Hvac Nov2008

HVAC Plant, Pumps, and Piping Calculations Compiled By: Prof. Dr. Essam E. Khalil Professor of Mechanical Engineering, Mechanical Power Engineering Department Faculty of Engineering, Cairo University, Cairo, Egypt Fellow ASME, Fellow AIAA, Distinguished Lecturer ASHRAE Chairman of the Arab Air Conditioning and Refrigeration Code, HBRC Convenor of TC205 WG2, ISO,Zurich

November 2008

HVAC – HVAC Plant, Pumps and Piping Calculations

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SCOPE This lecture includes tables and charts to size piping and pumps for fluid flow systems of the HVAC plant. There are two related but distinct concerns when designing a fluid flow system: sizing the pipe and determining the flow-pressure relationship to select the proper pumps. The two are often confused because they can use the same equations and design tools. Nevertheless, they should be determined separately. The emphasis in this lecture is on the problem of sizing the pipe, and to this end design charts and tables for specific fluids are presented in addition to the equations that describe the flow of fluids in pipes. Once a system has been sized, it should be analyzed with more detailed calculation methods to determine the pump head required to achieve the desired flow. Computer-based methods are well suited to handling the details of calculating the losses around an extensive system. WATER AND REFRIGERANT PIPING Most common heat-conveying media in air-conditioning and refrigeration systems are air, water, and refrigerants. This lecture concentrates on piping systems for water and pumps for motivating the flow of water in what are often called hydronic systems. This lecture also covers guidelines for selecting the size of refrigerant pipes. The requirements of a water-distribution system are that it provides the necessary flow rate to all the heat exchangers, that it be safe, and that its life-cycle (including both first and operating costs) be low. In selecting sizes of refrigerant pipes there are some standard recommendations which are heavily influenced by the refrigerant pressure drop. Some pressure drop is expected, but the pipe size should ensure that it will not be excessive, which result in higher operating would cost. This lecture first compares air and water as media for conveying heat, citing reasons why air would be used in one situation and water in another. Next, piping and pumps are described. Finally, the joint working of these elements is explored in the design of water-distribution. COMPARISON OF WATER AND AIR AS HEAT CONVEYING MEDIA The final transfer of heat is always either from or to the air in the conditioned space. The device for providing heat (heat source) is usually an electric or fuel fired furnace, and the device for extracting heat (heat sink) is a refrigeration unit. When a heat pump is used, the same equipment is both the source and sink of energy. While the source or sink of heat may sometimes be located in the conditioned space, the typical situation is that the source and sink be remotely located from the conditioned space, so that either air or water is heated or cooled at the source or sink and cooled or heated, respectively, at the conditioned space. Air could be heated or cooled at the source or sink and delivered directly to the conditioned space, or water could be heated or cooled and subsequently heat or cool the air in the conditioned space. The advantages of the water- over the air-distribution arrangement are

That the size of the heat source is smaller, That less space is required by water pipes than by air ducts, and Those higher temperatures of water than air are practical in heating, since the pipes, which are small, are easier to insulate than ducts.

The strengths and weaknesses of the two conveying media usually resolve into the following system choices. Small plants, such as residential and small commercial, use air throughout the system and no water whatsoever. The distance the heat must be transferred are short and the order of magnitude of the capacities does not result in excessive sizes of heat sources and ducts. Large air-conditioning systems, on the other hand, use hot- and chilled-water distribution. Fuel-fired or electric heat sources to heat air directly are not generally available in large sizes. The hot or chilled water may be piped directly to coils in the conditioned space; another popular arrangement is to pipe water to air heating and cooling coils, each serving one floor or one section of the building, Stoecker et al 1985. PIPING SYSTEM DESIGN Pressure Drop of Water Flowing in Pipes The equation for drop in pressure of a fluid flowing in a straight pipe is given as follows:


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The friction factor is obtained from Moody Chart, Figure 1. Many designers find the pre-calculated schedules and charts are convenient when many calculations must be performed. This method can apply only to one temperature of water, because the water temperature affects both the density and viscosity. The density appears directly in equation 1, and both density and viscosity influence the Reynolds number and thus the friction factor. The best single parameter besides the temperature that correlates the pressure drop at other temperatures is the velocity. Pressure drop in fittings One approach to computing the pressure drop caused by fittings (elbows, tees, open valves, etc.) in a piping system is to express their pressure drop in terms of the equivalent length of straight pipe that would cause the same pressure drop, Figure 2. The usefulness of this method is that the section of the piping system in which the diameter and flow are constant can be considered as one length of straight pipe.

