A 4-pipe heating and cooling plant contains both central heating and cooling equipment and is capable of delivering heating water and chilled water to the building simultaneously through four pipes (one heating water supply, one heating water return, one chilled water supply, and one chilled water return). Heating and cooling equipment within the building that is connected to a 4-pipe system will have four pipe connections, unless the equipment provides either heating only or cooling only. In this case, the equipment would have only two pipe connections.
Above Figure is a schematic diagram of the piping for a 4-pipe heating and cooling plant that utilizes two condensing hot water boilers and two water-cooled chillers. The pumping arrangement is primary-secondary for both the heating water and chilled water systems. Both the heating and chilled water systems are variable flow systems with variable frequency drives controlling the speed of the (secondary) heating and chilled water system pumps. One of the two pumps shown for the heating and chilled water system pumps and one of the condenser water pumps is a standby pump. A separate condenser water pump and cooling tower is dedicated to each chiller. Automatic shutoff valves are designed for the condenser water supply, return, and equalizer piping connections to isolate the idle cooling tower when only one chiller is operating.
A 2-pipe heating and cooling plant contains both central heating and cooling equipment but is not capable of delivering heating water and chilled water to the building simultaneously. It operates either in the heating mode or cooling mode and delivers either heating water or chilled water through two pipes (one dual-temperature water supply and one dual-temperature water return) to the building. Heating and cooling equipment within the building that is connected to a 2-pipe system will have two pipe connections.
Above Figure is a schematic diagram of the piping for a 2-pipe heating and cooling plant that ut:ili7.es two condensing hot water boilers and one watet'<ooled chiller. The pumping arrangement when the plant is operating in the heating mode is a primarysecondary pumping system with a primary pump dedicated to each boiler to ensure a constant flow of water through each condensing boiler. The dual-temperature water system pumps are constant speed and function as the secondary pumps.
One of the two pumps shown for the dual-temperature water system pumps and the condenser water pumps is a standby pump.
In the cooling mode, the plant operates in a primary~nly pumping arrangement. In this arrangement, the dual-temperature water system has to be a constant-flow system in order to maintain a constant flow of water through the chiller during cooling operation. If a primary pump were designed for the chiller, the dual-temperature water system could be a variable flow system with variable frequency drives controlling the speed of the (secondary) dual-temperature water pumps.
Design considerations for 4-pipe and 2-pipe heating and cooling plants are as follows:
It is common to design redundancy for the equipment in heating systems (such as boilers and pumps) because freezing of the building could occur if the heating system is lost. On the other hand, it is not common to design redundancy for the equipment in cooling systems (such as chillers and pumps) because comfort cooling is generally not considered critical. However, cooling systems serving critical functions, such as computer or health care facilities, may requite redundant cooling equipment.
Since some redundancy in the boilers is normally required, it is common for each of the two boilers in a 4-pipe or 2-pipe system to be sized for two-thirds of the peak heating load of the building. This provides 67% redundancy to keep the building temperature above freezing if one boiler fails.
For small systems, it is common to utilize a constant-flow, primary-only pumping system. However, for larger systems (where pumping energy is significant), a primary-secondary pumping system is recommended because the system (or secondary) flow can be varied to reduce the energy use of the secondary pump. In a primary-secondary pumping system, each piece of primary equipment, such as a boiler or chiller, has a dedicated primary pump. Energy savings are also achieved with primary-secondary pumping systems by staging on the primary equipment (and associated pumps) in response to the system load.
Figures above illustrate a constant-flow, primary-only pumping system and a primary-secondary pumping system. Note that a primary-secondary pumping system requires a common pipe that joins the primary and secondary pumping loops. The common pipe should be sized for the full secondary flow and should be a maximum of 10 pipe diameters long in order to reduce any unwanted mixing and to lceep the pressure loss through this pipe to an absolute minimum.
It is common to provide full redundancy for the system pump (or secondary pump in a primary-secondary pumping system) by designing two pumps, each sized to circulate the full flow. One pump will always be running while the other pump is available on a standby basis should the lead pump fail.
