بسم هللا الرحمن الرحيم. University of Khartoum Faculty of Engineering Mechanical Engineering Department

Transcription

1 بسم هللا الرحمن الرحيم University of Khartoum Faculty of Engineering Mechanical Engineering Department Usage and Sizing of Expansion Tanks in Chillers and Boilers A thesis submitted in partial fulfillment of the requirements for the degree of B.Sc. in Mechanical Engineering Presented by: 1/ Mohamed Amin Ahmed Eltayeb 2/ Obada Kamal Abd Alrahim Abd Allah Supervised by: Dr. Mohamed Ahmed Abdelbagi Sirag August 2015

2 Dedication. Acknowledgments i

3 Abstract Understanding how an expansion tank work and when and where to place one and how to select it is a must for a mechanical engineer specialized in any field that concerns with any types of closed systems. We answered all the above questions by studying the usage of expansion tanks in chillers and boilers (heating and cooling systems) in details and the advantages of each type over the other. We also studied sizing it which include how to select one and where to place it in your system. In the final we did a live application of our study by doing a case study in which we sized and selected expansion tank for forensic evidence building s cooling system. In the end in any closed system no one can argue about the importance of the expansion tank. It s one of the main components. ii

4 مستلخص فهم كيفية عمل خزان التمدد و مكان وضعه و كيفية اختيار المناسب امر ال بد منه لكل مهندس ميكانيكي متخصص في اي مجال متعلق باالنظمه المغلقه. لقد اجبنا عن كل هذه االسئله عن طريق دراسة استخدامات خزان التمدد في أجهزة التبريد والغاليات )أنظمة التدفئة والتبريد( بتعمق ومزايا كل نوع على اآلخر. و درسنا ايضا كيفية معرفة حجمها المناسب و اين توضع في النظام. ثم طبقنا دراستنا عمليا عن طريق اختيار خزان تمدد مناسب لنظام تبريد مبنى االدلة الجنائيه. في النهاية في أي نظام مغلق ال يستطيع أحد أن يجادل حول أهمية خزان التمدد. و انه واحد من المكونات الرئيسية. iii

5 Table of Contents Dedication... i Acknowledgments... i Abstract... ii... iii مستلخص List of symbols... vi List of figures... vii Chapter One Introduction Introduction Simple definition How an Expansion Tank Works A properly sized expansion tank The main goal... 3 Chapter Two Literature Review... 4 Literature Review... 5 Chapter Three Usage and Types of Expansion Tanks Usage of expansion tanks Expansion tank types Air cushion or Air-over-Water Pressure or plain steel tank Captive air expansion tank Chapter Four Sizing of Expansion Tanks Sizing Selecting the expansion tank Air Elimination System and its Components Air control System and its Components Sizing the Expansion Tank Sizing plain steel expansion tank Sizing captive air expansion tank Point of Connection to the System Chapter Five Case Study Case study for forensic evidence building: Sizing an expansion tank for the system Chapter Six Conclusion iv

6 6.1 Results Recommendation References Other references Appendices v

7 List of symbols V t volume of expansion tank, gal V s volume of water in system, gal t 1 lower temperature, F t 2 higher temperature, F Pa atmospheric pressure, psia P1 pressure at lower temperature, psia ( fill pressure ) P2 pressure at higher temperature, psia ( maximum design pressure ) V1 specific volume of water at lower temperature, ft3/lb V2 specific volume of water a higher temperature, ft3/lb linear coefficient of thermal expansion, in./in. - F α = 6.5 x 10-6 in./in. - F for steel = 9.5 x 10-6 in./in. - F for copper ΔT (t2 - t1), F vi