Figure 1: Moody Chart

Erosion Erosion in piping systems is caused by water bubbles, sand, or other solid matter impinging on the inner surface of the pipe. Generally, at velocities lower than 30 m/s, erosion is not significant as long as there is no cavitation. When solid matter is entrained in the fluid at high velocities, erosion occurs rapidly, especially in bends. Thus, high velocities should not be used in systems where sand or other solids are present or where slurries are transported. Allowances for Aging With age, the internal surfaces of pipes become increasingly rough, which reduces the available flow with a fixed pressure supply. However, designing with excessive age allowances may result in over sized piping. Age-related decreases in capacity depend on the type of water, type of pipe material, temperature of water, and type of system (open or closed) and include

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• Sliming (biological growth or deposited soil on the pipe walls), which occurs mainly in unchlorinated, raw water systems. • Caking of calcareous salts, which occurs in hard water (i.e., water bearing calcium salts) and increases with water temperature. • Corrosion (incrustations of ferrous and ferric hydroxide on the pipe walls), which occurs in metal pipe in soft water. Because oxygen is necessary for corrosion to take place, significantly more corrosion takes place in open systems. The HDR Design Guide (1981) increased the friction loss by 15 to 20% for closed piping systems and 75 to 90% for open systems. It is recommended increasing the design discharge by 1.55 for uncoated cast iron, 1.08 for iron and steel, and 1.06 for cement or concrete. Water Hammer When any moving fluid (not just water) is abruptly stopped, as when a valve closes suddenly, large pressures can develop. While detailed analysis requires knowledge of the elastic properties of the pipe and the flow-time history, the limiting case of rigid pipe and instantaneous closure is simple to calculate. The pressure rise caused by water hammer is ∆P = ρ Cs V (2) Where ρ = fluid density Cs = velocity of sound in fluid V = fluid flow velocity Plastic Pipe The maximum safe water velocity in a thermoplastic piping system under most operating conditions is typically 1.5 m/s; however, higher velocities can be used in cases where the operating characteristics of valves and pumps are known so that sudden changes in flow velocity can be controlled. The total pressure in the system at any time (operating pressure plus surge of water hammer) should not exceed 150% of the pressure rating of the system.

Figure 2: Equivalent lengths in straight pipe of several fittings


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PUMP CHARACTERISTICS AND SELECTION The most useful performance data of a pump are the pressure differences it is capable of developing at various flow rates. Of equal importance is the knowledge of the power requirement at the design condition and at other possible operation points. Performance of a centrifugal pump is commonly shown by a manufacturer’s performance curve, Figure 3. The ideal power of the can calculated as Pi = Q (∆P) = Q (ρ g H) (3) Where Pi =Ideal Power, W Q = Volume flow rate m3/s ∆P = Pressure rise, Pa H = Equivalent water head to the pressure rise, m The power P required in the actual pumping process where there are losses is P = Pi / η (4) Where: η is the efficiency

Figure 3: Performance Characteristics of a Centrifugal Pump [2]

The performance of the pump usually must be considered in combination with the characteristics of the pipe network it serves. The pump curve has the shape already shown in Figure 3. The pressure difference experienced by the pipe increases as the square of the flow rate. The relationship is predictable from Equation (1) for straight pipe and using Figure 2 to get the pressure drop in fitting.

Figure 4: System and Pump Curves [2]


