Fresh air volume design standards and calculation basis
The reasonable determination of fresh air volume is the foundation of hotel fresh air system design and directly affects indoor air quality and air-conditioning energy consumption. According to Article 3.0.6 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," the minimum fresh air volume per person in civil buildings shall comply with the following provisions: in five-star hotel guest room areas, the fresh air volume shall be no less than 50 m³ per person per hour; in lobbies and public areas, no less than 30 m³/h·person; in banquet halls and meeting rooms, no less than 30 m³/h·person; in gyms and SPA areas, no less than 50 m³/h·person; in restaurants and bars, no less than 30 m³/h·person. In actual design, calculations shall be made by two methods and the larger value taken: fresh air volume calculated by number of occupants (number of occupants × fresh air volume standard per person) and fresh air volume calculated by area (area × fresh air volume index per unit area). Five-star hotel guest rooms are generally calculated at 2 persons per room, and at 3 persons when the guest room area exceeds 50 m². The Marriott MIDS standard is stricter, requiring guest room fresh air volume to be designed at 60 m³/h·room (standard guest room) to 100 m³/h·room (suite), and the exhaust air volume shall be 85%-95% of the fresh air volume, maintaining slight positive pressure in guest rooms (5-10 Pa) to prevent infiltration of externally polluted air. In addition to meeting the fresh air demand of occupants, the fresh air system shall also consider building air pressure balance to ensure that public areas can achieve 100% outdoor air operation during transition seasons. When calculating the fresh air load, the proportion of fresh air load to the total air-conditioning load under design conditions is typically 25%-40%. Taking a five-star hotel in Shanghai as an example, the total cooling load under summer design conditions is 5,800 kW, of which the fresh air cooling load is approximately 1,740 kW, accounting for 30%. Therefore, reasonably controlling the fresh air volume standard and adopting energy-saving measures are of great significance for reducing system operating energy consumption. For kitchen areas, according to the requirements of GB 18483 "Emission Standard of Cooking Fume from Catering Industry," the kitchen exhaust air volume shall be calculated at 40-60 air changes per hour, and the fresh air volume shall be 80%-90% of the exhaust air volume, ensuring negative pressure operation in the kitchen and preventing cooking fume from spreading to dining areas. For large spaces such as hotel lobbies, GB 50736 recommends using the CO₂ concentration control method to regulate fresh air volume, maintaining indoor CO₂ concentration below 800 ppm (Marriott standard) or 1,000 ppm (national standard). Installing CO₂ sensors can dynamically regulate fresh air volume according to actual indoor occupant density, saving approximately 20%-30% energy compared with the fixed fresh air volume scheme.
Heat recovery unit selection and energy-saving benefit analysis
The heat recovery unit is the core energy-saving equipment of the fresh air system. It pre-treats fresh air by recovering cooling energy (summer) or heat (winter) from exhaust air, significantly reducing fresh air treatment energy consumption. According to Article 4.2.14 of GB 50189 "Design Standard for Energy Efficiency of Public Buildings," when the fresh air volume of an air-conditioning system is greater than or equal to 4,000 m³/h, a heat recovery unit shall be installed, with total heat recovery efficiency not less than 60%. Article 7.5.10 of GB 50736 further requires that sensible heat recovery units should be selected in severe cold and cold regions, and total heat recovery units should preferably be selected in hot summer and cold winter and hot summer and warm winter regions. The types of heat recovery units mainly include four types: rotary wheel heat recovery, plate-fin heat recovery, heat pipe heat recovery and liquid circulation heat recovery. The rotary wheel total heat recovery unit has the highest recovery efficiency, with sensible efficiency up to 75%-85% and total heat efficiency up to 60%-75%, suitable for systems with large fresh air volumes. Its working principle is to use the rotation of the wheel between the exhaust air and fresh air passages, whereby the hygroscopic material on the wheel absorbs cooling energy or heat from the exhaust air and transfers it to the fresh air. The plate-fin heat recovery unit has a simple structure, no moving parts and low maintenance cost, with sensible efficiency of 55%-70%, suitable for small and medium-sized systems, but there is a risk of cross-contamination (leakage rate approximately 1%-3%). The heat pipe heat recovery unit uses the phase change of the working fluid to transfer heat, with sensible efficiency of 50%-65%, no moving parts and long service life, but it can only recover sensible heat. Liquid circulation heat recovery (also called heat recovery loop) connects the fresh air handling unit and exhaust air unit through ethylene glycol