A few years ago, I got a chance to work on an air conditioning system for a large school hall that can hold one thousand students. Here is what I've learned.
Air conditioners for large halls require significantly more cooling capacity than large houses with the same floor area due to the number of people in the hall. Humans naturally generate heat. Hence, the AC system of halls that hold a few hundred to a thousand people must consider and overcome it in order to provide sufficient cooling.
Besides, the location of diffusers and grilles are important factors to the energy consumption of hall AC systems. Moreover, large air conditioners create significant vibration and noise that need to be controlled.
Hall Size, Location and Requirement
The hall is separated into two sections: a) main hall, b) stage. The main hall has a floor area of 11,000 sqft while the stage is 2,400 sqft.
The hall is located in Malaysia where a tropical rainforest climate is observable all year round. From WorldData, the average daytime temperature is 32°C and the average nighttime temperature is 24°C. The average relative humidity is 85% all year round.
The hall AC system is required to provide cooling to one thousand students on every school day. The noise and vibration should be controlled as per ASHRAE's guidelines. Finally, the supply air diffuser location was set at a height of about 3.5 meters.
Air Conditioner, Duct and Grille
Air-cooled split air conditioners are used in the school hall. The indoor units are ducted type air handling unit and they are connected to their outdoor units by copper pipes.
Two air-cooled ducted air handling units (AHU) were selected for the main hall. The cooling capacity of each AHU is 60 Tons at 18,000 CFM of airflow.
Each AHU is floor-mounted in a dedicated room behind the main hall stage. The outdoor units of both AHUs are located a few floors below the AHU at a well-ventilated place.
Each AHU is supplying air via a duct along the side of the main hall and side throwing the air into the main hall via double deflection grilles at a height of about 3.5 meters.
Each AHU has a total of 10 pieces of supply air grille. Each grille is supplying air at 1,800 CFM with a size of 1000x300mm. The AHU room has a big piece of the grille at a size of 1450x3000mm mounted on the wall between the AHU room and the main hall.l
The supply air duct is located at the outside of the hall where it is exposed to the outside temperature. Thus, the main duct with a size of 1250x700mm was insulated with polyethylene (PE) foams at a thickness of around 1 inch.
PE insulations were preferred over fiberglass insulations because they look better which is necessary for the hall due to people can see the duct from outside the school.
For the stage, two ceiling hung ducted units were used. Each unit has a cooling capacity of 18 Tons at 3,200 CFM of airflow.
Both units are hung in the same room with the AHUs and ducted out to the stage at a height of about 8 meters measured from the floor of the stage. The ceiling hung outdoor units are also located at the same place as the outdoor units of the AHUs.
The ducting of the ceiling hung units are only rigid ducts. No flexible ducts were used although short length of flexible ducts is viable. Because the ducted units are not visible, they are insulated with fiberglass insulations.
Each ducted unit has a total of 6 pieces of supply air grille with each supplying air at an airflow of 533 CFM. The return air grille is shared with the AHUs.
Despite supplying air from about 8 meters height, the grilles are double deflection type rather than jet diffusers because the idea was to let the cold air gently pour onto the stage so that there will be no potential disturbances to stage performers.
How to Calculate Cooling Capacity for Large Halls?
Many engineers and manufacturers estimate cooling capacity using a rule of thumb. The air conditioning rule of thumb generally refers to a constant number that used as a multiplier which gives a fairly accurate result within a significantly shorter amount of time compared to a traditional detailed calculation.
Different regions in the world use a different number as their rule of thumb based on climate. For instance, in hot climate countries like Malaysia, we often use 60 btu/hr.sqft for bedrooms while in America, Energy Star recommended 5,000 BTU for a 144 sqft room which is about 35 btu/hr.sqft only.
For the main hall, the total cooling capacity of the two AHUs is 120 Tons or 1,440,000 btu/hr (1 Ton = 12,000 btu/hr). By dividing the cooling capacity with the floor area (11,000 sqft), we get about a rule of thumb number of 130 btu/hr.sqft.
If we compare 130 btu/hr.sqft to the above cooling load check figures by Daikin Malaysia, for similar applications such as auditoriums and theatres, the rule of thumb is 120 btu/hr.sqft on the high side.
Furthermore, we know that humans generate heat. Energy Star recommended that we should increase the cooling capacity by 600 btu/hr for every additional person. So, if we take the most common rule of thumb number used in Malaysia which is 60 btu/hr.sqft and add the heat gain of 1000 person, we'll get the following result:
60 btu/hr.sqft x 11,000 sqft + (600 btu x 1000) = 1,260,000 btu/hr excluding all windows heat gain and energy losses. If we include a 10% safety factor, the resulted cooling capacity is more or less the same as the AC system in the school hall mentioned earlier which is currently in operation.