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The pump curve and the system curve can be plotted on the same graph. The intersection of the two curves (Figure 4) is the system operating point, where the pump’s developed pressure matches the system’s friction pressure. This operating point may or may not be the flow required to satisfy all the terminals in the system. In a typical hydronic system, a thermostat or controller varies the flow in a load terminal by positioning a two-way control valve to match the load. At full load the two-way valves are wide open, and the system follows curve A in Figure 5. As the load drops, the terminal valves begin closing to match the load (part load). This increases the friction and reduces the flow in the terminals. The system curve gradually changes to curve B. The operating point of a pump should be considered when the system includes two-way control valves. Point 1 in Figure 6 shows the pump operating at the design flow at the calculated design pressure of the system. But typically, the actual system curve is slightly different than the design curve. As a result, the pump operates at point 2 and produces a flow rate higher than design. To reduce the actual flow to the design flow at point 1, a balancing valve downstream from the pump can be adjusted while all the terminal valves are in a wide-open position. This pump discharge-balancing valve imposes a pressure drop equal to the pressure difference between point 1 and point 3. The manufacturer’s pump curve shows that the capacity may be reduced by substituting a new impeller with a smaller diameter or by trimming the existing pump impeller. After trimming, reopening the balancing valve in the pump discharge then eliminates the artificial drop and the pump operates at point 3. Points 3 and 4A demonstrate the effect a trimmed impeller has on reducing flow. Figure 7 is an example of a system curve with both fixed pressure loss and variable pressure loss. Such a system might be an open piping circuit between a refrigerating plant condenser and its cooling tower. The elevation difference between the water level in the tower pan and the spray distribution pipe creates the fixed pressure loss. The fixed loss occurs at all flow rates and is, therefore, an independent pressure as shown. Most variable-flow hydronic systems have individual two-way control valves on each terminal unit to permit full diversity (random loading from zero to full load). Regardless of the load required in most variable-flow systems, the designer establishes a minimum pressure difference to ensure that any terminal and its control valve receive the design flow at full demand. When graphing a system curve for a non-symmetrically loaded variable-flow system (Figure 7), the Dh (minimum maintained pressure difference) is treated like a fixed pressure loss (independent head), and becomes the starting datum for the system curve. The low and steep slope curves in Figure 7 represent the boundaries for operation of the system. The net vertical difference between the curves is the difference in friction loss developed by the distribution mains for the two extremes of possible loads. The area in which the system operates depends on the diverse loading or unloading imposed by the terminal units. This area represents the pumping energy that can be conserved with one-speed, two-speed, or variable-speed pumps after a review of the pump power, efficiency, and affinity relationships, ASHRAE HVAC Systems and Equipment 1996.

Figure 5: System Curve Change Due to Part-Load Flow [2]

Figure 6: Pump Operating Points [2]


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Figure 7: System Curve with System Static Pressure [2]

PUMPS SELECTION Substantial amounts of data are required to ensure that an adequate, efficient, and reliable pump is selected for a particular system. The designer should review the following criteria:

Design flow Pressure drop required for the most resistant loop Minimum system flow System pressure at maximum and minimum flows Type of control valve—two-way or three-way Continuous or variable flow Pump environment Number of pumps and standby Electric voltage and current Electric service and starting limitations Motor quality versus service life Water treatment, water conditions, and material selection

When a centrifugal pump is applied to a piping system, the operating point satisfies both the pump and system curves (Figure 4). As the load changes, control valves change the system curve and the operating point moves to a new point on the pump curve. Figure 8 shows the optimum regions to use when selecting a centrifugal pump. The areas bounded by lines AB and AC represent operating points that lie within the preferred pump selection range. But, because pumps are only manufactured in certain sizes, selection limits of 66% to 115% of flow at the BEP are suggested. The satisfactory range is that portion of a pump’s performance curve where the combined effect of circulatory flow, turbulence, and friction losses are minimized. Where possible, pumps should be chosen to operate to the left of the BEP because the pressure in the actual system may be less than design due to overstated data for pipe friction and for other equipment. Otherwise, the pump operates at a higher flow and possibly in the turbulent region (Stethem 1988). DESIGN OF A WATER DISTRIBUTION SYSTEM Piping Circuits Hydronic systems are designed with many different configurations of piping circuits. In addition to simple preference by the design engineer, the method of arranging the circuiting can be dictated by such factors as the shape or configuration of the building, the economics of installation, energy economics, the nature of the load, part-load capabilities or requirements, and others. Each piping system is a network; the more extensive the networks, the more complex it is to understand, analyze, or control. Thus, a major design objective is to maintain the highest degree of simplicity. Load distribution circuits are of four general types:

Full series Diverting series Parallel direct return Parallel reverse return


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Figure 8: Pump Selection Regions [2]

Figure 9: Flow Diagram of Simple Series Circuit [2]

Figure 10: Series Loop System [2]

Series Circuit. A simple series circuit is shown in Figure 9. Series loads generally have the advantage of both lower piping costs and higher temperature drops that result in smaller pipe sizes and lower energy consumption. A disadvantage is that the different circuits cannot be controlled separately. Simple series circuits are generally limited to residential and small commercial standing radiation systems. Figure 10 shows a typical layout of such a system with two zones for residential or small commercial heating.


