Abstract: Taking Xinxiang Central Hospital as an example, this paper briefly introduces the design of its building automation control system and the selection of system equipment. It focuses on the special processes involved in air conditioning commissioning and the issues that require attention.
Keywords: Fieldbus professional module, temperature, humidity, pressure difference, air volume
Building automatic design of XinXiang hospital
By Yu Guodong, Xu Tao, Gao Zhihong
Abstract: Taking the XinXiang hospital as an engineer background, the paper briefly introduced the design and device selection of its building automatic control system, with pay more attention to the special technics and caused problems need to care on the course of air condition testing.
Keywords: fieldbus ,special module ,temperature and humidity ,pressure difference ,discharge air volume
Xinxiang Central Hospital is the largest comprehensive hospital in Northwest Henan Province, integrating medical treatment, teaching, prevention, scientific research, and rehabilitation. To better serve patients, Xinxiang Central Hospital continuously improves its hardware facilities and medical environment. During the construction of the surgical building, the TECHCON intelligent control system was introduced to centrally manage and monitor the building's air conditioning, ventilation, water supply and drainage, and power systems. This aims to meet the stringent requirements of medical staff and patients regarding temperature, humidity, and ventilation conditions, reducing the likelihood of infection due to unclean air (bacteria), shortening treatment cycles, and improving cure rates. This initiative will create a brand-new intelligent treatment space, comprehensively enhancing the hospital's service level, and while improving service quality, it will also conserve energy, achieving a dual advantage in both service and energy efficiency.
The building control system at Xinxiang Central Hospital primarily manages the air conditioning in the ICU wards, all inpatient wards, hospital offices, and some operating rooms across 19 floors. In addition to cleanroom air conditioning, the system design includes numerous electromechanical devices such as fresh air handling units, supply and exhaust fans, water supply and drainage systems, heat and cold sources, and medical gas systems. These devices must be capable of independent operation while also coordinating with related equipment, and simultaneously transmitting data back to the host computer for analysis and reference. The design and commissioning of the building control system for this project focused on three main aspects.
I. System Structure
Given the large number and highly dispersed nature of hospital equipment, we adopted the Techcon 04 control system. The following diagram shows the system topology:
The Techcon 04 system network structure is a three-layer architecture: the bottom layer is the module connection bus, the middle layer is the station connection bus, and the top layer is the management network. The module connection bus connects the main control module and the I/O modules; its network topology is based on a CAN physical layer bus structure with a communication rate of 57600bps. The middle-layer station connection bus also uses the CAN physical layer, with a communication rate of 38400bps.
The site connection bus adopts the CANBUS standard at the physical layer and follows the BACnet definition rules at the link layer, enabling the connection of 64 master controllers with a site communication rate of 38400bps. This communication connection method is particularly suitable for building control systems, where a single network segment is typically sufficient for the communication network of a building. Each master controller can manage up to 15 I/O modules via the module connection bus. These modules can exchange data via cables extending up to 1200 meters without the need for any additional equipment (such as dedicated bases or dedicated power modules). A new type of CAN hub was used at Xinxiang Hospital to solve the communication problems of a large system. It breaks the limitations of the traditional linear topology and can form complex topology networks such as star and tree topologies. With the help of the CAN hub, the site connection bus can achieve network segment extension within a 4-layer range, with each CAN network segment reaching a maximum distance of 1.2 kilometers. Therefore, the CAN-type site connection bus can achieve data communication within a 5-kilometer range.
With this network topology, the system can easily cover tens of square kilometers without relying on Ethernet. In Xinxiang Hospital, a CAN hub was used to directly divide the system into two segments: one bus manages the basement equipment, and the other manages the electromechanical equipment above ground. This division makes the system layout very clear, while also leaving one bus for redundancy, allowing for the laying of communication cables as needed to reduce investment in upgrades.
II. Module Features
While various I/O modules exist in mainstream control systems today, comprehensive I/O modules are relatively few. At Xinxiang Central Hospital, the Techcon series of new modules with multiple reasonable placement points were adopted, as detailed in the table below:
The table shows that one GCA module can completely manage one fresh air handling unit; one MCA module can control four water pumps or blowers and monitor related feedback; one MCC module can control two motors and manage corresponding liquid levels; and one VDA module can control four dual DO butterfly valves, and can effectively monitor valves regardless of whether they use DI or AI feedback. In this project, corresponding professional I/O modules were used based on the system characteristics, which greatly saved investment costs. Furthermore, the main equipment can be controlled with only one or two modules, and the layout is highly replicable and referable. Since one main control module can carry 15 I/O modules, only 10 I/O modules are typically managed during configuration. This approach creates true redundancy in the system. If the client raises new requirements or overlooks certain aspects during design, these can be easily addressed without disrupting the original system structure.