8 List of figures figure 1 expansion tank in automotive industry... 8 Figure 2 expansion tank in large pumping station... 9 figure 3 plain steel expansion tank figure 4 captive air diaphragm expansion tank from outside figure 5 captive air diaphragm expansion tank from inside figure 6 working process of a captive air diaphragm expansion tank Figure 7 captive air bladder expansion tank from inside figure 8 captive air bladder expansion tank from outside figure 9 work process of captive air bladder expansion tank Figure 10 Air Elimination System figure 11 Air control System figure 12 expansion tank located on the suction side of the pump figure 13 expansion tank located on the discharge side of the pump Figure 14 two expansion tanks in one system Figure 15 phase one of chiller water system figure 16 phase two of chiller water system vii

9 Chapter One Introduction 1

10 1.1 Introduction All liquids used in hydronic heating systems expand when heated. This thermal expansion is unavoidable and extremely powerful force of nature. Upon heating, each of the trillions of fluid molecules contained in the system becomes slightly larger. From a macroscopic perspective, one might think there was an increase in the amount of fluid in the system. This is not true. The same molecules just take up more space when at a higher temperature. The volume of the fluid has increased, but the total mass of the system s fluid has not changed.for all practical purposes, liquids are incompressible. A given number of liquid molecules cannot be compacted or squeezed into a smaller volume without tremendous force. Any container completely filled with a liquid and sealed from the atmosphere will experience a rapid increase in pressure as the liquid is heated. If this pressure is allowed to build, the container will burst, in some cases violently.to prevent this from occurring, all hydronic heating systems must be equipped with a means of accommodating the volume increase of their fluid as it is heated. In systems that are open to the atmosphere, such as a non-pressurized thermal storage tank, the volume increase can be accommodated by extra space at the top of the tank. This allows the expanding fluid to park its extra volume Same thing in cooling system the main reason of the expansion tank is to maintain a balanced pressure and to keep enough water in the system. 1.2 Simple definition An expansion tank or expansion vessel is a small tank used to protect closed (not open to atmospheric pressure) water systems from excessive pressure. 1.3 How an Expansion Tank Works An expansion tank is only partially full of water on start up. The rest of the tank contains air. As the system water expands, the added system volume moves into the expansion tank, compressing the air, thereby increasing the air pressure, which pushes back to increase the system pressure. 1.4 A properly sized expansion tank Limits the system operating pressure increase under the hot condition and Provides a safe system pressure without relying on the relief valve to discharge also Insures that 2

11 pump NPSH requirements are met, and Establishes a point of "zero pressure change" for the system, ensuring that there will be no negative pressure points anywhere in the system. 1.5 The main goal The objective of this paper is to show the importance of the expansion tanks in any closed system especially in chillers and boilers. 3

12 Chapter Two Literature Review 4

13 Literature Review happens: In the old days systems didn t have an expansion tank. Here is what When the fluid in the system expand it has nowhere to go and the system will over pressurize and fail unless you have some kind of a relief valve like spring loaded valve or so. An expansion tank can take all the expanded water not only that, but can give it back when the system needs it. This days a lot of companies are competing in this area due to that there are many expansion tanks brand names in the market. Every one of them developed an electronic automatic calculator to calculate the size and to select the right expansion tank. But for any engineer the basics of any science must be learned and clearly known to get the job perfectly done. 5

14 Chapter Three Usage and Types of Expansion Tanks 6

15 3.1 Usage of expansion tanks The main usage of expansion tanks is to pressurize the closed system to insure those three requirements: 1. The pressure at the fill point must be high enough to push the fluid to the top of the system. This is normally only a concern when the fill point is in the basement, but you can understand why there would be problems if there were literally no fluid in the top part of the system. 2. Maintain enough pressure to satisfy the NPSH requirements of the circulation pumps, and prevent cavitation in control valves. For systems operating below about 230 degrees F, centrifugal pumps will require a minimum of 4 psig at the pump suction. Control valves, particularly in heating systems, must be evaluated to determine the pressure required at the outlet to prevent cavitation. 3. Keep all points in the system above atmospheric pressure to prevent the ingestion of air, and to allow air-venting devices to work properly. Good air removal is crucial to the proper operation of any closed system. But also expansion tank has other application such in automotive industry particularly in cooling internal combustion engines. This is exactly what happens:- When the coolant starts to come up to operating temperature, it expands. The radiator cap is designed to hold a certain system pressure. The coolant can run at a higher temperature without boiling with this higher pressure. Anything above this pressure will be vented from the radiator through the radiator cap. It contains a spring loaded valve. In newer equipment, this excess pressure coolant goes into the expansion tank. Without the expansion tank, the coolant would be lost and fall to the road. When you turn of the engine and it cools down, the coolant contracts, causing a partial vacuum in the radiator and coolant is drawn from the expansion tank back into the radiator. The expansion tank helps keep the radiator full, all the time. This eliminates any air in the coolant system and makes the radiator more effective at doing its job. 7

