Fresh air system design and Energy conservation analysis of an office building in Nanjing

Jiangsu Jienengyuan Environmental Science Co., Ltd. Li He Wang Jianzhong

Jiangsu Architectural Design Research Institute Co., Ltd. Wang Ying, Sun Jing and Wu Bin

Absrtact: This paper briefly introduces the air conditioning design of an office building in Nanjing. The air-cooled heat pump unit is used as the cold and heat source. In view of the large fresh air volume of the project, the fresh air system of the project is emphatically introduced. The exhaust air and heat recovery equipment is selected, the runner type total heat exchanger is adopted, and the Energy conservation and economic analysis are made.

Key words: Energy conservation of runner type total heat exchanger in fresh air system

1. General situation of Engineering

An office building is located in Hexi Central Business District, Nanjing, with a total land area of 13 300. The office building is situated on the east side of the plot with a total height of 126.10m. The ground floor area is 43500 m2, including 7500 m2 communication room (air conditioning system is set up separately). There are 2 floors underground for motors parking and equipment rooms. There are 28 floors above ground, 5-9 floors for communication room, 13 floors for refuge, 28 floors for multi-function hall, and the rest floors for office and conference.

2. Air conditioning load calculation

2.1. Indoor and outdoor design parameters:

Indoor: Dry bulb temperature 25 ℃, relative humidity 60%, enthalpy 55.8 kJ/kg in summer

Dry bulb temperature in winter is 20 ℃, relative humidity is 40%, enthalpy is 35.0 kJ/kg.

Fresh air volume 30 m3/h.p, per capita area index 8 m2/p 

The illumination adopts fluorescent lamp hidden in ceiling glass cover of air-conditioning room. The illumination power density index is 13W/m2.

A 250W computer per person, a 300W printer per layer and a 500W photocopier per layer

Outdoor: dry bulb temperature is 35 ℃, wet bulb temperature is 28.3 ℃, enthalpy 91.5 kJ/kg in summer

Dry bulb temperature in winter - 6 ℃, relative humidity 73%, enthalpy - 1.7kJ/kg in winter

1.2.Calculation parameters of enclosure structure:

(1) Outer windows (including transparent curtain walls):

(2) Roof: waterproof and thermal insulation flat roof, not suitable for people. Light-colored brick veneer, of which solar radiation heat absorption coefficient P = 0.50

Heat transfer coefficient K = 0.676W/(m2.K)

(3) Exterior wall:

Heat retaining reinforced concrete walls with dry-hanging granite in the South and north, whose heat transfer coefficient K=0.87W/(m2.K)

Insulating shale porous brick or reinforced concrete wall with dry-hanging granite on the east and west, whose heat transfer coefficient K=0.864W/(m2.K)

2.3. Load Statistics and Analysis

HDY-SMAD HVAC load calculation and analysis software was used for this building. The results are as follows:

The maximum total cooling load (including fresh air/total heat) in summer is 3972 kW.

The maximum indoor cooling load in summer is 1988 kW

The cooling load of fresh air in summer is 1984 kW

The maximum total heat load (including fresh air/total heat) in winter is -3107 kW.

The maximum indoor heat load in winter is -784 kW

Winter fresh air heat load is -2323 kW

The total wet load in summer is 697.1 kg/h.

The total wet load in winter is 492.7 kg/h.

Calculated cooling index of air conditioning load is 110.3 W/m2

Calculated heat index of air conditioning load is 86.3 W/m2

The fresh air cooling load accounts for nearly 50% of the total cooling load in summer and nearly 75% in winter. When calculating winter load, the correction coefficient of winter heat gain was deducted from 0.1. After energy conservation measures are taken to maintain the structure, the cooling and heating loads are obviously reduced. Because of the large number of personnel, relatively large total fresh air volume, relatively high proportion of fresh air load to total load, and relatively high proportion of fresh air energy consumption to total air conditioning energy consumption, energy conservation design of fresh air system is particularly important.

