Many newcomers in the industry, when evaluating a system, tend to choose a project's system from the perspective of HVAC (Heating, Ventilation, and Air Conditioning) professionals. In fact, not only newcomers do this, but many veteran engineers with years of experience also follow the same approach. Many people are also keen on adopting advanced energy-saving solutions.
However, in recent years, as the market continues to evolve, selecting a system for a project is no longer solely the responsibility of HVAC professionals. First and foremost, we need to recognize the role of HVAC – it serves the building, supports its functions, and caters to the future operation of the project. When choosing a system, factors related to serving the project’s function have already surpassed those inherent to the HVAC field itself. Some representative issues include: Should high-end hotels adopt four-pipe systems? For high-rise office buildings, should water systems or multi-split systems be used?
Especially regarding the first question, many high-end hotels, particularly guest room areas, don’t have internal zones that require heating and cooling simultaneously. From a professional HVAC perspective, a four-pipe system isn’t necessary. However, hotel management companies consider the needs of guests worldwide, so their design manuals specify that four-pipe systems must be used.
For super-large complex projects, which may include hotels, office buildings, and shopping malls, setting up a centralized energy center might seem optimal from an HVAC standpoint. However, since these are developer-led projects that will be managed by multiple parties in the future and assets can be sold or leased, designing separate heating and cooling sources based on functional zones has become more common. Many people may ask, why not just design according to load requirements? Take, for example, a famous hotel with two independent sections—Phase I and Phase II. There was once a plan to sell Phase I, but because the HVAC system shared resources between Phase I and Phase II, no hotel management company was willing to take over. Why do houses emphasize having separate kitchens and bathrooms? Many management requirements take precedence.
If high-rise office buildings are intended for leasing or sale in the future, systems like multi-split or water-loop heat pumps that allow easy metering rise to the top of the list.
Therefore, when doing design work and determining the system, designers should communicate more with clients, fully understand the future operation and management model, and the asset division mode of the project, then determine the system according to the characteristics of the HVAC field.
Having worked in design for over a decade, in the early years, air conditioning load calculations mainly relied on estimation, but later detailed software-based calculations became the norm. With the accumulation of so many projects, now it is still necessary to perform detailed calculations, with calculation being the main method and estimation as a supplement. This does not apply to heating loads; heating loads cannot rely on estimation alone—they must be calculated.
A tight design schedule is not an excuse. Our current design schedules are tighter than ever, but we still need to find time for detailed load calculations. This is also a requirement during the review process.
Many newcomers start by asking what the indicators for *** are? The time spent waiting for a reply would be better spent calculating themselves. Below, let me elaborate:
1. Accurate load estimation requires long-term project accumulation and rich life experience.
Everyone agrees with the former, but the latter emphasizes life experience rather than project experience. For example, in many traditional data handbooks, the density of people in exhibition halls is 1 person per square meter. However, for urban construction exhibition halls or internal corporate exhibition halls, how often are there that many people present? The handbook indicates a meeting room density of 1 person per square meter, which generally refers to cinema-style meeting rooms. However, in most corporate meeting rooms, once a large conference table is placed, how many people could fit?
Some indicators in handbooks have a wide range. For a certain functional room, the indicator ranges from 120 to 250, a difference of twice the value. Whether you choose the higher or lower end depends on the characteristics of the current project. Even for traditional functional areas, you must understand their specific features within the context of the current project. Without experience, you can only rely on detailed calculations.
