A LL -W ATER S YSTEMS
8.4 TWO-PIPE SYSTEMS .1 Basic Two-Pipe System
This system is composed of central water cooling and heating equipment, terminal units, pumps, distribution piping, and system and terminal controls. Each terminal is connected to a single supply pipe and a single return pipe. In the cooling mode, chilled water cir-culates to the terminals to accomplish both sensible and latent cool-ing. In the heating mode, hot water circulates to the terminals for sensible heating. Room temperature is controlled at the terminal by automatically or manually regulating air or water flow or both. In its simplest form, the entire system must be in either cooling or heating mode. Some spaces served by such a system, however, may require cooling, while others need heating, resulting in unsatisfac-tory performance. It may be possible to group spaces with common heating or cooling requirements together into a single zone. By design of the circulating system and its control, some zones can be operated on cooling while others are on heating. The zoning, piping arrangement, and controls to achieve this limited flexibility within a two-pipe arrangement demand careful design.
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Zones can frequently be divided according to exposure. Zoning is practical for spaces where solar heat gains during spring and fall are the predominant factor in establishing a need for heating or cooling. Shadows from other buildings or wings that vary through-out the day (not just at peak-load time), however, can defeat this approach. Occasionally each major piping branch, perhaps a single set of risers, is designed to serve one zone. The mode (heating or cooling) for terminals connected to that branch may be indexed to a key space thermostat or discriminator control circuitry. This config-uration is actually a cross between the two-pipe concept and the four-pipe concept described later.
The piping arrangement illustrated in Figure 8-1 contains both a heating and a cooling pump. When zones require both condition-ing modes, both hot and chilled water are available. The chilled-water pump must have adequate capacity to maintain the minimum required flow through the chiller. If only cooling is needed, the hot-water pump can be turned off. If only heating is needed, the chilled-water pump can be turned off. It is feasible to use the two pumps operating in parallel to circulate chilled water. Operating in this mode, hot water is not available to the system, but total pumping capacity can be shared. During intermediate seasons, and possibly throughout the heating season as well, partial system water flow is usually adequate to meet the space thermal needs. It is important to
Figure 8-1. Multizone two-pipe all-water system (Carrier 1965).
lay out the return piping in a manner that will avoid the blending of hot and chilled water upstream of the chiller or hot water generator.
Control valves on both supply and return circuits are necessary to switch to the appropriate source for a zone. Changeover from one mode to the other can be a difficult choice, since the required mode depends upon several factors, such as outdoor temperature and solar load. Solar-compensated outdoor temperature sensors lagged to sim-ulate the building’s thermal storage or “flywheel” characteristics have been employed. Keying to selected room thermostats and manual switching are other techniques of zone cooling/heating changeover.
Unfortunately, all these control concepts have shortcomings.
The foregoing discussion suggests that system design may become quite complicated in order to provide appropriate zoning.
The additional expense for piping, pumps, and controls may rival or exceed that of an inherently superior system concept, which is to provide a cooling/heating alternative at each terminal using a four-pipe system. The temperature in rooms equipped with fan-coil units is maintained by fan ON/OFF or speed control, by ON/OFF or variable water flow control, or by both. Vertical riser units with series-connected terminal units can be controlled by bypassing air around the heat transfer surface using face and bypass dampers.
Varying unit water flow as a method of room temperature control is not feasible through units arranged in series.
Unless the conditioned space is dehumidified by a supplemen-tal air-conditioning system, both sensible and latent cooling are per-formed by the terminal unit coil. The sensible heat ratio of the cooling process in a given space will vary widely over time. This variation will be most extreme if ventilation air is introduced through windows or unit intakes in an exterior wall. Since room temperature is the basis for unit control, humidity control is inci-dental. Relative humidity can vary widely, as depicted in Figures 8-2 and 8-3. Control solely by manually operating the fan is unsatisfactory. Automatic control of fan operation with manual selection of fan speed results in intermittent airflow through the occupied space, temperature swings near the outside wall, and potentially annoying sound pressure level variations. These charac-teristics are similar, however, to those of residential forced-air heat-ing systems, and in many multi-residence applications of fan-coil units they have been accepted as tolerable. As Figures 8-2 and 8-3 indicate, superior control of humidity is more likely with fan con-trol than with water flow concon-trol because humid outdoor air may continue to be brought in even though the thermostat is satisfied
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and the chilled-water flow is at a minimum while the fan continues to run. Tight-sealing dampers on wall openings should be automati-cally closed when the unit fan is off. Use of water flow control in combination with ventilation via wall apertures incurs a real risk of coil freeze-up in cold weather.
Manual fan control along with ON/OFF or modulating control of water flow can be viewed as an improvement to automatic fan con-trol in other respects (although this approach can lose concon-trol over humidity). This is true of cost (which is higher) as well as perfor-mance. ON/OFF water flow control can cause frequent cycling of the supply air temperature—although potentially annoying, this is the same effect as seen with thermostatic control of a DX system. Qui-Figure 8-2. Room temperature and humidity variation under manual three-speed fan control (Carrier 1965).
etly operating solenoid valves can minimize the bothersome clicks that occur as the valves are actuated by the thermostats. A room thermostat is preferred over a return air thermostat, since it will sense the occupied space temperature even if the fan is off. When the system mode is changed, a second thermostat that senses system water temperature (an aquastat, available with the terminal) must reverse the action of the room thermostat.
8.4.2 Two-Pipe System plus Supplemental Heating Fan-coil units can be obtained with auxiliary electric heating coils. This approach can materially reduce the difficulties of changeover between cooling and heating modes. As long as cooling by chilled water circulated through the terminals is necessary to counter solar heat gain and other transient cooling loads, overcool-ing of other spaces with a net heat loss on the same system can be prevented by energizing the electric coil. This will probably elimi-nate the need for the complex zoning layout previously described.
Even if total heating by electric resistance heat is not cost-effective, Figure 8-3. Room temperature and humidity variation under auto-matically modulated water control with continuous fan operation (Car-rier 1965).
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electric heating during the intermediate seasons may be justified by reduced piping and control costs as well as by a significant perfor-mance improvement balanced against increased unit and electrical distribution costs.
Control of cooling must be by chilled-water flow valves or by face and bypass dampers. Such control is then sequenced with ON/
OFF control of the electric heater energized through a magnetic contactor. Both cooling and heating are under control of a common thermostat. When hot water is supplied for heating, the unit aqu-astat reverses the control action of the room thermostat on the water valve and de-energizes the electric heater circuit. Only heat-ing by hot water is then available. The electric heater is de-ener-gized whenever the fan is not operating. The system mode changeover decision is less critical but still involves the numerous variables previously mentioned. More often than not, the pivotal rationale for the decision is economic. Owners are reluctant to incur monthly electrical demand charges for short-period operation of their refrigeration plants. Therefore, the changeover decision is governed by the calendar—so many months on cooling and so many on heating. Not infrequently, the months when cooling will be available become a lease provision, but many tenants will not accept such a lease restriction.
8.4.3 Two-Pipe Systems with Total Electric Heat
If electric heating is economically justifiable throughout the year, the need to change water supply from cooling to heating and vice versa is eliminated. Unit control is simplified; reversa1 of the room thermostat action is unnecessary. Chilled-water circulation and refrigeration can be discontinued when there is little or no need for cooling, and zoning of distribution circuits is unnecessary. A significant advantage is that the electric power source for heating and air circulation can be via the tenant connection and can thus be separately metered. For some occupancies, in some jurisdictions, this is a legal requirement as an energy conservation measure.