solution circulation, enabling long-distance transmission and being suitable for spatially separated fresh and exhaust air outlet arrangements. The energy-saving benefit of the total heat recovery unit is considerable: in hot summer and cold winter regions (taking Shanghai as an example), after installing a total heat recovery unit with 65% efficiency, the fresh air treatment energy consumption is reduced by approximately 55%-65%, saving approximately 18-25 kWh/m²·fresh air volume of air-conditioning electricity consumption annually. The investment payback period is typically 2-4 years. The Marriott MIDS standard requires that hotel fresh air systems must be equipped with heat recovery units, with total heat recovery efficiency not less than 65%, and the leakage rate of rotary wheel heat recovery units not exceeding 5% (on the fresh air inlet side). In terms of antifreeze control, antifreeze protection measures such as fresh air preheating coils and antifreeze circulation pumps shall be provided before and after the heat recovery unit in severe cold regions. Article 8.3.4 of GB 50736 stipulates that bypass ducts and motorized air dampers shall be provided on both the fresh air side and the exhaust air side of the heat recovery unit, allowing bypass operation during maintenance of the heat recovery unit or free cooling in transition seasons. For five-star hotels, considering maintenance convenience during operation, it is recommended to select modular heat recovery units to facilitate sectional cleaning and repair.
Transition season 100% outdoor air operation and control strategy
Transition-season full fresh air operation is an important energy-saving strategy that uses outdoor natural cooling sources to provide cooling for hotels, significantly reducing chiller operating hours and lowering the overall energy consumption of the air-conditioning system. According to Article 7.3.10 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," air-conditioning systems shall be capable of transition-season full fresh air operation, and the cross-sectional dimensions of fresh air ducts and fresh air valves shall meet the air volume requirements for full fresh air operation. According to Article 4.2.12 of GB 50189, air-conditioning systems in public buildings should be capable of full fresh air operation or adjustable fresh air ratio operation, with corresponding exhaust systems provided. The core of the full fresh air operation control strategy is to compare the outdoor air enthalpy with the indoor return air enthalpy: when the outdoor enthalpy is lower than the indoor return air enthalpy, the full fresh air operating mode is adopted. Specific control methods include: enthalpy control (most energy-efficient), temperature control (simplest), and dew-point temperature control (most suitable for hot and humid regions). The implementation of full fresh air operation in five-star hotels requires the following design conditions: the fresh air duct cross-section is designed for 100% fresh air volume, the exhaust system capacity matches the full fresh air operating capacity of the fresh air handling unit, and the indoor temperature is controlled within an adjustable range of 22-26°C during transition-season full fresh air operation. The Marriott MIDS standard requires that the fresh air valve, return air valve, and exhaust air valve of the hotel air-conditioning system all be equipped with electric actuators, with a modulating stroke of 0-90°, an actuator response time not exceeding 30 seconds, and a three-valve linked control function (when the fresh air valve opens wider, the return air valve closes down and the exhaust air valve opens wider). In terms of control logic, a simple control method based on outdoor temperature can be adopted: when the outdoor temperature is below 22°C, close the chilled water valve and enable full fresh air mode; when the outdoor temperature is above 26°C, switch to minimum fresh air mode and start the chiller. More precise is the enthalpy control method: install an outdoor air enthalpy sensor and an indoor return air enthalpy sensor, and automatically switch when the outdoor enthalpy is lower than the indoor enthalpy. The energy-saving effect of transition-season full fresh air operation is significant: taking the Beijing region as an example, the period during which full fresh air operation can be utilized in transition seasons is from mid-April to the end of May and from mid-September to the end of October each year, approximately 120 days. During this period, chillers can be shut down or operated at low load, and the air handling units only need to run the fans, with a power saving rate of approximately 40%-60%. However, for hot and humid regions (such as Guangzhou and Shenzhen), the transition-season full fresh air utilization period is shorter (approximately 60-80 days/year), and attention must be paid to the risk of condensation under high-humidity weather conditions. Article 7.3.11 of GB 50736 specifically states that during transition-season full fresh air operation, condensation on indoor surfaces shall be prevented, and fresh air shall be dehumidified when necessary.
