to a minimum since flow depends only upon gravity. 2. Circulation is difficult to maintain to There is a slow response to changes in heat radiators on the same level as the boiler. 3. demand. 4. The maximum water temperature is limited to approximately 77 C 170 F. HVAC 6018 2 FORCED CIRCUL

9、ATION SYSTEMS The disadvantages listed for the gravity circulation system can be overcome by using a closed expansion tank and a pump to force the water through the system. With forced circulation, smaller pipes may be used and positive circulation can be established to all heating units. In additio

10、n, the response to changes in heating load is more rapid than with gravity systems. The use of a pump, however, means noisier operation, increased power costs, and increased maintenance. The closed expansion tank, besides allowing for expansion and contraction of the water, also provides a cushion o

11、f air above the water level in the tank and allows the system to operate at pressures well above that of the atmosphere. As a result, the boiling point of the water can be raised considerably so that the system can operate at increased water temperatures without producing steam. For domestic applica

12、tions, the temperature of the water in the system may be as high as 102 C 215 F while for commercial and industrial applications, water temperatures may even exceed 150 C 302 F. Forced circulation hot water systems are cl assified according to the methods of piping. Systems may be considered to be o

13、f two general types, upfeed and downfeed. The supply main of the upfeed system is installed at a low level and feeds the various risers upward. The risers, in turn, feed the radiators and convectors on the floors above. The return main is installed at the level of the supply main and receives the wa

14、ter returning from the heating units through the return risers. The downfeed system has its supply main installed overhead in the mechanical room or pipe space and supplies the risers which feed the convectors on the various floors - downward. The return risers usually feed downward to the return ma

15、in installed at a low level which then returns the water to the boilers. These systems may be further classified into the following types: 1. Loop system 2. One-pipe system 3. Two-pipe direct return system 4. Two-pipe reverse return system All other kinds of systems are nearly always variations HVAC

16、 6018 3 BG_3_0_1.jpg P and/or combinations of the above types. Loop System In the loop system, the heating units usually of the baseboard or finned tube convector type are connected in series so the water circulates from the hot water supply main directly through all the convectors to the return mai

17、n. This is the cheapest piping system possible. Its application is restricted to fairly low heating capacities since the water passes through the entire loop getting progressively cooler, and the heat emission from each consecutive section becoming less. Where more capacity is needed, two or more lo

18、ops are supplied by a trunk main, as is shown in Fig. 1. This diagram also shows the circulating pump usually called the circulator installed in the return main. HVAC 6018 4 Figure 1 Loop System Courtesy of Dunham-Bush The disadvantage of a loop system is that individual convectors cannot be shut of

19、f. However, when dampers are fitted on the convector casings, room temperatures can be sufficiently regulated. BG_3_0_2.jpg G One-Pipe System In the one-pipe system, also called a “ single main system,” a pump is also used to circulate the water through the boiler, through a single main, and back to

20、 the boiler again. The water supply to each convector is tapped off the main and the return from each convector is connected back to the main again downstream from the supply connection. Special diverter fittings are used to ensure adequate flow through each convector. A basic diagram of this system

21、 is shown in Fig. 2. HVAC 6018 5 Figure 2 One-Pipe System Courtesy of Dunham-Bush This diagram also shows how the closed expansion tank is connected to the system by means of an air separator in the supply main near the boiler outlet. This separator is a fitting which removes any air entrained in th

22、e water leaving the boiler and releases it into the expansion tank. The one-pipe system is quite simple in design. Zone temperature control can be achieved by installing thermostatically controlled valves on individual convectors. However, it has the disadvantage that the water entering each followi

23、ng convector is somewhat cooler than the previous one. To compensate for this temperature drop, it is sometimes necessary to increase the size of the convectors toward the end of the system. BG3_fig2.gif Two-Pipe Direct Return System This system, illustrated in Fig. 3, uses two mains: a supply main

24、and a return main. Each convector is directly connected to these mains, and the temperature of the water entering each convector is the same for them all. It is referred to as a direct return system because the return from each convector flows by the shortest and most direct route back to the boiler