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Diverting Series. The simplest diverting series circuit diverts some of the flow from the main piping circuit through a special diverting tee to a load device (usually standing radiation) that has a low-pressure drop. This system is generally limited to heating systems in residential or small commercial applications.

Figure 11: One-Pipe Diverting Tee System [2]

Figure 11 illustrates a typical one-pipe diverting tee circuit. For each terminal unit, a supply and a return tee are installed on the main. One of the two tees is a special diverting tee that creates a pressure drop in the main flow to divert part of the flow to the unit. One (return) diverting tee is usually sufficient for up-feed (units above the main) systems. Two special fittings (supply and return tees) are usually required to overcome thermal pressure in down-feed units. Special tees are proprietary; consult manufacturer’s literature for flow rates and pressure drop data on these devices. Unit selection can be only approximate without these data. One-pipe diverting series circuits allow manual or automatic control of flow to individual heating units. On-off rather than flow modulation control is preferred because of the relatively low-pressure drop allowable through the control valve in the diverted flow circuit. This system is likely to cost more than the series loop because extra branch pipe and fittings, including special tees, are required. Each unit usually requires a manual air vent because of the low water velocity through the unit. The length and load imposed on a one-pipe circuit are usually small because of these limitations. Because only a fraction of the main flow is diverted in a one-pipe circuit, the flow rate and pressure drop are less variable as water flow to the load is controlled than in some other circuits. When two or more one-pipe circuits are connected to the same two-pipe mains, the circuit flow may need to be mechanically balanced. After balancing, sufficient flow must be maintained in each one pipe circuit to ensure adequate flow diversion to the loads. Parallel Piping. These networks are the most commonly used in hydronic systems because they allow the same temperature water to be available to all loads. The two types of parallel networks are direct return and reverse return (Figure 12). In the direct-return system, the length of supply and return piping through the sub-circuits is unequal, which may cause unbalanced flow rates and require careful balancing to provide each sub-circuit with design flow. Ideally, the reverse-return system provides nearly equal total lengths for all terminal circuits. Direct-return piping has been successfully applied where the designer has guarded against major flow unbalance by

Figure 12: Direct- and Reverse-Return Two-Pipe Systems [2]


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1. Providing for pressure drops in the sub-circuits or terminals that are significant percentages of the total, usually establishing pressure drops for close sub-circuits at higher values than those for the far sub-circuits

2. Minimizing distribution piping pressure drop (In the limit, if the distribution piping loss is zero and the loads are of equal flow resistance, the system is inherently balanced.)

3. Including balancing devices and some means of measuring flow at each terminal or branch circuit 4. Using control valves with a high head loss at the terminals

Expansion Tank The expansion chamber (also called an expansion or compression tank) serves both a thermal function and a hydraulic function. In its thermal function the tank provides a space into which the non-compressible liquid can expand or from which it can contract as the liquid undergoes volumetric changes with changes in temperature. To allow for this expansion or contraction, the expansion tank provides an interface point between the system fluid and a compressible gas. Note: By definition, a closed system can have only one such interface; thus, a system designed to function as a closed system can have only one expansion chamber. Expansion tanks are of three basic configurations, Figure 13:

1. A closed tank, which contains a captured volume of compressed air and water, with an air water interface (sometimes called a plain steel tank);

2. An open tank (i.e., a tank open to the atmosphere); and 3. A diaphragm tank, in which a flexible membrane is inserted between the air and the water (another

configuration of a diaphragm tank is the bladder tank). In the plain steel tank and the open tank, gases can enter the system water through the interface and can adversely affect system performance. Thus, current design practice normally employs diaphragm tanks. Sizing the tank is the primary thermal consideration in incorporating a tank into a system. However, prior to sizing the tank, the control or elimination of air must be considered. An equation for the expansion tank size adapted from references [1,6].

