Purpose
The makeup water assembly (Fig. 4-27) for hydronic systems is the source of water supply for the initial system fill and ongoing replenishment of the system water to make up for leaks. The makeup water assembly consists of a backflow preventer (Fig. 4-28),
Figure 4-27 Floor plan representation of a makeup water assembly.
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pressure-reducing valve (Fig. 4-29), and shutoff valves. A backflow preventer and pressure-reducing valve are necessary components of the makeup water assembly because the makeup water supply pressure, whether originating from a municipality or private source, fluctuates.
During periods when the makeup water pressure is lower than the hydronic system pressure, it is necessary to protect the potable water supply from the contamination that would occur from reverse flow (back siphonage) of the hydronic system water into the potable water system. The backflow preventer is specially designed to prevent the reverse flow of nonpotable hydronic system water from entering the potable water sys- tem due to back siphonage or back pressure. The most common type of backflow pre- venter used for hydronic systems is a reduced pressure zone assembly, which consists of two check valves separated by a relief valve assembly.
During periods when the makeup water pressure is higher than desired for the hydronic system, it is necessary to protect the hydronic system from overpressuriza- tion. The pressure-reducing valve accomplishes this and maintains a constant (adjust- able) makeup water pressure for the hydronic system.
The purpose of the shutoff valves is simply to isolate each of the components for maintenance or replacement.
Physical Characteristics
Makeup water assemblies range in pipe size from ¾ to 2 in., depending upon the water flow that is desired for the initial hydronic system fill. A ¾-in. makeup water piping connection is common because the flow through a ¾-in. pipe is usually adequate to fill the system within a reasonable amount of time. Larger hydronic systems require larger makeup water assemblies in order to reduce the time required to fill the system. A ¾-in. backflow preventer is approximately 18 in. long and 9 in. high.
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Connections
Connection of the makeup water assembly to the hydronic system piping is through a piping connection only; there are no electrical or ATC connections. The makeup water assembly is usually connected to the hydronic system at the system air separa- tor. The connection to the system piping should be the same size as the backflow preventer. Backflow preventers are typically fitted with shutoff valves on the inlet and outlet connections and have a relief valve drain connection. The backflow pre- venter is also equipped with multiple pressure test connections to facilitate the test- ing of each of the two check valves and the relief valve assembly within the backflow preventer. Most jurisdictions require testing of backflow preventers on an annual basis. Testing of the backflow preventer is performed with a portable differential pressure gauge.
The direct-acting pressure-reducing valve, located downstream of the backflow preventer, also requires shutoff valves to isolate it for maintenance or replacement. The desired outlet pressure is set through a manual adjustment screw equipped with either a hex head or handwheel.
Design Considerations
Once the hydronic system has been filled, the flow through the makeup water assembly is equal to the loss of system water due to leaks, which should be practically zero. Whenever the backflow preventer operates to prevent back siphonage, some water will be discharged through the relief valve drain connection. Therefore, it is necessary to provide a drain pipe (the same size as the drain connection) that should be routed to a suitable location, such as a floor drain. Sufficient access should be provided for the annual testing of the backflow preventer and maintenance or replacement of the makeup water assembly components, as required. The makeup water assembly should not be mounted so high as to prevent the connection of the test gauge or to make adjust- ment of the system pressure difficult for the maintenance personnel.
Although it is not necessary, a globe valve bypass may be designed around the pressure-reducing valve, allowing the makeup water assembly to remain in opera- tion while the pressure-reducing valve is repaired or replaced. However, this is not necessary because the hydronic system will operate satisfactorily for a limited amount of time while the makeup water assembly is shut off. A bypass should never be designed around the backflow preventer because this would defeat the purpose of the backflow preventer and would make back siphonage possible if the bypass valve were opened.
Pumps
Purpose
The purpose of a pump in a hydronic system is to circulate the system fluid.
Physical Characteristics
There are many types of pumps including end-suction (Figs. 4-30 through 4-32), close-coupled, in-line (Figs. 4-33 and 4-34), horizontal split-case, vertical split-case, and positive displacement pumps. The most common types of pumps used for hydronic systems are end-suction and in-line pumps, which are both centrifugal pumps.
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Figure 4-30 Floor plan representation of end-suction pumps.
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Figure 4-33 Photograph of an in-line pump.
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End-suction pumps range in size from 3 to 6 ft long and from 1 to 3 ft wide. The motor shaft is connected to the impeller shaft through a coupling.
