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' 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: 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: 当室外温度在 0 到 50°F 之间变化时，供暖供水温度在 140 到 90°F 之间成比例变化。 冷凝式锅炉的供暖水温度重置只需根据室外温度重置锅炉的供暖供水温度即可完成。如前所述，冷凝锅炉的效率随着回水温度的降低而提高。 最好对用于分配系统的中央设备采用相同的管道尺寸标准。 所有封闭系统的补给水组件由回流防止器、减压阀和截止阀组成。 出于前面讨论的原因，锅炉应安装在供暖水系统泵产生的最低压力点（泵的吸入侧）。 对于由多个水冷式冷水机组成的冷却设备，每个冷水机通常都有一个专用的冷却塔（或多单元冷却塔内的冷却塔单元）和一个专用的冷凝器水泵。每两个冷凝水系统可以额外配备一台冷凝水泵作为备用泵，前提是系统需要相同的水流量，并且安装了适当的阀门来隔离泵。 对于只有一台冷水机和一台冷却塔的中央冷却设备，第三台泵可以充当冷冻水和冷凝水系统的备用泵，前提是该泵具有适合这两个系统的工作点。 两管加热和冷却系统的一个主要缺点是每年春季完成从加热操作到冷却操作的转换需要时间，因为冷水机组通常不能容忍进入蒸发器的水温高于 70°F。因此，双温水回路必须从至少 140°F（对于非冷凝锅炉）的供暖水温度冷却至 70°F，然后双温水才能循环通过冷水机蒸发器并产生冷冻水。 这样做的问题是，当建筑物需要制冷时，却没有供暖需求。因此，双温水系统中的温水无法散发热量。由于绝缘双温水管道的热量损失，双温水回路必须冷却，这可能需要长达 2 或 3 天的时间，具体取决于系统的大小。 如果冷水机采用水冷，则可以解决此问题。通过采用双温水冷却系统，可以大大缩短转换时间。该系统利用冷却塔作为双温水系统在加热模式下的排热源。要完成这种操作模式，需要添加板框式换热器、三通分流阀和控制器，其详细信息超出了本书的范围。设计考虑因素
HVAC 设计资料手册 - W. Larsen Angel，PE，LEED AP，是 MEP 咨询工程公司 Green Building Energy Engineers 的负责人
