When the hotel's cooling and heating source system provides simultaneous cooling and heating, how should the zoning and switching strategy of the air-conditioning water system be designed to maximize energy savings and reliability?
The configuration and operating strategy of a hotel's heating and cooling source system directly affect the hotel's energy costs and operational quality. The following is a comprehensive professional analysis from the dimensions of design selection, operating strategy, and control logic.
I. Basis of Codes and Standards
Article 8.1.2 of the Design Code for Heating Ventilation and Air Conditioning of Civil Buildings (GB 50736-2012) sets out basic requirements for the zoning and switching of air-conditioning water systems—"when heating and cooling conditions operate alternately or simultaneously, heating and cooling pipelines shall be provided separately, or flexible system forms such as zoned two-pipe systems shall be adopted." Article 8.1.4 sets out requirements for the number of heating and cooling source units configured—"the operating efficiency under partial load shall be considered, and it is advisable to adopt a combination of large and small units." Article 4.2.9 of the Design Standard for Energy Efficiency of Public Buildings (GB 50189-2015) sets out energy-saving requirements for the zoning and system forms of air-conditioning systems—"air-conditioning systems should be provided separately according to the load characteristics of the interior zone and the perimeter zone." Article 4.6.2 encourages the adoption of "water-side economizer" (Free Cooling) technology. Article 3.2.10 of the General Code for Energy Conservation and Renewable Energy Utilization in Buildings (GB 55015-2021) requires that "air-conditioning systems shall have the capability to adjust cooling and heating output according to load changes," which is a mandatory code provision. Regarding international brands—Section 7.3 of the Marriott MEP Design Guidelines requires that "areas requiring year-round cooling (interior zones, IT rooms, etc.) must be provided with an independent cooling source or adopt a four-pipe system."
II. Impact on Hotel Operations (Pros and Cons)
Advantages: Reasonable zoning and switching strategies ensure year-round comfort in all areas of the hotel—the ballroom (perimeter zone with full glass curtain wall) has sufficient heating in winter and strong cooling in summer, with uniform temperature in large spaces (temperature difference ≤2℃); the interior-zone IT rooms and kitchens (which require cooling in some areas) have stable year-round cooling; in transition seasons (spring and autumn), free cooling (cooling tower cooling) keeps interiors cool without manually starting compressors, significantly reducing electricity costs. Operational flexibility is improved—guest rooms of different brands and grades can be set to cooling/heating modes separately (no need for a "one-size-fits-all" approach), meeting the differentiated needs of different guests.
Disadvantages: An improperly designed heating/cooling zoning system instead brings problems—the "zoned two-pipe system" creates a contradiction in the spring and autumn transition seasons where "guest rooms on the east side need cooling while guest rooms on the west side need heating" (due to different solar orientations), while the same pipeline system can only supply either cooling or heating, resulting in "half the areas comfortable and the other half complaining"; in a four-pipe system, the terminal units (fan coil units and AHUs) have their chilled-water coils and hot-water coils positioned too close together, and during cooling/heating changeover, condensate on the coil surface may not fully evaporate due to the influence of the hot-water coil, leading to mold and bacterial growth.
III. Pros and Cons of Later Maintenance
Advantages: When the four-pipe system operates, the chilled-water and hot-water systems are independent, facilitating fault isolation—when the chilled-water system fails, the hot-water system still works normally, so guests at least still have hot water available (heating); equipment selection flexibility is high—the maintenance cycles of chillers and boilers can be staggered (shutting down hot-water boilers for maintenance in summer and chillers for maintenance in winter) without affecting hotel operations. The use of the cooling tower free-cooling system in transition seasons reduces compressor operating time and extends the service life of chillers (each year of operation of a centrifugal compressor consumes approximately 2-3 years of design life).
Disadvantages: The doubling of piping in a four-pipe system increases the pipe insulation workload (insulation cost approximately 50-100 yuan per linear meter of pipe), and the probability of pipe leakage increases (the number of pipe joints doubles). The seals of cooling/heating changeover valves, being under long-term alternating cooling and heating conditions (temperature variation ranging from 7℃ to 60℃), have shortened service life (reduced from the normal 5 years to 3-4 years). The cooling tower free-cooling system carries a risk of cooling water freezing when used in winter (anti-freeze circulation and electric heat tracing are required, increasing maintenance workload).
IV. Cost Pros and Cons
Short-term investment: Compared with a two-pipe system, a zoned four-pipe system increases investment by approximately 2-4 million yuan (for a hotel of 50,000 m²), including: an additional set of supply and return water pipelines (1-2 million yuan), cooling/heating changeover valves (0.5-1 million yuan), and an additional set of coils and valves at the terminals (0.5-1 million yuan). A total heat recovery chiller increases investment by approximately 0.3-0.8 million yuan (15-25% more expensive than an ordinary centrifugal chiller). The cooling tower free-cooling system increases investment by approximately 0.2-0.4 million yuan (including plate heat exchangers, changeover valves, piping, etc.).
Long-term benefits: Free cooling in transition seasons saves approximately 0.1-0.3 million yuan in electricity costs per year (varying by climate zone—northern cities save more); the total heat recovery system saves approximately 50,000-150,000 yuan in gas costs per year (heat recovery heats domestic hot water, replacing part of the boiler load); precise heating/cooling zoning control reduces repair coordination costs caused by "overcooling/overheating" complaints (20,000-50,000 yuan per year). The cumulative energy-saving benefit over a comprehensive 5-year operating period is approximately 0.8-2 million yuan.
