Performance investigation of indoor thermal environment and air handling unit in a hub airport terminal

: Airport terminal is a typical large space building with large high interspaces and multitudinous functional zones. The task of heating, ventilation and air conditioning (HVAC) system is to ensure the thermal environment of these zones with different functions. A typical terminal in cold region of China is investigated in present paper, where indoor environment and performance of air conditioning system in summer are emphasized. The results indicate that the indoor temperature and humidity parameters at a height of 1.5 m in each zone of the terminal can be effectively controlled. The indoor temperature at a height of 1.5 varies in the range of 24~27°C, and the temperature difference is 0.5~4.9°C in the vertical direction. The temperature at the top of the terminal is greatly affected by the outdoor temperature where is higher and fluctuates obviously, while the temperature at the height of the occupant area is lower and more stable under the control of the air-conditioning system. The indoor carbon dioxide concentration is also at a reasonable level, all below 900 ppm, indicating that the equivalent outdoor air volume per capita is greater than 35 m 3 /h. Even in the case of high load caused by flight delays, the thermal environment in the terminal can be basically controlled at a comfortable level. As for the air handling unit, air handling process is tested to investigate the cooling capacity. The results show that the actual water supply of the AHU is relatively higher than the set value, but the air volume is almost equal to the set value which leads to an unsatisfactory energy efficiency. The tested energy efficiency ratio (EER) of AHU is only 5~7, almost approaching to that of the cooling plant. Reducing air flow rate could greatly improve the EER of the current air-conditioning system.


Introduction
In recent years, civil aviation industry is in the rapid conditioning is provided indoors in summer, the temperature difference between indoor and outdoor is significant, causing the hot air outdoors infiltrating indoor through skylights due to the effect of thermal pressure [10]. In order to maintain a good indoor environment in summer, the air conditioning system needs to operate in a good condition. However, there are still limited tests on the actual performance of airconditioning systems in terminals [11][12][13][14][15].
In the current study, on-site measurements in summer were performed in the large space building of a hub airport terminal in China (with annual passengers throughout of more than 68 million). Both indoor environment and performance of the air handling units are measured to reflect the actual situation in the typical summer condition.

Description of the airport terminal
The case airport for measurement is the Terminal 2 of a hub airport in the cold region of China, officially opened in 1999. As shown in Fig. 1  The air-conditioning system normally runs 24 hours every day in the cooling season with a design supply water temperature of 7 ºC. And the indoor design air temperature is 26 ºC. As shown in Fig. 2

Test results of indoor environment
On-site measurements of the indoor environment in this airport terminal were carried out in July 2018, which could be regarded to represent typical summer conditions as shown in  Table   1. The air temperature and relative humidity were measured using a combined resistance temperature detector (Pt RTD) and a lithium chloride relative humidity sensor (LiCl RHS) respectively.

CO2 concentration distribution
CO2 is usually taken as a reference index of indoor environment in buildings [16][17][18]. The emissions of CO2 are approximately proportional to the amount of CO2 emitted by human respiration in terminal buildings. The concentration of indoor CO2 are usually suggested to be lower than 1,000 ppm in many countries, which is beneficial to take the influence of both other pollutants and odors caused by human activities into consideration at the same time.      The cooling capacity of the AHU can be calculated as Eq.
(1), and energy efficiency ratio (EERt) of the AHU is defined as Eq. (2): where subscripts in and out refer to the supply and return water respectively; Qc is cooling capacity of the AHU, kW; G is the water flow rate, m 3 /h; Wt is the power consumption of the AHU, kW.
The total heat exchange efficiency is the effective indicator for surface cooler which can be calculated as Eq. (3): where subscripts M and S refer to the mixed air and supply air, respectively.  Fig. 11.
Generally, the rated EERt of the AHUs are higher than 10, while EERt of all the tested AHUs are lower than 10, far below the rated EERt. It is found that the air volume of the AHU is approximately equal to the rated air volume, but the water flow rate is smaller and the water temperature is higher, which leads to the lower cooling capacity and the low energy efficiency ratio. Unbalance rate (%) 9.8 Total heat exchange efficiency (%) 34 Fig. 11 Energy efficiency ratio of AHUs.

Conclusions
In this paper, on-site measurements of the indoor environment and air handling units were conducted in a typical Chinese hub airport. The main conclusions can be summarized as follows: 1) The indoor temperature and humidity parameters can be effectively controlled when the air 3) The test results for the AHUs show that the supply water temperature is high, and the actual water flow rate of the AHUs is lower than the set value.
While the air volume is equal to the set value, resulting in the lower energy efficiency of the AHUs. As a result, the EERt of the AHU is only about 5~10, which is at a relatively low level and restricting the performance of the entire system.

Funding
The research described in this paper is supported by the Dear Reviewers, Thank you so much for your elaborate work with our paper. We are especially grateful for reviewers' time and effort for reviewing the previous version of the manuscript. These valuable comments and suggestions have considerably improved the clarity of the paper. The manuscript has been revised in response to the comments with the main modifications listed below.
• Reviewer #1: The paper is focused on studying an air conditioning system of a large airport terminal in China. The paper is well structured but needs some minor revisions according to my comments below. (Accept) 1) The references in text must be changed from superscript to normal 2) The numbering in the References chapter must be size 10 3) Please replace "the-air conditioning" with "the air conditioning" (page 2) Reply: Thank you so much for the suggestion. The references in text and the numbering in the References chapter have been adjusted to the correct format. And the spelling mistake in page 2 has been corrected.
• Reviewer #2: operation of the air conditioning system of the large space buildings in the airport, and at the same time have a certain guiding role in the design of air conditioning systems with up to space.
2) the paper is not respecting the format accordingly to E3S publication requests Reply: Thank you so much for the suggestion. The format of the paper has been adjusted to meet the E3S publication requests.
3) there are many grammar mistakes

4) images quality is very poor
Reply: Thank you so much for the suggestion. The language has been polished in the revised manuscript.
• Reviewer #3: 1) English can be improved, especially in Chapter 4.1. and other chapters containing detailed explanations.
Reply: Thank you so much for the suggestion. The language has been polished in the revised manuscript.
Reply: Thank you so much for the suggestion. The FCU (fan coil units) has been defined in Chapter 2.
3) Chapter 3: The first paragraph declares measurements to have been done in typical summer conditions. Typical summer conditions as defined by which criteria/standard/etc.? open and connected directly to the outdoor environment. Is this not a mechanical supply?

Reply
Is it negligible? Further clarification required.
Reply: Thank you so much for the suggestion. The mechanical supply of fresh air in this article refers only to the part that is supplied to the room through MAUs (make-up air units) or AHUs, and the part that enters the room through the opened entrances is defined as air infiltration. It cannot be ignored, which is also the reason for the lower CO2 concentration in the space without the mechanical fresh air supply. Figure 9.b: Judging from the chart the supply air point is around the 85% RH curve. What is the impact on interior air quality in this case?

5)
Reply: Thank you so much for the suggestion. It can be seen from Fig. 9(b) that the air is dehumidified and sent into the room after being treated by the AHU, and has no effect on the indoor air quality.