2. Detailed load calculations can be inaccurate due to incorrect input parameters.
Many people believe that detailed load calculations are inaccurate, often resulting in underestimations, leading them to dismiss detailed calculations. This is wrong. The input parameters for detailed load calculations are controlled by the calculator. Any deviation in the results is caused by inaccuracies in the input parameters. Different types of malls have significant differences. For instance, furniture and building materials malls may have very few people, so a density of 0.1 people per square meter might suffice. Luxury goods malls have slightly more people, so a density of 0.25 people per square meter might work. Central business district comprehensive commercial buildings might have densities of 0.6 people per square meter on the ground floor, 0.5 people per square meter in the basement and second floor, and 0.4 people per square meter on the third floor, decreasing with each level. If it's a bustling supermarket, refer to the design manual because foot traffic can vary significantly at different times. Lighting loads in different malls also differ greatly. Similarly, offices vary widely. We’ve encountered cases where the final indicator reached 350W/m² for a bank office with ten computers per person. Therefore, at the beginning of the design process, the positioning and characteristics of the project must be thoroughly understood.
3. Reasons to force yourself into detailed calculations.
There are some reasons why moving from estimation back to detailed calculations is beneficial:
a. Buildings are becoming increasingly complex, with more unconventional shapes. There are fewer precedents from past projects to reference in terms of building envelope structures. Additionally, with societal development, the characteristics of functional areas have changed significantly compared to previous projects. Even if it's just an office space, personnel and equipment may differ from previous projects.
b. Through some engineering calculations, orientation plays an important role. For example, west-facing and south-facing office spaces with similar curtain wall designs can differ significantly. Simply relying on indicators could lead to errors.
c. Many special functional areas, such as kitchens and laundry rooms that require 100% fresh air, lack standard indicators. Only comprehensive calculations can address these.
Given the above reasons, once calculations begin, they should cover all aspects comprehensively.
In many discussion threads, someone lists the air volume of an air conditioner, such as a fan parameter of 35,000 cubic meters per hour, with cooling power of 432kW and heating power of 728kW. Subsequently, others use this cooling capacity to verify the unit cooling index of the served rooms while ignoring the operating conditions of the air conditioning unit. Using the example provided, these parameters correspond to a general air conditioning box sample with 35,000 cubic meters per hour airflow under new air conditions with six rows of tubes, assuming inlet air parameters of 35°C dry bulb/28°C wet bulb. When applied to indoor rooms under indoor design conditions, the return air and fresh air mixed in a certain proportion might not reach this point. If the inlet air temperature is lower, would the air conditioning box output the specified cooling capacity? Think about whether the textbooks teach to determine airflow based on sample cooling capacity or calculate airflow based on supply air temperature difference. Therefore, if the specific operating condition corresponding to the selected cooling capacity cannot be provided, the airflow should still be verified.
After completing the calculations for terminal rooms, many people immediately check the sample based on the calculated values, matching the cooling capacity from the sample without considering the operating conditions or verifying the supply air volume. This could lead to selection errors. Selecting fan coil units at mid-range cooling capacity can sidestep this issue from another angle, but VRV indoor units cannot avoid it.
Take, for example, Daikin model 28 indoor units with a rated cooling capacity of 2.8kW. Have you noticed that the 2.8kW condition assumes an inlet air temperature of 27°C dry bulb/19°C wet bulb, whereas our typical design indoor temperature is 25°C, meaning our design condition requires the indoor unit's inlet air condition to be 25°C dry bulb/18°C wet bulb. Under this design condition, the cooling capacity of this model 28 indoor unit decreases by approximately 20%. Comparing samples of imported Daikin machines with domestic ones, the same model number might be labeled as 28 in domestic samples but 25 in imported samples. This is not because Daikin exaggerates machine models but because national standards differ—China requires a standard condition of 27°C dry bulb inlet air, while other countries require 25°C dry bulb inlet air.
Thus, when selecting terminal equipment (fan coil units or indoor units), either consult variable condition performance tables or verify the supply air volume based on the supply air temperature difference. You can simply calculate the supply air volume using the sensible heat formula based on the indoor sensible heat load. It is recommended to verify or directly calculate the supply air volume since not every sample includes a variable condition performance table.
For rooms,
Q(kw) = G(kg/s) * 1.02 * (t1 - t2)
Here, Q represents the indoor sensible heat load (not total heat load); G is the supply air volume; t1 is the indoor temperature; and t2 is the supply air temperature.
