Building automatic control system

Facility managers of buildings must consider many factors when investing in building automation systems to manage the building environment. Cost is usually the first factor, but other factors such as system reliability, maintenance cost, and whether ready-made DDC can be provided can not be ignored. In the long run, all these factors will affect operating costs.

The most obvious way that system reliability affects the cost is that when the DDC in the network needs to be replaced, if the DDC is easy to fail, the replacement cost will increase. The configuration of DDC also plays a vital role in the reliability of the whole system. In the control application in the early years, most or all tasks were usually assigned to a large DDC with strong processing capacity within the scope permitted by wiring. This method usually assigns unrelated functions to the same general controller. This means that once the DDC fails, all related functions will be lost, thus affecting the normal work of multiple devices. Assigning multiple functions to a large DDC also means that finding and repairing faults is a time-consuming process, because the system is configured in logical groups and has no functions, and replacing a large DDC is more expensive than replacing a small DDC. Based on the above factors, the current practice has developed in a more modular direction. In order to overcome the shortcomings of using large DDC, the concept of dedicated DDC appeared. In other words, each AS DDC is dedicated to a certain logic-related specific function, such as DDC dedicated to VAV junction box, fan coil unit, area lighting control, etc. This DDC focuses on a specific function, that is, modularity. Once a fault occurs, it will be easy to find and repair. A device failure will be found immediately and the corresponding controller will be found, and it will be cheaper to replace a small controller. Because of its "single" function, large-scale multi-functional DDC has achieved multi-functional tasks. Once it fails, it needs to be replaced, which leads to an increase in cost.

A small controller does not mean that it is weaker than a large controller. With the development of technology, small controllers have the same decision-making ability as large controllers, and their ability comes from the more intelligent control strategy of software (also known as firmware) solidified on each dedicated controller. Each modular controller can monitor the input quantity and independently make appropriate control output for the controlled equipment, which is the intelligent DDC. This kind of intelligence and decision-making ability independent of the system server is very important to the reliability of the system. As long as the power supply is not interrupted, the DDC independent of the server can continue to work, ensuring that the control of the equipment is not affected. Even if the server works abnormally or the network communication is interrupted.

When considering simple equipment such as fan coil and lighting, dedicated one-to-one DDC is an ideal application. Many manufacturers produce controllers for simple equipment such as VAV terminals, but only a few manufacturers are willing to produce special controllers for large HVAC equipment such as coolers and related pumps and cooling towers. Part of the reason is that one-to-one application will need more DDCs to realize different control functions in water chillers compared with the number of lighting or fan coil DDCs needed in buildings, but the small number naturally means that DDCs dedicated to large equipment are expensive, so the control of many water chillers is still the domain of general DDCs (relative to dedicated DDCs). The control is realized by the system integrator on-site programming as required.

The result of this practice is that the cost is still high, not for DDC manufacturers, but for owners. Specially programmed universal DDC is not modular. Direct one-on-one replacement may cause problems. If the complete programming logic is not fully documented, the failure of DDC is expensive. Because it will be a complicated process to replace the configuration of general DDC before recovery, its programming does not follow specific standards, but is written by the integrator's engineers according to the situation, that is to say, each configured system may be different. If the original system integration engineer no longer undertakes the replacement work, the problems encountered will directly lead to expensive repair costs.

Dedicated DDC is different, because the replacement is simple, direct and modular, so it is a plug-and-play way. In addition, if the newly-built system chooses widely used open communication protocols, such as LonWorks network, it can even choose different brands to replace DDC, thus saving costs, because the owners can choose products with price advantages.

The main reason for the lack of dedicated DDC for large equipment is that there is no in-depth study on the module division of different equipment groups. The new method adopted by Mike to intelligently control UCP2484L is to group modules according to logical functions, instead of assigning a DDC to each set of equipment of the water chiller.

Each chiller must be equipped with corresponding pumps, which are logically related and run in a certain order. Grouping these devices into "each cooler" means that modularity is indeed possible. DDC with this function, UCP2484L intelligently controlled by Mike, becomes a special DDC in this way, which controls all equipment in each water chiller, including chilled water pump, cooling water pump and cooling tower.

It should be noted that logical grouping is vertical, not horizontal. That is, packets span different types of devices. Horizontal grouping of devices of the same type can also achieve the purpose of exclusive grouping of logical functions. But at the cost of reliability. Because of the possibility of DDC failure, although this possibility is very low as an industrial digital product. Horizontal grouping of equipment means that after any DDC fails, the whole refrigeration room will automatically fail. For example, when DDC, which is responsible for the operation of chilled water pump, fails, all chillers will not be started, and the whole cold and heat source system will not work automatically. The failure rate is equivalent to the total failure rate of all monitoring chillers DDC in the freezer.

From the vertical grouping of equipment, it can be seen that any failure of Mike-controlled UCP2484L or a set of internal equipment (such as cooling tower) will not lead to the automatic failure of the whole system. It will only affect a certain group of units. In all buildings with more than one water chiller, more than one Mike can be used to intelligently control UCP2484L in the refrigeration room. The other controller UCL0882L is an intelligent group control controller, which coordinates the operation of multiple UCP2484L and determines the number of water chillers to be started according to the cooling load demand. Contrary to the practice that starting water chillers on schedule often leads to excessive cooling capacity when the weather gets warmer, this control greatly reduces energy consumption, because the power consumption of water chillers accounts for more than 50% of the whole building air conditioning system. This control method has achieved remarkable energy-saving effect in the control of water chillers in Singapore Management University, and its air conditioning system is VAV system.

The group controller DDC can also start each chiller in sequence (according to a fixed sequence or operating hours), ensuring that the wear rate of each chiller is basically the same, minimizing machine failures and facilitating maintenance. However, in reality, the configuration of water chillers is often not static. This frequent difference makes it difficult to realize the concept of special DDC for water chillers. There are many different situations, such as using air-cooled units without cooling towers, standby pumps, secondary pumps, and water chillers with different capacities. In some projects, due to limited space, the cooling water pump and cooling tower are placed on the roof, and the water chiller is far away from the ground. Other systems use a balanced piping configuration without a bypass valve.

The second piping diagram is taken from the actual engineering configuration, showing that Mike intelligent control controllers UCP2484L and UCL0882L can also control water systems with chillers and secondary pumps with different capacities (large-capacity units and pumps are marked with the letter "B", small-capacity units are marked with "S", and equipment with the same number is connected to the same DDC). This design also applies two other special DDCs: Mike intelligent control USP2484L, which is used to control the secondary pump group, one is used to control the large-capacity pump group, and the other is used to control the small-capacity pump group.

Even for the design of chilled water systems with different configurations, the flexibility demonstrated by this application is practical and convincing, and it also ensures the application of special DDC. With the current educational trend focusing on biotechnology and information technology, it is difficult to find HVAC engineers with necessary professional experience in equipment automation. Therefore, it is more meaningful to choose the special type than before. All intelligent control strategies are written into the firmware of DDC, so that DDC can handle the control and save manpower.

To be sure, the control of water chillers is gradually developing in the direction of dedicated DDC, because it has various benefits for users and can achieve cost savings.