16 figure 1 expansion tank in automotive industry (the brown plastic tank with white lid in top of picture) Also expansion tanks are used in large-scale pumping stations, where they may be called expansion chambers or hydrophores, to maintain an even pressure and to reduce the effects of water hammer. 8

17 Figure 2 expansion tank in large pumping station 9

18 3.2 Expansion tank types In general there are two types of expansion tanks Air cushion or Air-over-Water Pressure or plain steel tank In this tank it is desirable to direct the separated air from the air separator to the space above the water level in the expansion tank. The air from the air separator is piped to the expansion tank through a special tank fitting. This fitting directs the air to the top portion of the tank, and discourages air from migrating back into the system, when the system cools. Note that since the air is recycled to provide a cushion in the expansion tank, this system is called an Air Control system. The air cushion in the tank can be depleted due to absorption of air into the water. It can also be depleted by loosing air through air vents in the piping. Care must also be taken to insure that piping between the air separator and the plain steel expansion tank is pitched at least 3 degrees to facilitate the migration of captured air back into the expansion vessel. Systems with plain steel expansion tanks must not have automatic air vents installed as this will lead to the loss of the expansion tank air cushion. If air is lost in the tank then the tank will become water logged. With a water-logged expansion tank, the expanded water must now seek a new outlet which can be the relief valve on one of the major components. As note previously the tank must be sized for the expansion of the water in the system plus the initial charge of water to compress atmospheric air in the tank to the fill pressure. This makes the tank much larger. The tank is also subject to corrosion with the presence of air and oxygen in the tank. Disadvantages: they are larger, use more water, and provide no more capacity than smaller, newer tanks. These tanks will get water-logged over time and therefore no longer popular and the air cushion could be depleted as mentioned above Advantages: Lower cost, Ceiling mounted to save floor space figure 3 plain steel expansion tank 10

19 3.2.2 Captive air expansion tank It has basically two types: diaphragm and bladder Advantages over the Air-over-Water Pressure Tank:- Closed vessels removing any risk of evaporation and frost. Diaphragm in natural rubber, food processing grade, easily interchangeable. Eliminates the need for expensive air replacement systems. Speedy, easy installation. No maintenance. Large useful water reserve avoiding starting the pump too often in pressure boosting configuration. I. Captive Air Diaphragm Expansion Tank In a diaphragm tank the air is held captive by the use of a diaphragm with the expanded water being held on one side of the diaphragm and air on the other. This permanent separation allows the tank to be precharged on the air side to the minimum operating or fill pressure. This eliminates many gallons of water to compress atmospheric pressure air in an air cushion or plain steel tank to the fill pressure. This allows the reduction in Captive Air expansion tank sizes of up to 80% compared to air cushion or plain steel tanks. In a diaphragm tank the diaphragm is attached to the tank wall and cannot move inside the tank. As a result the tank has a limited acceptance volume. In addition, there is some water in contact with the tank wall providing an opportunity for corrosions. 11

20 figure 4 captive air diaphragm expansion tank from outside figure 5 captive air diaphragm expansion tank from inside 12