III. Special Control Technology
Xinxiang Central Hospital uses a semi-centralized air conditioning system, which is very suitable for large hospitals like Xinxiang Hospital with high simultaneous usage of operating rooms. This system divides the air conditioning systems of the operating department and inpatient wards into several zones. High-level air conditioning systems, due to their large air volume and relatively low usage frequency, use separate air conditioning units; low-level systems share the same air handling system among several rooms, with fresh air centrally processed and then delivered to the air conditioning units of each system. To ensure the entire operating department remains under control, a standby fan is installed, which operates only when not in use to maintain the pressure gradient within the operating department. This system configuration allows for easy adjustment of temperature and humidity in each room, ensuring that adjustments at different pressure levels do not interfere with each other, while also achieving good pressure control throughout the entire operating department.
Xinxiang Central Hospital has one or two cleanroom air conditioning units on each floor. Cleanroom air conditioning units differ significantly from conventional combined air conditioning units. The monitoring equipment mainly includes: return air temperature, return air humidity, primary filter clogging alarm, fresh air primary filter clogging alarm, medium-efficiency filter clogging alarm, high-efficiency filter clogging alarm, cold coil water temperature regulation, hot coil water temperature regulation, steam humidifier control, fresh air damper opening control, return air damper opening control, supply fan operating status, supply fan manual/automatic control status, supply fan pressure difference before and after, supply fan start/stop control, supply fan fault status, supply fan frequency converter control signal, operating room temperature and humidity, operating room slightly positive pressure, return air fan operating status, return air fan fault status, return air fan pressure difference before and after, return air fan start/stop control, return air fan manual/automatic control status, anti-freeze switch, etc.
The authors have given considerable thought to the environmental control solutions for ICUs, operating rooms, and wards. The bio-cleanliness technology employed has shifted from the previous approach of directly killing pathogenic microorganisms in the room, focusing instead on eliminating or controlling factors that threaten sterile spaces. This has resulted in a comprehensive set of technical measures, primarily reflected in the following aspects:
1. Proper temperature and humidity control [3]
Most specifications only provide the range of temperature and humidity: 22℃～25℃ for temperature and 30%～60% for relative humidity. The author also made adjustments based on these values during the design and debugging process. However, during the trial operation and service period, it was found that many settings needed to be adjusted, and generalizations should not be made.
During discussions with the hospital, we learned that appropriate air conditioning can promote treatment and recuperation, and even be an important treatment method in some situations. However, different patients often have very different requirements for air conditioning. For example, hyperthyroidism patients cannot tolerate high temperatures and humidity; chronic lung disease patients need to inhale hot and humid air to effectively prevent dehydration due to the accumulation and increased viscosity of secretions; burn patients need a high environmental requirement, with room temperature generally adjusted to 32°C and relative humidity to 95%; and cardiovascular and cerebrovascular patients can receive good treatment in cool and dry rooms.
Because different wards have significantly different temperature requirements, and patients also have different environmental needs, the authors zoned the air conditioning systems according to actual usage. A closed-loop control algorithm was used in areas with similar functions to meet the basic needs of patients within those areas. For patients with special needs, indoor heating or localized humidification was used to meet their requirements.
2. Air filtration[6]
The project employed a three-stage air filtration system. According to the national standard GB/T14295-93, air filters are divided into five levels. This project used pre-filters, medium-efficiency filters, and high-efficiency filters. However, in actual use, the performance of the high-efficiency filters in the project was essentially equivalent to that of the medium- and high-efficiency filters in the national standard. The table below shows the performance of the three-stage filters used in the project:
Since bacteria cannot survive independently and must attach to larger airborne particles, filtering these particles carrying microorganisms removes them. Air filtration is the most economical and effective method for removing bacteria.
3. Air volume
According to relevant international standards, the permissible air supply volume for operating rooms with return air is approximately 20-25 air changes per hour. This standard is equivalent to Class III clean operating rooms in the national standard GB50333-2002, "Technical Specification for Clean Operating Room Buildings in Hospitals". However, in the current general concept of operating rooms, from a purification principle perspective, increasing the air supply volume does not improve the indoor conditions for operating rooms with high-efficiency filters at the terminal. The author considers setting a minimum air change rate of 6 times per hour for 100% fresh air intake. In this project, the air supply volume should be determined based on the indoor load, but it should generally not be lower than 6 air changes per hour, and is generally controlled between 6 and 8 air changes per hour. This level is equivalent to the air supply and ventilation requirements for trauma operating rooms abroad. When controlling the air conditioner, enthalpy control is used to determine the minimum allowable air volume. The specific calculation method is as follows: Indoor state point N, dry bulb temperature tN=25.5℃, relative humidity φ=50%, iN=51.9kJ/kg; Outdoor state point W, dry bulb temperature tW=33℃, wet bulb temperature tWS=27.9℃, iW=89.7kJ/kg; fresh air ratio 0.252, supply and return air are mixed to point C before treatment, tC=27.1℃, iC=59.5kJ/kg. Given a summer heat-to-moisture ratio ε = 12000, and dew point air supply at point S (where φ = 95% relative humidity) along the heat-to-moisture ratio line from point N, with tS = 12.5℃, the enthalpy difference between points N and S is Δi = iN - iS = 51.9 - 33.4 = 18.5 kJ/kg. However, based on the summer cooling load and total air supply volume, the required enthalpy difference is Δi' = Q/G = 18.5 kJ/kg. Therefore, the preset air change rate is too low. By reverse calculation, G = Q/Δi = 896 m³/h, and an air change rate of 6.7 times/hour (fresh air ratio 0.252) is suitable for dew point air supply, thus achieving the minimum air volume.