Fresh air purification filtration grade configuration and maintenance management
The rational configuration of the fresh air filtration and purification system is a key defense line for ensuring indoor air quality in five-star hotels, especially in the post-pandemic era, when filtration grade requirements have been significantly raised. According to Article 7.5.9 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," fresh air systems shall be equipped with air filtration devices, and the arrangement of filters shall meet the following requirements: the resistance of air filters shall be calculated based on final resistance, which is generally twice the initial resistance; filters shall be installed for easy cleaning and replacement. GB/T 18801 "Air Cleaners" and GB/T 14295 "Air Filters" classify air filters by filtration efficiency into three grades: coarse (G1-G4), medium (M5-M6, F7-F9), and high efficiency (H10-H14, U15-U17). The minimum configuration standard for five-star hotel fresh air systems is: coarse filter (G4) + medium-efficiency filter (F7) two-stage filtration. The G4 coarse filter is used to capture particles ≥5μm, with an initial resistance of approximately 50-80Pa and a final resistance of approximately 150-200Pa. The F7 medium-efficiency filter is used to capture particles ≥1μm, with an initial resistance of approximately 80-120Pa and a final resistance of approximately 250-300Pa. The filtration grade requirements of the Marriott MIDS standard are more stringent: the unit section is configured with G4+F7+F9 three-stage filtration, and the standard updated after the pandemic added ultraviolet germicidal lamps (UV-C) or plasma sterilization devices. For high-risk areas in hotels (such as ballrooms and meeting rooms), it is recommended to configure HEPA H13 high-efficiency filtration, with a filtration efficiency of ≥99.97% for particles ≥0.3μm. In terms of filter maintenance management, a regular replacement system should be established: G4 coarse filters cleaned or replaced every 1-2 months (adjustable according to air quality conditions), F7 medium-efficiency filters replaced every 3-4 months, and F9 sub-high-efficiency filters replaced every 6 months. Filter differential pressure monitoring is the scientific basis for determining replacement timing: install differential pressure switches on air handling units and fresh air handling units, and issue an alarm signal when the filter resistance reaches the set value (generally twice the initial resistance). Article 7.2.14 of GB 50243 "Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works" stipulates: the installation direction of filters shall be correct, the filter material shall be installed tightly and flatly, and the seal between the filter and the frame shall be good to prevent bypass. The following technical details should be noted when installing a fresh air purification and filtration system: the unit casing shall have sufficient maintenance space, and the filter section shall be provided with an access door; filters should be designed for drawer-type installation for quick replacement; Article 7.5.12 of GB 50736 requires that fresh air intakes be located in places with clean outdoor air, away from pollution sources such as exhaust outlets, chimneys, and garbage stations, with a horizontal distance of not less than 10m and a vertical distance of not less than 3m between them. Filter selection also needs to consider the balance between resistance and energy consumption: the higher the filter grade, the greater the resistance and the higher the fan energy consumption. Upgrading from G4+F7 to the G4+F7+F9 scheme increases fan energy consumption by approximately 15%-20%, and the fan residual pressure must be correspondingly increased in the design.