21 figure 6 working process of a captive air diaphragm expansion tank 13

22 II. Captive Air Bladder Expansion Tank Same thing as in the diaphragm tank the air is held captive only the difference it uses a field replaceable bladder to permanently separate the air and water In a bladder tank the bladder is not attached to the tank wall like a diaphragm tank. Rather it is suspended inside the tank very much like a balloon. Expanded water flows into the inside of the bladder. Air is on the outside of the bladder between the bladder and the tank. As a result no water is in contact with the tank wall minimizing corrosion. In a partial acceptance bladder tank the bladder is of limited acceptance volume and does not stretch. As a result, if there is an overpressure condition in the system the bladder will burst, again, very much like a balloon. Bladder tank has two types one is partial acceptance and the other is full acceptance the difference is that the bladder in the full acceptance is of full acceptance volume and can expand to the full volume of the tank unlike the partial acceptance one. As a result, the bladder will not burst if the system experiences an overpressure condition. Figure 7 captive air bladder expansion tank from inside 14

23 figure 8 captive air bladder expansion tank from outside 15

24 figure 9 work process of captive air bladder expansion tank 16

25 Chapter Four Sizing of Expansion Tanks 17

26 4.1 Sizing Data Required for Sizing the Expansion Tank To properly size an expansion tank, we must know the following values: I. System volume II. Fill temperature, III. Fill pressure, IV. Maximum design pressure, V. Maximum design temperature. Let s consider each of these factors: Determine System Volume by adding the water-holding capacities of all the components of the piping system, including boilers, chillers, coils, piping, air separators, etc. Note that in determining system volume, it is best to be safe. An undersized expansion tank results in the problems. An oversized tank results in no operational problems. Fill temperature: The temperature of the water available to fill the system. In our climate, use about 40 F. Fill pressure: The pressure to which the system will be initially filled at start up. The fill pressure setting on the fill valve establishes this pressure. (This valve admits water to the system whenever the system pressure falls below the fill valve setting). Two factors impact the chosen fill pressure for a system. 1. The fill pressure must lift the water to the highest point in the system feet of water column equals a pressure of 1 PSI, so a system with a high point in the piping of 23 above the fill valve requires a pressure of 10 PSIG at the valve (23 /2.31). To this minimum pressure, add an additional 5 PSIG safety margin. The reason: as the system fills, the water displaces the air, which rises to high points in the system. At start up this air must be manually vented using manual air vents. The pressure in the piping needs to be greater than atmospheric pressure to insure that the air will readily move from the pipe, through the air vent, and into the atmosphere. In no case, should the fill pressure be less than PSIG, even for one-story buildings. Systems operating at lower pressures simply take longer to vent. Example 1: What is the fill pressure recommended for a 23 high system? 18

27 Solution: (23 /2.31) + (5 PSIG) = 15 PSIG 2. The fill pressure must prevent cavitation. As a rule of thumb, perform the NPSH calculations when designing a system for 210 degrees or greater, and the pump NPSHr is greater than feet. If the fill pressure determined by the building height is insufficient to prevent cavitation, find a lower NPSHr pump or resort to a higher fill pressure. Maximum Design Pressure: Use a maximum operating pressure is normally input at about 5-10 PSIG below the relief valve setting. (Relief valves often weep at settings below their relief setting. The 5-10 PSIG margin minimizes the chance of weeping). The relief valve setting is determined by a combination of factors including: 1. The maximum pressure rating of equipment in the system, such as boilers, chillers, pumps and accessories. Though relief valves may be ordered for any setting, distributors stock relief valves set at 30#, 50#, 75# and 125#, so one of these pressures is normally chosen. All other factors being equal, the higher the maximum design pressure, the smaller the expansion tank. 2. The relative price of available backflow preventers. Using a 30# relief valve results in an inexpensive backflow preventer. In small buildings, this often favors a setting of 30# in spite of the fact that other items in the system would withstand a higher pressure. Remember the pressure will be higher than at other points in the system than it is at the expansion tank if the tank is properly located at the pump suction. For example, the pressures at the discharge of the pump will be higher by the amount the of pump head. Therefore, when selecting the relief valve setting, take into account the location of the valve and the pressures at other points in the system to avoid exceeding equipment pressure ratings. Example: The hydronic components of a system carry a rating of 125 PSIG. The designer selects a relief valve setting of 125 PSIG and sizes his expansion tank accordingly. The contractor installs the relief valve on the suction side of the pump. The pump is provides a head of 70. When the system heats up, the pressure on the suction side of the pump (point of connection to the expansion tank) reaches 120 PSIG. Think about the pressure on the discharge side of the pump with the pump in operation. Is the system adequately protected against over pressurization? Maximum Design Temperature For heating systems use either the maximum expected normal operating temperature of the boiler (or the high limit setting on the boiler for a bit more safety). For chilled water systems, use the maximum expected temperature of the water system on a summer day with the cooling system is turned off (perhaps degrees). 19