Figure 13: Tank Pressure related to “System” Pressure [2]

The point of connection of the tank should be based on the pressure requirements of the system, remembering that the pressure at the tank connection will not change as the pump is turned on or off. Figure 14 shows alternative locations for connecting the expansion tank. With the tank on the pump suction side, when the pump is turned on, the pressure increases on the discharge side by an amount equal to the pump pressure (Figure 14 A). With the tank connected on the

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discharge side of the pump, the pressure decreases on the suction side by the same amount (Figure 14 B). Other considerations relating to the tank connection include the following:

A tank open to the atmosphere must be located above the highest point in the system. A tank with an air/water interface is generally used with an air control system that continually re-vents the air into the tank. For this reason, it should be connected at a point where air can best be released. Within reason, the lower the pressure in a tank, the smaller is the tank. Thus, in a vertical system, the higher the tank is placed, the smaller it can be.

REFRIGERANT PIPING It is appropriate to discuss here the three major pipe sections in a basic refrigeration system. They are the piping for the discharge gas, for liquid, and for suction gas. Somewhat different considerations apply to the selection of sizes of these three different pipe sections.

Figure 14: Effect of Expansion Tank Location with

Respect to Pump Pressure [2] Discharge line The pressure drop in this line is a penalty on compressor power, since for a given condenser pressure any increase in the line pressure drop requires a higher discharge pressure from the compressor. Beyond a certain optimum pipe size, however, additional enlargement adds more to the first cost than is recovered in the lifetime pumping cost of the compressor. Liquid line Since this pipe carries liquid with much higher density than the vapour in the other sections, its diameter will be smaller that that of the others. Pressure drop in this line does not penalize the efficiency of the cycle because what pressure drop does not occur in the liquid line will take place in the expansion device anyway. The pressure drop in the liquid line is limited for a different reason, however: if the pressure drops so much that some liquid flashes into vapour, the expansion device will not work properly. Suction line Pressure drop in this line, as in the discharge line, imposes a penalty on efficiency because this pressure drop reduces the entering pressure to the compressor. There is a limitation on how large a suction line can be chosen, however, imposed by the need in many refrigeration systems to carry lubricating oil from the evaporator back to the compressor. Velocities in vertical suction lines are often maintained at 6 m/s and higher in order to facilitate oil return. The pressure drop in refrigerant lines can be computed using Equation (1) with refrigerant properties. The pressure drop corresponding to various refrigerating capacities can also be obtained from reference [5] PIPES AND FITTINGS Steel Pipe Steel pipe is manufactured by several processes. Seamless pipe, made by piercing or extruding, has no longitudinal seam. Other manufacturing methods roll sheet into a cylinder and weld a longitudinal seam. A continuous-weld (CW) furnace butt-welding process forces and joins the edges together at high temperature. An electric current welds the seam in electric resistance welded (ERW) pipe.


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Copper Tube Because of their inherent resistance to corrosion and ease of installation, copper and copper alloys are often used in heating, air-conditioning, refrigeration, and water supply installations. Joining Methods

Soldering and Brazing Flared and Compression Joints Flanges Welding Reinforced Outlet Fittings Unions Other Joints

VALVE DESIGN

Valves are basically used for achieving the following functions:

1. Regulation 2. Isolation 3. Back flow prevention 4. Pressure Relief Valve Actuators

Figure 15: Valve Actuator

Figure 16: Diaphragm Actuator


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Figure 17: Position Indicator

Examples are given here below to valves electrically driven by motors to change their respective duty and discharge.

Figure 18: Mechanical Valves

Valves shown here in figure 4 are different in design .These can be classified as: A: Quick Opening B. Linear C: Equal Percentage


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Figure 19: Body of Valve

The basic valve construction is shown in figure 20 to comprise the following main components:

1. Valve body 2. Valve plug 3. Valve bonnet 4. Plug stem 5. Packing Box 6. Packing Rings 7. Valve seat

Figure 20: Valve Construction


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Figure 21: Valve Action

Figure 22: Valve Characteristics

Trim design will affect how the valve capacity changes as the valve moves through its complete travel. Because of the variation in trim design, many valves are not linear in nature. THE RELATIONSHIP BETWEEN VALVE CAPACITY AND VALVE TRAVEL IS KNOWN AS THE FLOW CHARACTERISTIC OF THE VALVE. Valve trims are specially designed, or characterized, in order to meet the large variety of control application needs. This is necessary because most control loops have some inherent nonlinearity, which you can compensate for when selecting control valve trim.