In-line pumps are either vertical or horizontal, which describes the orientation of the motor/impeller shaft. The motor shaft is connected directly to the impeller shaft. In-line pumps range in size from 1 to more than 3 ft high (dimension from the impeller to the end of the motor) and 1 to 3 ft between the suction and discharge connections.
Connections
End-suction pumps are attached to an integral steel base frame that is field-mounted to a concrete base. The concrete base can be a 4-in.-high housekeeping pad to which the pump base frame is mounted with spring isolators. However, the preferred mounting is a concrete inertia base to which the pump base frame is bolted. A concrete inertia base is a steel-framed concrete block that is approximately 6 in. high and 6 in. larger than the pump base on all sides which is supported off of the floor by spring isolators. The con- crete inertia base provides a rigid base to maintain alignment of the pump shaft and reduce the vibratory motion caused by the rotating pump motor.
The pump suction pipe connection is parallel to the impeller shaft and the discharge pipe connection is perpendicular to the impeller shaft. Flexible pipe connectors are used on the suction and discharge pipe connections for end-suction pumps to isolate the vibration that is generated by the pump from the piping system.
The suction and discharge connections for in-line pumps are in line with each other and are perpendicular to the pump/impeller shaft. Small in-line pumps are supported by the piping system. Large in-line pumps require pipe hangers to be installed near the suction and discharge connections. Very large in-line pumps will be supported from the building floor, usually on a 4-in.-high concrete housekeeping pad.
The piping connections required for pumps include shutoff valves on the pump suction and discharge, balancing valve on the pump discharge, check valve and flow meter on the pump discharge, and pressure gauges. As an option, a multipurpose valve, which performs the duties of a shutoff valve, balancing valve, and check valve, may be installed on the pump discharge. It is common for a suction diffuser, which is similar in size to that of a long radius 90° pipe elbow, to be used on the suction pipe connection for end-suction pumps. This allows the suction pipe to drop vertically into the suction diffuser. Otherwise, it is necessary to provide five pipe diameters of straight pipe upstream of the pump suction connection. If a suction diffuser or the necessary length of straight pipe upstream of the pump suction connection is not provided, undesirable turbulence in the fluid flow will occur at the pump suction connection, which will com- promise the performance of the pump and may also damage the pump.
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In addition to the piping connections, the motors for pumps require an electrical connection. Typically, motors smaller than ½ hp require single-phase electrical power; motors ½ hp and larger require three-phase electrical power if it is available within the building. All motors require a starter, which can be a motor-rated manual switch for small motors or an across-the-line magnetic motor starter for larger motors. Some appli- cations may also require that the pump motor be controlled by a VFD, which modulates the speed of the motor and functions as the motor starter.
If automatic control of the pump is required through the ATC system, the magnetic motor starter or VFD must be equipped with the capability to receive a signal, such as a start/stop or motor speed signal, from the ATC system. This is discussed in more detail in Chap. 9.
Design Considerations
The pump must be designed to overcome the pressure losses within the system, which include losses associated with the supply and return piping, fittings, valves (shutoff, balancing, and control), strainers, heat transfer equipment (coils, heat exchangers), flow meters, and any other component within the system. The actual pressure drop from the manufacturer’s product data should be used for each component. However, 50% of the pressure drop through the piping system is normally used as an allowance for the pres- sure drop through the pipe fittings, and the shutoff and balancing valves. The elevation head must only be taken into account for an open system (refer to the Cooling Towers section earlier); the elevation of the piping system is not a consideration for a closed system. The following is a sample pump head calculation for a primary-only, closed, chilled water system:
Component Pressure Drop (ft w.c.)
1. Air separator (2 psi) 4.6
2. Strainer (4 psi) 9.2
3. Suction diffuser (0.5 psi) 1.2
4. Flow meter (5 psi) 11.6
5. Chiller cooler 10.0
6. Piping (400 ft × 4 ft w.c./100 ft) 16.0 7. Valves and fittings (50% of piping) 8.0
8. Control valve (5 psi) 11.6
9. Chilled water coil 10.0
Subtotal 82.2
Safety factor (5%) 4.1
Total head 86.3 (round to 86 ft w.c.)
The preferred selection range for a centrifugal pump is between 85 and 105% of the flow at the best efficiency point (BEP) on the pump curve. Refer to Chap. 43 of the 2008
ASHRAE Handbook—HVAC Systems and Equipment for more information on the physical characteristics and selection procedures of centrifugal pumps.
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Sufficient access must be provided around the pump for proper maintenance and testing. Typically, 12 to 18 in. of clearance on all sides of an end-suction pump provides sufficient access for maintenance.