25、. HVAC 6018 6 Figure 3 Direct Return System Courtesy of Dunham-Bush As in the one-pipe system, a pump is used to circulate the water through the boiler and around the system, and the expansion tank is connected to the air separator installed in the supply main near the boiler. The direct return syst

26、em is difficult to balance because of the different circuit lengths to each convector. For example, the water circuit in Fig. 3 from the boiler through number 1 convector and back to the boiler again is much shorter than the circuits to the other convectors. Similarly number 2 convector circuit is s

27、horter than numbers 3, 4, and 5. It is therefore necessary to size the system piping carefully in order to obtain equal flow through each convector. Because of the difficulty of balancing and the need of balancing valves and special piping connections, the direct return system is seldom used unless

28、other factors such as cost of installation play a more important role. BG3_fig3.gif BG_3_0_3.jpg G Two-Pipe Reverse Return System The reverse return system shown in Fig. 4 uses both a supply main and a return main, as does the direct return system. The difference between the two systems is that in t

29、he reverse system the return from each convector takes the long way around to return to the boiler. This method has the advantage that all circuits have the same length. by the water in flowing through number 1 convector For example, the total distance travelled and then back to the boiler is the sa

30、me as the distance travelled by the water flowing through each of the other convectors. This feature means that the system tends to be self-balancing, thus all connectors receive the same amount of water and at the same temperature. Also pipe sizing is not as critical as the direct return system. HV

31、AC 6018 7 Figure 4 Reverse Return System Courtesy of Dunham-Bush As with other forced systems described this far, the pump is located in the return line and discharges into the boiler. The expansion tank is the closed type, installed with an air separator. The reverse return system is suitable for h

32、eating systems in multistory buildings as shown in the piping arrangements illustrated in Figs. 5 and 6. The supply main and the return main of the system in Fig. 5 are located in the basement. The supply risers are fed upwards by the supply main. The system in Fig. 6 has an overhead supply main fee

33、ding downward into the risers. BG3_fig4.gif HVAC 6018 8 Figure 5 Multi-Story Upfeed Reverse Return System Courtesy of Dunham-Bush Figure 6 Multi-Story Downfeed Reverse Return System BG3_fig5.gif BG3_fig6.gif In both systems the circulating pump is located in the supply main near the boiler in order

34、to obtain maximum pressure in the supply main. The branch circuits in both diagrams are indicated by single convectors connected between supply and return risers. In reality, each branch circuit may contain several heating units serving a number of rooms in a suite or office. When a branch circuit c

35、ontains only a small number of heating units, it is usually arranged as a loop or one-pipe system. For a larger number of units, the branch circuit should be arranged as a reverse return system. Multi-Zone Systems In many buildings, the hot water heating system is divided into several separate circu

36、its, each supplying heat to a different section or zone of the building. The flow of water through each zone is then controlled in one of the following ways: 1. Each zone circuit is equipped with its own circulating pump, a zone thermostat controlling the operation of the pump. 2. A single circulato

37、r is used for the entire system but the flow through each circuit is regulated by a thermostatically controlled motorized valve. 3. A primary pump circulates the hot water through the main headers continuously and each zone draws the water off the main when needed by means of a secondary or zone pum

38、p controlled by the zone thermostat. Fig. 7 shows a diagram of a three-zone heating system for a three story building, circuit. The hot water supply of each circuit is controlled each floor having its own one-pipe by a zone thermostat which starts and stops the circulating pump for that particular z

39、one. Thus the supply to each zone is completely independent from the other zones. Any or all of the zones may be operating at one time. The boiler maintains the water temperature at the required setting. Each circuit also contains flow control valves which close when the pump is not running and so p

40、revent natural circulation which could cause overheating of that zone. The advantage of such a multi-zone system is that each zone can be heated to a different temperature depending on the occupancy requirements. For instance, if the system shown in Fig. 7 applies to a building where the basement is

