Hot Water Circulating

A centrifugal pump best meets the requirements of this service. Water is usually used in a closed circuit so that there is no static head. The only resistance to flow is that from friction in the piping and fittings, the heater, the heating coils or radiators, and the control valves. In selecting the pump, the total flow resistance at the required flow rate should be calculated as accurately as possible, with some thought as to how much variation there might be as a result of inaccuracy of calculations or changes in the circuit because of installation conditions. It is not good practice to select a pump for a head or capacity considerably higher than that required, as this is likely to result in a higher noise level as well as increased power.

When hot water is used for radiation in a single circuit, through several radiators, the water temperature variation is usually only about 20 F° (11 C°) at the time of maximum requirements, so there is not too great a difference in heat output between the first and last radiator in the circuit. With the flow rate based on water at 180° to 200°F (82 to 93°C) to heat air to about 75°F (24°C), a 10% reduction in the flow would have little effect, as the actual difference would increase to only 22 F° (12°C), and the reduction in the heat output of the radiator with 178°F (81°C) water would be only about 2%. Reference to Section 8.1, on the selection of pumps and the prediction of performance from the head-capacity pump curves and system head-flow rate curves, will show that a rather large undercalculation of circuit head loss would be necessary to produce a flow rate 10% less than desired.

Greater temperature differences are frequently used for other radiation circuits, and a reduced flow rate may have a greater temperature differential than in the single circuit. Whatever the condition, the pump should be selected only after full consideration of all the factors, and not by use of so-called safety factors, which are likely instead to be "trouble factors."

Air in the Circuit Initially, the entire circuit will be full of air that must be displaced by the water. Arrangements should be provided to vent most of the air before the pump is operated. Even if all the air is eliminated at the start, more will be separated from the water when it is heated. Any water added later to replace that lost to revaporation will result in additional trapped air when the water is heated. Means must be provided for continuous air separation, but this cannot be accomplished by vents at high points in the piping because the flow is usually turbulent and the air is not separated at the top of the pipe.

A separator installed before the pump intake will remove the air circulating in the system. In a heating system, an air separation device is often provided at the point where the water leaves the boiler or other heating source. If the pump intake is immediately after this point, this is the point of lowest pressure and highest temperature in the system, and therefore it is the point where separation of air from the water can be most effectively achieved.1

If there are places in the system where the flow is not turbulent, air may accumulate and remain at these points and interfere with heat transfer. Automatic air vents should seldom or never be used. If they are used, it is important that they be located only where the pressure of the water is always above that of the surrounding air, whether the pump is operating or idle. Otherwise, the air vent becomes an air intake.

Several important factors influence the choice of a pump for a hot water system with a number of separate heating coils, each having a separate control. Many systems in the past used three-way valves to change the flow from the coil to the bypass. When two-way valves are used, low-flow operation may occur for a large portion of the operating time. For this type of operation, therefore, the pump selected should have a flat performance curve so the head rise is limited at reduced flows. A very high head rise can cause problems when many of the valves are closed. Excessive flow rates through the coils and greater pressure differences across the control valves are some of the problems that can be avoided with a flat pump curve. A centrifugal pump should not operate very long with zero flow, for it would overheat. This condition is controlled by using one or two three-way valves, a relief bypass, or a continuous small bleed between the supply and the return line. Whichever means is used to control minimum flow, the circuit must be able to dispose of the heat corresponding to the pump power at that operating condition, without reaching a temperature detrimental to the pump.

Types of Pumps Many pumps for hot water circulation are for flow rates and heads in the range of in-line centrifugal pumps that are supported by the pipeline in which they are installed. Such pumps are available up to at least 5 hp (3.73 kW) and operate with good efficiency. More important than the type of pump are the performance and efficiency.

For greater flow rates and heads (and even for the smaller ones), the standard end-suction pump can be used. In the intermediate range, the use of an in-line or end-suction pump is a question not of one being better than the other but whether one or the other is better suited to the overall design and arrangement. Practically all the in-line or end-suction pumps for this service use seals instead of packing.

If the hot water system is of the medium- or high-temperature type, above 250°F (121°C), the pump must be carefully selected for the pressure and temperature at which it will operate.

FIGURE 1 A series loop system

FIGURE 1 A series loop system

FIGURE 2 A one-pipe system
FIGURE 3 A two-pipe reverse-return system.A circuit with primary-secondary pumping provides variable temperature at constant flow rate for two or more coils.

Types of Water Circuit There are several types of water circuits. Those shown in Figures 1 and 2 are suitable for smaller systems and can be used for larger systems by having several of these circuits in parallel. The one shown in Figure 3 is suitable for small or

FIGURE 4 A circuit with primary-secondary pumping to provide variable temperature at constant flow rate for two or more coils

very large circuits, but the reverse return would add considerably to the cost if the circuit extended in one direction instead of in a practically closed loop as shown. For the extended circuit, a simple two-pipe circuit, with proper design for balancing, would be used.

There are a number of reasons for using other circuits, particularly primary-secondary pumping where the system is more extended or complicated, such as continuous circulation branches with controlled temperature. When a coil heats air, part or all of which may be below freezing, the velocity of the water in the tubes and its temperature at any point in the coil must be such that the temperature of the inside surface of the tube is not below freezing. The circuit shown in Figure 4 makes this possible.

Primary-secondary pumping permits flow rates and temperatures in branch circuits to be different from those in the main circuit without the flow and pressure differences in the mains or branches having a significant effect on each other. There are many possible primary-secondary circuits to meet different requirements.

Steam Heating Systems No pumping is required with the smallest and simplest steam systems if there is sufficient level difference between the boiler and condensers (radiators, heating coils, and so on) to provide the required flow. When insufficient head exists between the level of the condensate in the condenser and the boiler to produce the required flow to the boiler, a pump must be introduced to provide the required head. Because the condensate in the hot well will be at or near its saturation temperature and pressure, the only NPSH available to the pump will be the submergence less the losses in the piping between the hot well and the pump. A pump must be selected that will operate on these low values of NPSH without destructive cavitation.

In many cases, particularly for very large systems, vacuum pumps are used to remove both the condensate and air from the condensers. This permits smaller piping for the return of condensate and air, more positive removal of condensate from condensers, and, when high vacuums—above 20 in (0.5 m)—are possible, some control of the temperature at which the steam condenses. The use of vacuum return, particularly with higher vacuums, helps reduce the possibility of frozen heating coils exposed to outside air or to stratified outside and recirculated air. Vacuum return pumps are available for handling air and water. Vacuum, condensate, and boiler-feed pumps with condensate tanks are all available in package form.

Most condensate pumps are centrifugal. Vacuum pumps may be rotary, including a rotary type with a water seal and displacement arrangement.

Fuel Oil When oil burners are fairly far from the oil storage tank or when there are a number of burners at different locations in a building, a fuel oil circulating system is required. The flow rate is relatively low—1 gpm (3.8 liters/min) would provide more than 8,000,000 Btu/h (560,000 Gcal/s or 2,343,000 W)—and a small gear pump is usually used.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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