28 4.2 Selecting the expansion tank The system designer first decides whether to use an air elimination system or an air control system Air Elimination System and its Components The system below illustrates air elimination using an air scoop and air vent. The captive air expansion tank allows for expansion. The air scoop separates air from the water, and the vent discharges that air to the equipment room. Figure 10 Air Elimination System Air control System and its Components The air control system operates with an air separator that is not equipped with air vent. Therefore, it does not vent the air, but instead sends it through a special tank fitting into a plain steel expansion tank. Hence it saves the separated air to help provide an air cushion. The tank fitting works in concert with the air separator and prevents air from re-entering the system on a cool down cycle. 20

29 figure 11 Air control System 4.3 Sizing the Expansion Tank This was formerly a manual calculation ( but we still going to explain how to do it manually ), but today we plug the system volume, fill temperature, fill pressure, maximum design temperature, and maximum design pressure into the TacoNet software to select multiple sizes and types of tanks for our consideration Sizing plain steel expansion tank The ASHRAE formula for plain steel expansion tank sizing is: Vt = Vs ((V2 V1 ) 1) 3α t ( Pa P1 ) ( Pa P2 ) Vt = volume of expansion tank, gal Vs = volume of water in system, gal t1 = lower temperature, F t2 = higher temperature, F Pa = atmospheric pressure, psia 21

30 P1 = pressure at lower temperature, psia P2 = pressure at higher temperature, psia V1 = specific volume of water at lower temperature, ft3/lb V2 = specific volume of water a higher temperature, ft3/lb α = linear coefficient of thermal expansion, in./in. - F = 6.5 x 10-6 in./in. - F for steel = 9.5 x 10-6 in./in. - F for copper ΔT= (t2 t1), F Chilled water sizing example: Sizing a plain steel tank for a chilled water system with a temperature range of 40 F to 100 F (ambient temperature). System fill pressure of 10 psig, System volume of 3000 gallons, with steel piping system, System fill pressure of 65 psig and a 90 psig maximum operating pressure. Given: Vs = 3000 gallons V1 = ft3/lb (40 F) V2 = ft3/lb (100 F) Pa = 14.7 psia P1 = 65psig +14.7psia = 79.7psia P2 = 90psig+14.7 psia = psia α = 6.5x 10-6 in/in F for steel Δt = 60 F (using the above equation for plain steel expansion tank ) Vt = Vs (V2 V1 ) 1 3α t ( Pa P1 ) ( Pa P2 ) Vt = gallon 22

31 4.3.2 Sizing captive air expansion tank The formula is the same as plain steel formula only Pa = P1 because the captive air expansion tank is pre charged: Then we have the final formula: Vt = Vs (V2 V1 ) 1 3α t 1 ( Pa P2 ) 4.4 Point of Connection to the System The point where the expansion tank connects to the system is called the point of zero pressure change. The reason is that the pressure in the tank and at the point of connection is the same whether the pump is off or on. The diagram below shows system pressures throughout a system when the expansion tank properly connected to the suction side of the pump. 23

32 figure 12 expansion tank located on the suction side of the pump The next diagram shows what will happen to the system pressures at various points when the expansion tank is improperly connected to the discharge side. Note that with the tank connected to the discharge side of the pump, the pressure can become a vacuum at some points in the system. This could create NPSH problems. It could also result in air being drawn into the system. 24