However, large halls have a very high ceiling at about 9 meters or 12 meters or more. As mentioned earlier, the ACs are supplying air into the main hall at a height of about 3.5 meters. If the ACs are supplying air at a higher level, for example, 8 meters, the ACs need to cover more air volume and thus, the cooling capacity should be increased. Generally, the cooling capacity is increased by 10% if the ceiling height is more than 10 ft.
Air Distribution in Large Halls
Cold air is denser and thus, it naturally sinks while warm air floats to the top. Hence, placing AC supply air diffusers at a reasonably low level can reduce the total cooling capacity needed and therefore, save cost. Contrarily, placing AC supply air diffusers at high level waste energy on unnecessary cooling in exchange for better aesthetics.
However, the major drawback of having side-throw supply air grilles rather than supplying air from above is uneven air distribution. Despite high airflow, cold air still unable to reach the center of the hall. Furthermore, noise is greater near the grilles while supplying air from high up often have inaudible noise.
In high-end or luxury halls where events such as ceremony and music performance are held, the air conditioning system mostly supplies air from above. Jet diffusers are mostly used to push air out at high velocity so that the cold air can travel to about 4 meters above the floor and also, reduce energy wastage on unnecessary cooling.
It is worth mentioning that the cold air should not travel too fast and too far so that people sitting in the hall will not feel the cold air on their head which will make them extremely uncomfortable.
Noise and Vibration Control for Large Halls
For such an air conditioner system, the majority of the noise is coming from the return air grilles but, it's not due to high air velocity. As mentioned earlier, each AHU room has one big piece of return air grille. Thus, the noise of the AHU is transmitted through the return air grille to the main hall.
Initially, the design was to utilize the AHU room as a return air plenum to save cost. Later, because of the noise issue, the return air grille was ducted to the AHU. On top of that, the return air duct was insulated internally using rockwool insulations to absorb noise and it successfully reduced the amount of noise transmitted from the AHU to the main hall.
The main hall is located on the first floor while the AHU room is located on the second floor. Since we are supplying air at a low level, the AHU duct must penetrate the second floor concrete slab.
During operation, AHUs inevitably generate a decent amount of vibration. The vibration can transfer to the nearby slab and wall, causing the decorations and TVs in the main hall shaky. Thus, we not only sit the AHU on vibration isolators, but we also placed rockwool insulations at the slab opening around the gaps between the supply air duct and the concrete to prevent the vibration of the duct from transmitting to the slab.
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What are the Other Options to Cool Large Halls?
I went to many halls to attend events such as wedding dinners and ceremonies. Other than ducted units, I've seen some halls installed ceiling exposed type of indoor unit air conditioner. The ceiling exposed type is also an air-cooled split system.
A ceiling exposed is a ductless type similar to a wall-mounted unit but usually has much a higher cooling capacity. Besides, the ceiling exposed units also have a higher airflow. Thus, a large hall may install about 20 units of ceiling exposed split air conditioners to provide cooling.
The main benefit of using the ceiling exposed split units is low cost and simplicity. However, the major drawback is similar to the ducted air conditioner which is poor air distribution. Furthermore, unlike the ducted AHUs, ceiling exposed units can't reduce much of their noise because they are placed inside the hall rather than somewhere outside the hall where we can soundproof them.
Another common way to cool a large hall is using multiple ceiling ducted units just like the ducted unit used for the stage as mentioned earlier. Using ceiling ducted units can supply air from above and provide better air distribution and noise control.
However, a large hall may need multiple units of ceiling ducted air conditioner. In a hall, the ceiling ducted units are installed very high up which increased the cost of installation and future maintenance. Furthermore, the risk of water leakage is increased dramatically due to the number of air conditioning units.
Large halls require an exceptionally high cooling capacity to cater to a huge number of people. Noise and vibration control are critical, especially in high-end halls. Slab opening insulations and duct internal insulations can drastically reduce the noise and vibration of ducted air conditioners.
In a hall, supplying air from a reasonably low level can reduce the cost of air conditioning in exchange for more noise and uneven air distribution. Conversely, supplying air from above provides better comfort.
Therefore, large budget halls can supply air from the side if using AHUs. Otherwise, the ceiling exposed type is an option for an even lower cost. On the other hand, large luxury halls should always supply air from above for the best air distribution and the lowest noise for a premium feel. Hence, a centralized air conditioning system such as ducted AHUs usually is the best solution for high-end halls.
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