When preparing the air conditioning unit equipment table, the inlet temperature of the coil and the outlet temperature of the coil (generally the supply air temperature) should be provided. In a primary return air condition, the coil inlet temperature is the mixed temperature of indoor return air and fresh air.
Soundproofing and vibration isolation are issues that are easily overlooked in current designs. Some people completely ignore them, causing serious consequences, while others add silencers and flexible joints but still encounter severe problems. Newcomers should remember that in air conditioning systems, issues related to insufficient cooling or heating are rare, but noise problems are the most common.
The first topic to discuss here is the ZP100 silencer, currently the only silencer available for selection by design personnel that meets national standards, with national drawing collections. Some northern provinces and municipalities also have drawing collections for other types of silencers, which we won't delve into here. However, precisely this ZP100 panel silencer has serious issues:
1. The indicators in the drawing collection are overly exaggerated, and no manufacturer's product can meet the collection's indicators. The sound attenuation at 250Hz frequency in the collection is 17dB, while the best silencer currently available in the market attenuates only 9dB at this frequency. Professional noise control manufacturers (not silencer manufacturers, these companies specialize in solving various noise problems) scoff at this collection, unsure how the data was derived. Moreover, the collection lacks parameters for the 63Hz frequency.
2. The drawing collection does not specify the panel spacing, and many manufacturers take advantage of this omission by cutting corners. For example, in a 1m wide silencer, only one panel is placed. According to acoustics experts, the panel spacing should be 250mm (center-to-center).
The ZP100 is a resistive silencer, effective against medium and high-frequency noise but less effective against low-frequency noise. Therefore, designers should prioritize impedance composite silencers with sound-absorbing cotton that have been professionally designed.
The second topic to discuss is ceiling-mounted units. In many designs, ceiling-mounted units either connect directly to the ceiling without ducts for return air or connect directly to a plenum box with return air grilles underneath. Remember, noise has no directionality—it doesn't spread only through the supply air ducts just because the airflow is directed that way. Return air grilles are close to fans and are weak points where noise can easily leak out. Plenum boxes typically provide only 5dB of sound attenuation. Noise control for ceiling-mounted units requires special attention. A paper by Chen Wei from Shanghai Baifuqin Company (who has since left the company) provides three representative examples of air conditioning unit noise control, which are highly illustrative.
It is recommended to apply soundproofing treatment to the supply and return air ducts entering and exiting the mechanical room for a distance of 10m outside the mechanical room in areas with high noise requirements. Fan noise can easily penetrate the ducts and ceilings after entering the ducts upon leaving the mechanical room, spreading to occupied spaces.
Additionally, avoid using T-type three-way fittings or right-angle bends. In a certain building in Pudong, Shanghai, noise issues were severe, and both local and Japanese noise control units failed to resolve them. The local company performed slightly better, controlling the noise at the nearest outlet to 46dB. The farther the outlet, the greater the noise, reaching over 50dB near the curtain walls. The exact cause wasn't investigated, but they estimated that the straight three-way fittings and bends didn't naturally attenuate the noise but instead enhanced the reflection sound.
Silencer selection should be determined based on professional calculations of the noise spectrum of the ordered equipment. Given everyone's busy schedules and limited expertise, it's recommended to leave this task to specialized manufacturers by including such a note in the design specifications.
Regarding vibration isolation, everyone knows that rubber flexible joints should be installed at pump inlets and outlets, etc., but have you ever calculated whether the displacement of the flexible joint can withstand the vibration of the unit? Know that flexible joints in typical refrigeration rooms are often ineffective. Similarly, vibration isolators should be determined based on professional calculations of the weight and parameters of the ordered equipment. This includes flexible joints at the inlets and outlets of chillers and pumps, vibration isolators for chillers, pumps, and air conditioning units, and vibration-reducing hangers in refrigeration rooms.