4. Positive pressure
In order to maintain its sterility, positive pressure was considered in the control of the operating room of Xinxiang Hospital. The positive pressure control of the room was affected by the micro positive pressure sensor installed in the operating room, and even the airflow direction and orderly pressure distribution between areas with different cleanliness levels in the operating room were maintained to avoid the influence of the outside on the inside and the lower level on the higher level environment. Only by ensuring that the orderly gradient pressure distribution in the clean operating room can be maintained under any circumstances (especially under non-design conditions or abnormal operating conditions) can the risk of cross-infection in the operating area be effectively reduced. The standard recommended by the US CDC (Centers for Disease Control and Prevention) and HICPAC (the Healthcare Infection Control Practices Advisory Committee) in 2003 set the negative pressure value of the negative pressure isolation ward at 2.5 Pa (0.01 in H2O) [7].
To date, there are no clear regulations in my country's relevant design standards regarding the negative pressure value of negative pressure isolation wards, and the national standard "Code for Design of Infectious Disease Hospitals" (discussion draft) does not specify a specific negative pressure value.
During the commissioning process, positive pressure plays a crucial role in three aspects: First, when doors and windows are closed, it prevents contaminants from outside the cleanroom from seeping into the cleanroom through gaps; second, it ensures sufficient airflow to the outside when doors open, reducing interference from the moment doors open or people enter, but it does not guarantee positive pressure when doors are open for extended periods; and finally, it ensures a reasonable and orderly airflow direction and volume within the clean area. In Xinxiang Hospital, during trial and operation, the authors set the minimum static pressure difference between the negative pressure isolation ward and the buffer room to 8 Pa, the minimum static pressure difference between the ward and the restroom to 4 Pa, and the minimum static pressure difference between the buffer room and the medical staff work corridor, as well as between the semi-contaminated area and the clean area, to 4 Pa. This achieved good results and was also approved by the hospital.
Engineering Design and Debugging Experience
The primary concern for hospital building control systems, derived from the Xinxiang Central Hospital model, is communication. After establishing the system using a robust bus topology, the focus of the entire project shifts to the specific needs of the hospital: a conducive indoor environment for patient recovery while effectively preventing cross-infection. National standards only stipulate that the supply air volume in clean areas must be greater than the exhaust air volume, creating positive pressure in clean areas, while contaminated areas must have an exhaust air volume greater than the supply air volume, creating negative pressure. Furthermore, negative pressure rooms are required to have an exhaust air volume at least 10% greater than the supply air volume (an air volume difference of no less than 85 m³/h). The standards do not specify the required positive or negative pressure values for each area, nor do they specify requirements for the building envelope or the sealing of doors and windows. Without a basis for setting a micro-pressure difference during the design and commissioning phases, adjustments can only be made during practical commissioning and trial operation. The actual commissioning results are difficult to verify, relying solely on theoretical calculations. Moreover, relying solely on the supply and exhaust air volume difference without specifying requirements for room airtightness during construction and operation makes it difficult to guarantee the achievement of the pressure gradient.
References
[1] Techcon System User Guide
[2] Techcon 409 IO module User Guide
[3] ASHRAE handbook, HVAC Application Chapter 7 healthcare facilities, 1999
[4] Chen Yin, Discussion on the design of air conditioning system for operating room purification, Air Conditioning and Heating Technology, 2001(1).1-4
[5] Brian Wisemen, PE room pressure for critical environment ASHRAE Journal February 2003 vol.109,34-39
[6] Qiu Jifu. Research on Evaluation Methods of Antibacterial Filters: [Master's Thesis]. Shanghai: Tongji University, 2005
[7] CDC. Guidelines for preventing the transmission of mycobacterium tuberculosis in health care facilities[R]. Morbidity and Mortality Weekly Report, 1994,43, No RR-1