A primary-secondary pumping system is almost always used for high-efficiency (condensing) boilers because of their need for constant water flow. Some high-efficiency boilers are equipped with primary pumps installed within the boilers themselves to ensure that the heat exchangers receive the minimum required water flow. As mentioned earlier in this chapter, some condensing boilers no longer require a minimum flow rate for proper operation. As a result, these boilers can be connected to a heating water system that utilizes a variableflow, primary-only pumping arrangement.
A common control strategy for heating water systems is to reset the temperature of the heating water supplied to the heating equipment in the building based on outdoor temperature. This strategy allows for better control of space temperature and also reduces the heat loss from the heating water piping system during part-load operation.
A common heating water reset schedule for noncondensing boilers is as follows:
- 180°F heating water supply temperature when the outdoor temperature is 0°F.
- 140°F heating water supply temperature when the outdoor temperature is 50°F.
The heating water supply temperature varies proportionally between 180 and 140°F as the outdoor temperature varies between 0 and 50°F.
However, as mentioned earlier, noncondensing boilers must maintain a minimum of 140°F returning water temperature; thus it would not be possible to achieve the reset schedule listed above by resetting the heating water supply temperature from the boilers. Therefore, the addition of a 3-way mixing valve to blend heating water return with heating water supply is required to reset the heating water supply temperature based on outdoor temperature.
A common heating water reset schedule for condensing boilers is as follows:
- 140°F heating water supply temperature when the outdoor temperature is 0°F.
- 90°F heating water supply temperature when the outdoor temperature is 50°F.
The heating water supply temperature varies proportionally between 140 and 90°F as the outdoor temperature varies between 0 and 50°F.
Heating water temperature reset is accomplished with condensing boilers simply by resetting the heating water supply temperature from the boilers based on the outdoor temperature. As mentioned previously, the efficiency of condensing boilers increases as the returning water temperature decreases.
It is best to utilize the same pipe sizing criteria for the central plant that is used for the distribution system.
The makeup water assembly for all closed systems consists of a backflow preventer, pressure-reducing valve, and shutoff valves.
The boiler should be installed at the point of lowest pressure developed by the heating water system pump (suction side of the pump) for the reasons discussed earlier.
For cooling plants consisting of multiple water-cooled chillers, it is common for each chiller to have a dedicated cooling tower (or cooling tower cell within a multiple-cell cooling tower) and a dedicated condenser water pump. An additional condenser water pump can serve as a standby pump for every two condenser water systems, provided the systems require the same water flow rate and appropriate valves are installed to isolate the pumps.
For central cooling plants having only one chiller and one cooling tower, it is possible for a third pump to function as a standby pump for both the chilled water and condenser water systems, provided the pump has a suitable operating point for both systems.
One major disadvantage of 2-pipe heating and cooling systems is the time that it takes to accomplish the changeover from heating operation to cooling operation in the spring of each year because chillers generally cannot tolerate an entering water temperature to the evaporator that is greater than 70°F. Therefore, the dualtemperature water loop must cool down from a heating water temperature that is at least 140°F (for noncondensing boilers) to 70°F before dual-temperature water can be circulated through the chiller evaporator and chilled water can be produced.
The problem with this is that when the building is calling for cooling, there is no demand for heat. Thus, there is no way for the warm water in the dualtemperature water system to reject its heat. The dual temperature water loop must cool down as the result of heat losses from the insulated dual-temperature water piping, which can take up to 2 or 3 days, depending upon the size of the system.
A solution to this problem is available if the chillers are water-cooled. The changeover time can be greatly reduced through the incorporation of a dualtemperature water cool-down system. This system utilizes the cooling tower as a source of heat rejection for the dual-temperature water system when it is in the heating mode. The addition of a plate and frame heat exchanger, 3-way diverting valves, and controls are necessary to accomplish this mode of operation, the details of which are beyond the scope of this book.
HVAC Design Sourcebook - W. Larsen Angel, P.E., LEED AP, is a principal in the MEP consulting engineering firm Green Building Energy Engineers