41、 used for storage, the first floor as office space, occupied only during the daytime for five days a week, while the second floor contains living quarters, it should be obvious that the heating requirements for each floor are quite different. HVAC 6018 9 HVAC 6018 10 Figure 7 Three-Zone Multi-Circui

42、t, One-Pipe System Fig. 8 shows a basic diagram of a multi-zone system with primary and secondary pumping circuits indicating the many types of secondary zones that may be used. Figure 8 Multi-Zone Hot Water System with Primary and Secondary Pumping Circuits Courtesy of Bell and Gossett BG3_fig8.gif

43、 HOT WATER HEATING COMPARED TO STEAM HEATING Advantages 1. The temperature of the supply water can be varied in relation to the changing outdoor temperature much more readily than steam temperature can be varied. This allows for variations in load without shutting off and opening up the supply to th

44、e radiators. 2. A hot water boiler is smaller and more compact than a steam boiler of the same output, since no boiler steam space is required. In pipe sizes are generally smaller in a hot water system than in a steam system, and fewer addition, and less expensive fittings are required. 3. The pipin

45、g pitch or slope is not as critical in a forced circulation hot water system as in a steam system where proper drainage of condensate is important. 4. Maintenance costs are usually less in a hot water system, due to the fact that traps are not required and also because there is less corrosion in the

46、 piping. Disadvantages 1. When a hot water system is used in a multistory building and the boiler has to be located in the basement, the boiler must be designed to withstand high pressure due to the high head of water in the system. 2. In a large hot water system, considerable power is required to c

47、irculate the water. 3. There is a greater danger of damage due to freezing in a hot water system. 4. If a leak does occur more damage will result with a hot water system than with a steam system. HVAC 6018 11 SPECIAL HEATING SYSTEMS Radiant Panel Heating Heating rooms by means of warmed sections of

48、ceilings, walls or floor panels is known as radiant panel heating. In contrast with steam and hot water heating systems which supply heat to the spaces to be warmed mainly by convection, the radiant panel system supplies the larger part of the heat by radiation and only a small part by convection. T

49、o warm these sections or panels, several methods can be used such as: 1. Passage of warm air, heated by unit heaters, through hollow floor tiles or through the space above hung ceilings. 2. Electrical heating cables embedded in the panels. 3. Piping or tubing, embedded in the panels, through which h

50、ot water is circulated. Since the last method is most frequently used, only this method will be discussed. Temperatures used in panel heating are limited in order to prevent discomfort to occupants and cracking of plaster in walls and ceilings. The surface temperature of ceilings should not exceed 4

51、6 C 115 F, of walls 38 C 100 F, and floors should be held below 30 C 86 F. The panels are usually heated by continuous serpentine coils made of thin copper tubing. Where greater ruggedness is required, as in concrete floor slabs, steel piping laid out in grids is often used. Some systems use plastic

52、 tubing. The temperature of the water in the coils is lower than tha t used in conventional hot water heating systems. It should not be higher than 66 C 150 F for ceilings and 60 C 140 F for floors. A basic diagram of a two-zone, panel heating system is shown in Fig. 9. HVAC 6018 12 HVAC 6018 13 Fig

53、ure 9 Two-Zone Panel Heating System Note that supply and return piping is basically similar to that used in other hot water systems. The supply and return piping of the zone containing panels 2, 3 and 4 is reverse return as is the piping of the other zone containing panels 1 and 5. All panels are eq

54、uipped with a balancing fitting to obtain even heat distribution. Some of the advantages of radiant panel heating over a conventional heating system are: 1. A more even heat distribution throughout the rooms. 2. More comfort for buildings built on concrete slabs without crawl space or cellar when he

55、ated floor panels are used. 3. Heating elements are completely out of sight and do not interfere with decorations, furniture, etc. However, there are also some disadvantages, which make the system unsuitable for use in certain types of buildings. These disadvantages are: 1. Due to their large heat storage capacity, the panels are fairly slow in heating up and cooling down. This causes problems maintaining change quickly. a steady room temperature when outside temperatures HVAC 6018 14 2. Any leakage in the panels is co