Charts similar to Figure 22 are used to illustrate various control valve flow characteristics. The percent of full flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers. These curves are based on CONSTANT PRESSURE DROP across the valve and are called INHERENT FLOW CHARACTERISTICS.


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The quick-opening characteristic provides large changes in flow for very small changes in lift. It usually has too high a valve gain for use in modulating control. So it is limited to on-off service, such as sequential operation in either batch or semi-continuous processes.

The majority of control applications are valves with linear, equal-percentage, or modified-flow characteristics.

• Linear - flow capacity increases linearly with valve travel. • Equal percentage - flow capacity increases exponentially with valve trim travel; equal increments of valve

travel produce equal percentage changes in the existing Cv. • A modified parabolic characteristic is approximately midway between linear and equal-percentage

characteristics. It provides fine throttling at low flow capacity and approximately linear characteristics at higher flow capacity.

When valves are installed with a pump, pipes, fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel. When the actual flow in a system is plotted against valve opening, the curve is called the INSTALLED FLOW CHARACTERISTIC. These can be classified as:

1. Quick opening 2. Linear 3. Modified percentage 4. Equal Percentage

Figure 23: Valve Plugs


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Figure 24: Valve Electrical actuators

Figure 25: Valve direct and reverse acting A Control Valve is a power-operated device used to modify the fluid flow rate in a process system. Well, what happens if the power is cut off? When a Control Valve is sized or selected to do a particular job, one of the first questions you should consider is how that valve will respond in the event of a loss of signal or power. This is called its "fail-safe mode" and knowing the fail-safe mode is the key to troubleshooting it. In most applications (about 80%), it is desirable for valves to fail closed. In other applications, you might want a valve to fail open or fail in place. Safety concerns and process requirements will mandate the fall mode of the valve. When a valve is not sitting in its fail position, is being told how and when to move by some external signal. By the comments one hears, you would be led to believe that control valves sit around and think up things to do on their own. Perhaps this will some day be true when all control valves are "smart." If a Control Valve is observed in an unstable condition or appears to not be responding correctly to an input signal, remember that something is telling the valve to behave that way.


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There are two basic types of control valves: rotary and linear. Linear-motion control valves commonly have globe, gate, diaphragm, or pinch - type closures. Rotary-motion valves have ball, butterfly, or plug closures. Each type of valve has its special generic features, which may, in a given application, be either an advantage or a disadvantage.

Linear Valve Features

• TORTUOUS FLOW PATH • LOW RECOVERY • CAN THROTTLE SMALL FLOW RATES • OFFERS VARIETY OF SPECIAL TRIM DESIGNS • SUITED TO HIGH-PRESSURE APPLICATIONS • USUALLY FLANGED OR THREADED • SEPARABLE BONNET

Rotary Valve Features • STREAMLINED FLOW PATH • HIGH RECOVERY • MORE CAPACITY • LESS PACKING WEAR • CAN HANDLE SLURRY AND ABRASIVES • FLANGELESS • INTEGRAL BONNET • HIGH RANGEABILITY

Control Valve Classification


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TYPICAL PRIMARY-SECONDARY PUMPING SYSTEMS

EXAMPLES

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EXAMPLES OF PUMP CONNECTIONS

REFERENCES This lecture is based mainly on the references [1,2,3] [1] Stoecker, W.F., and Jones, J.W., 1985, Refrigeration and Air Conditioning, Chapter 7, TMH Edition, Second Edition, TATA McGraw Hill, 1985. [2] ASHRAE HVAC Systems and Equipment, 2008, ASHRAE. [3] ASHRAE Fundamentals, 2005, AHRAE. [4] Stethem, W.C. 1988. Application of constant speed pumps to variable volume systems. ASHRAE Transactions 94(2):1458-66. [5] Wile, D. D., 1977, Refrigerant Line Sizing, ASHRAE Spec. Publications. 185. [6] Handbook and Product Directory, Systems Volume, 1980, Chapter 15, ASHRAE


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CHILLERS EFFICIENCY INDICATORS

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