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"According to the 'Practical Heating and Air Conditioning Design Handbook,' page 186, 'Table 3.2-1 lists the meteorological parameters of 270 stations as specified in the Code for Design of Heating Ventilation and Air Conditioning (GB50019-2003). These data meet the statistical requirements of the regulations.' However, the compiler of Table 3.2-1 in this handbook appears to have misunderstood the original intent of the 'Code for Design of Heating, Ventilation and Air Conditioning,' resulting in inaccurate values. As a result, most design agencies do not recognize or adopt these figures. Until new authoritative data are available, the values listed in the appendix of GBJ19-87 remain more appropriate.
It is worth noting that the 'Practical Heating and Air Conditioning Design Manual' is currently revising Table 3.2-1. In practice, technical data such as design manuals, standard drawings, and technical specifications should not carry the same normative authority. This distinction is essential for ensuring accurate and reliable engineering practices.
There are numerous key issues in heating and air conditioning systems, including:
1. Problems in the heating (air conditioning) water system
2. Constant pressure and water supply in the water system
3. Hydrostatic test pressure
4. Thermal expansion in pipes and compensation
5. Vibration reduction and noise control design
6. Proper use of various regulating valves
7. Issues in public building ventilation
8. Several 'edge' issues in smoke prevention design
9. Selection of heat sources, cold sources, and heating/air conditioning modes
10. All-air terminal variable air volume systems
11. Cool and warm radiation air-conditioning heating
12. Addressing inner and outer area cooling challenges
13. Long-standing cooling issues
14. Ventilation and air conditioning design for biosafety laboratories
15. VRV systems for atmospheric boilers and radiant heating
16. Plastic pipes
17. Ground source heat pumps and geothermal cascading systems
18. Multi-angle thinking on electric heating
19. Pump hydraulics: features, common faults, and misunderstandings
20. Several aspects of better regulation control methods
The complexity of heating and air conditioning engineering lies in its dual demands: achieving thermal balance for multiple spaces simultaneously and adjusting dynamically to changing external conditions. Unlike simple heating, which focuses on balancing heat gain and loss, air conditioning also requires managing humidity. The challenge becomes even greater when dealing with dynamic systems where thermal performance varies across different rooms.
In real-world applications, the actual process of a heating or air conditioning water system does not follow an isothermal temperature drop as assumed in design calculations. Instead, it involves variable temperature drops due to hydraulic imbalances. Parallel loops often fail to achieve full hydraulic balance, leading to differences in return water temperatures and affecting heat delivery. This deviation can significantly impact indoor comfort.
The goal of hydraulic balance is to approach an isothermal temperature drop as closely as possible. For example, in a hot water heating system designed for 85/60°C with a 25°C temperature drop, any deviation from this will affect the system’s performance. Similarly, in an air conditioning system with a 7/12°C temperature rise, improper hydraulic balance can lead to deviations in both temperature and humidity, making it difficult to maintain design conditions.
Despite the importance of hydraulic balance, many engineers still rely on simplified methods, such as selecting pipe diameters based on flow rate and friction loss, rather than following the principles of hydraulic balance. This often leads to uneven heating or cooling in buildings.
While automatic adjustment systems exist, they are not always economically viable or practical. In many cases, careful design and proper configuration of the system can achieve acceptable results without overcomplicating the system.
According to GB50019-2003, the relative difference in pressure loss between parallel loops in a hot water heating system should not exceed 15%. For air conditioning systems, similar rules apply. Although the exact reason for the 15% threshold is not fully explained in the code, it is based on practical experience and ensures acceptable performance without excessive complexity.
Achieving this level of balance typically involves dividing the system into evenly sized loops, selecting appropriate pipe sizes, and using hydraulic balancing devices where necessary. By following these steps, engineers can ensure that the system performs reliably and efficiently.
In summary, while there are many challenges in designing heating and air conditioning systems, a solid understanding of hydraulic balance and proper implementation of design principles can greatly improve system performance and user comfort."