V. Specific Technical Details and Engineering Experience
1) Design conditions for "Cooling Tower Free Cooling": The cooling water loop on the chiller side is connected to the load-side air-conditioning water loop via a plate heat exchanger. When the control system detects that the outdoor wet-bulb temperature is ≤10℃ (conservative value 12℃), it automatically switches the cooling tower outlet water to flow directly into the plate heat exchanger, cooling the air-conditioning return water (12℃) to 7-10℃ before supplying it to the terminals. Key parameters for heat exchanger selection and design: The heat exchange area of the plate heat exchanger is designed based on a cooling water side inlet/outlet temperature difference of 4-6℃ and an air-conditioning side inlet/outlet temperature difference of 5-8℃. Brazed plate heat exchangers are recommended (more compact than traditional gasketed types, with better pressure resistance, but inconvenient to maintain—once a leak occurs, the entire unit must be replaced). Note: In free cooling mode, when the cooling tower water temperature is below 10℃, the cooling tower fan should be stopped (to prevent freezing of the cooling tower basin).
2) Control strategy for the "zoned two-pipe" system: The hotel is divided into 8-12 "air-conditioning zones" by orientation (east/south/west/north) and by inner/outer zone. Each zone is provided with independent supply/return water branch pipes, electric switching valves, and variable-frequency pumps. The control logic is as follows: The "outer zone temperature sensor" of each zone determines whether the zone currently requires cooling or heating (the changeover setpoint is an outdoor air temperature of 16-18℃), and the BA system automatically switches the electric valves based on the judgment result. Although this system is called "two-pipe," it is essentially "zoned variable flow direction." It saves approximately 30% of piping investment compared with a full four-pipe system, but the control logic is complex (prone to "changeover oscillation"—frequent zone switching when the outdoor temperature fluctuates around 16℃). Optimization measures: Set a "changeover dead band" (no changeover within the range of 16℃±2℃) and add a "timed lockout" (lock for at least 2 hours after each changeover).
3) Key selection points for heat recovery chillers: Heat recovery is divided into "partial heat recovery" (recovering 20-30% of exhaust heat, with outlet water temperature of 40-45℃) and "full heat recovery" (recovering more than 90% of exhaust heat, with outlet water temperature of 55-65℃). For hotels, a combined scheme of full heat recovery plus auxiliary heating is recommended: The full heat recovery unit provides heat to meet the base load of domestic hot water (accounting for approximately 60-80% of annual domestic hot water demand), with the shortfall supplemented by gas boilers. When selecting full heat recovery units, note the following: ① Under heat recovery conditions, the cooling COP of the chiller decreases by approximately 10-15% (because the discharge pressure increases to correspond to a 60℃ condensing temperature), but the comprehensive energy utilization efficiency (equivalent COP) can reach 8-10 (due to the "free" acquisition of additional heat); ② When retrofitting a centrifugal chiller for heat recovery, the standard condenser (corresponding to a 37℃ condensing temperature) needs to be replaced with a high-temperature condenser (corresponding to 55-60℃), or a separate "heat recovery condenser" needs to be added externally.
4) "Zero-leakage" technology for cooling/heating changeover: Traditional butterfly valves or gate valves have a leakage rate of approximately 0.1-0.5% in cooling/heating changeover applications, which is insufficient to eliminate "cooling loss and heating crossover." It is recommended to use eccentric hemispherical valves (Type C eccentric spherical seal, with high contact specific pressure, automatic compensation after seal face wear, and leakage rate <0.01%), or triple-eccentric butterfly valves (multi-layer sealing rings + eccentric conical seal, with a leakage rate that can meet the "zero-leakage" requirement, i.e., ANSI Class VI). Actuators should be spring-return type (automatically switching to a preset "safe mode" upon power failure—avoiding cooling/heating water crossover caused by uncertain valve position after power failure).
5) Considerations for "lithium bromide absorption chillers": For hotels that simultaneously have "large amounts of waste heat or cheap gas," consideration may be given to introducing lithium bromide absorption chillers (double-effect COP approximately 1.2-1.4, triple-effect up to 1.6-1.7). Absorption chillers achieve "energy flexibility" between heat sources and electricity—used during summer air-conditioning electricity peak hours (reducing electricity costs), while absorption chillers can utilize waste heat from generators (the "combined heat and power" scheme). However, absorption chillers have high initial investment (approximately 1.5-2 times that of centrifugal chillers), large footprint (increasing plant room area by 50-80%), and complex maintenance (solution requires periodic regeneration, and high vacuum maintenance requirements). For most urban five-star hotels, electric-driven centrifugal/screw chillers remain the most cost-effective solution.
6) Insulation and freeze protection design for pipe trenches: When cold and hot water pipes are laid in a common trench, the insulation thickness of cold water pipes (7-12℃) is calculated based on anti-condensation requirements (generally ≥50mm rubber-plastic insulation), and the insulation thickness of hot water pipes (60-85℃) is calculated based on energy-saving requirements (generally ≥80mm glass wool or ≥60mm rubber-plastic). The spacing between the two types of pipes shall be ≥100mm (to avoid increased cooling load of cold pipes due to thermal radiation). The outer surface of the insulation layer shall be provided with an "aluminum foil + FRP" protective layer (for basements or pipe trenches with high humidity) to prevent insulation failure due to water absorption. Drainage trenches and automatic drainage pumps shall be provided at the lowest points of underground pipe trenches (to prevent accumulated water from soaking the insulation layer).