33 figure 13 expansion tank located on the discharge side of the pump In the next figure is a system with two expansion tanks. The point of no pressure change will be somewhere between the two tanks. Therefore, the general rule of thumb in hydronic systems is that Multiple expansion tanks in a system is not recommended since unstable pressure conditions will result. 25

34 Figure 14 two expansion tanks in one system 26

35 Chapter Five Case Study 27

36 5.1 Case study for forensic evidence building: We did a case study by selecting a proper expansion tank for the chilled water system of the building. The chilled water system in the building was divided into two phases. Look at the two pics of the two phases. Every phase has here pumps (two operating and one is backup) and has 2 chillers Figure 15 phase one of chiller water system figure 16 phase two of chiller water system 28

37 So we had to select two expansion tanks one for each phase in order to do so we calculated the total volume of piping in each phase in order to get the total volume of all the system. Here is the volumes in cubic meter :- Phase 1 Phase 2 Basement m m 3 Ground m m 3 First Floor m m 3 Second Floor m m 3 Third Floor m m 3 Fourth Floor m m 3 Fifth Floor m m 3 Roof 2.74 m m 3 Risers m m 3 Total volume of phase one = m m 3 = m 3 Total volume of phase two = m m 3 = m 3 (We added 0.5 m 3 to the whole system for safety because an undersized expansion tank results in a lot of problems but an oversized tank results in no operational problems). 5.2 Sizing an expansion tank for the system Since we have two phases we are going to have two expansion tanks for each phase. We also going to use the captive air diaphragm expansion tank to the due reasons:- We don t want to take a lot of space To minimize the risk of evaporation ( excessive heat on the roof ) Information about the system:- Minimum water temperature = 45 F Maximum water temperature = 115 F Specific volume of water at lower temperature = ft 3 /lb Specific volume of water at higher temperature = ft 3 /lb Initial fill pressure = 3.5 bar Maximum design pressure = 8 bar α=6.5 x 10-6 in./in. - F ( for steel ) 29

38 Sizing formula for captive air expansion tank :- Vt = Vs (V2 V1 ) 1 3α t 1 ( Pa P2 ) P a = P 1 = 3.5 bar = psi = psi. P 2 = 8 bar = psi = psi Phase one :- Vt = (( ) 1) ( ) 1 ( ) Vt =34.68 gallons Phase two :- Vt = (( ) 1) ( ) 1 ( ) Vt = gallons We are going to use two captive air diaphragm expansion tanks sized 53 gallons one for each phase. 30

39 Chapter Six Conclusion 31

40 6.1 Results Since the paper is actually a study of an object there s is no results for that. But it does have a result for the case study. In which we mentioned before that s we selected two captive air diaphragm expansion tanks sized 53 gallons one for each phase of chilled water system. 6.2 Recommendation Those points are recommend in sizing and installing any expansion tank: To use captive air expansion tank over plain steel one whenever possible because of the many advantages. Expansion tank calculation must be accurate and to put in consideration that having oversized tank is always better than having undersized tank. Best location of the expansion tank is at the suction of the pump. 32

41 References o ASHRAE. (2012) ASHRAE Handbook--HVAC Systems and Equipment, Chapter 13. Atlanta: American Society of Heating, Refrigerating and Air- Conditioning Engineers, Inc o Home Reference ebook - The Encyclopedia of Homes (2012) by Alan Carson o HVAC: Equations, Data, and Rules of Thumb, Second Edition (2007) by: Arthur A. Bell Jr o Mechanical Equipment of Buildings: A Reference Book for Engineers and Architects, Volume 2 (2010) By Louis Allen Harding, Cutts Willard o Plant engineer s reference book (2009) Dennis Snow o Pumping Station Design, By Garr M. Jones, Robert L. Sanks, Bayard E. Bosserman, George Tchobanoglous. Other references o o control/expansion_tanks/index.html o 33

42 Appendices 34

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