3. Design of fresh air and exhaust system for air conditioning

Each floor of the building is equipped with fresh air units. The fresh air volume on the standard floor is 7000 m3/h, the nineteenth floor or above 66000 m3/h. Because the 5-9 floors of the building are used for communication rooms, the ventilation shaft cannot be connected due to the restrictions of building conditions. The ventilation fan is installed on each floor above 10th floor, of which ventilation volume is 4250 m3/h, and the total design ventilation volume is 63750 m3/h. The ventilation goes to the roof through the ventilation shaft. The energy recovery unit is settled on the roof, the exhaust air is discharged after pre-treating the fresh air, and the pre-treated fresh air enters the fresh air shaft. In the transitional season, the fresh air is brought by turning on the electric valve on the fresh air and exhaust shaft or opening the windows at the same time turning off the energy recovery unit. The fresh air units in other layer directly induce outdoor fresh air. The fresh air volume should be about 3.5% bigger than the exhaust air volume, so as to avoid the leakage of the exhaust air into the fresh air. The flow chart of fresh air and exhaust system is shown in the figure.

4. Design and Selection of Energy Recovery Unit

4.1 Selection of air conditioning exhaust heat recovery device:

The exhaust heat recovery device in air conditioning system includes plate type (plate-fin type), runner type, heat pipe type and heat pump type according to the type of air heat exchanger, and sensible heat recovery and total heat recovery in the light of the nature of energy recovery. Based on their different working principles, these heat recovery devices are suitable for different occasions.

4.1.1 Plate (Plate-Fin)

This sensible heat recovery device is mainly made of aluminum foil, and the total heat recovery device is made of paper and other hygroscopic materials. The use effect depends mainly on the type of heat transfer matrix and the level of structure and technology. Total heat exchanger has higher efficiency, smaller air resistance and longer service life. It is suitable for situations of small air volume or setting up heat recovery devices regionally.

4.1.2 Heat pipe type

Heat pipe bundles are used as heat exchangers for sensible heat recovery. Heat is transferred by working fluid phase change, larger heat transfer capacity can be obtained under smaller temperature difference, which is very suitable for the heat transfer system of air conditioning with small temperature difference between supply air and exhaust air. Most of them are used in small and medium-sized public buildings with centralized placements of heat recovery devices.

4.1.3 Runner Type

The sensible core material is mostly aluminium foil, while the core material of total heat recovery device has developed to molecular sieve core. The runner type heat recovery device is characterized by its mature technology, which is suitable for placing heat recovery devices in large and super large public buildings, especially large air volume and relatively small structure size. At 3 m/s surface wind speed, the maximum heat transfer efficiency can reach 80%.

4.1.4 Heat pump

Its working principle is that heat pump is used to obtain high-grade heat energy at the same time air-conditioning exhaust cooling (heat) is used as a low-temperature cold (heat) source. The heat recovery device has high efficiency, does not need a centralized cold and heat sources, so that the air conditioning water pipeline system is reduced. It is suitable for situation of small air volume and layered settling of heat recovery devices.

According to the comparison of various heat recovery types and the actual situation of this project, the runner type total heat recovery device was chosen for this project.

4.2. Selection of Runner Energy Recovery Unit

The designed fresh air volume of the unit is 66 000 m3/h and the exhaust air volume 63 750 m3/h. The unit is equipped with primary effect filter, runner heat exchanger, and fan section. The power of fan motor is 30 kW, the power of runner drive motor is 0.75 kW, and the external pressure is 350 Pa. The figures are shown as follows:

5. Energy conservation analysis

5.1. Energy Recovery Calculation

The enthalpy efficiency and temperature efficiency of the air energy recovery device are the most important technical indexes. The heat transfer efficiency and recovery energy of the total heat exchanger are calculated according to formulas (1) and (2): 

η= 100 * (h1-h2)/ (h1-h3)      (1)

Q =ρV (h1-h2) / 3600      (2)

In which, η - Enthalpy efficiency of the device, %; 

h1 - Enthalpy of fresh air inlet, kJ/kg;

h2- Enthalpy of outlet air, kJ/kg;

h3 - Enthalpy value of exhaust inlet air, kJ/kg.

Q - Unit heat recovery, W;

V-Air volume, m3/h;

ρ-Air density, kg/m3.

Article 5.3.14 of Standard for Energy conservation Design of Public Buildings GB50189-2005 requires that the rated heat recovery efficiency of exhaust heat recovery devices (total heat and sensible heat) should not be less than 60%. The design takes enthalpy efficiency as the index. The enthalpy efficiency should be more than 60% in refrigeration and 65% in heating. Because the designed fresh air volume is not equal to the exhaust air volume, the exhaust air volume should be used as the calculation air volume of energy recovery. Under the design conditions, the runner energy recovery device can recover 436 kW cooling capacity when refrigerating, about 11% of the total cooling load and 532 kW heat when heating, about 17% of the total heat load. all of them are used for pre-cooling and pre-heating fresh air.

5.2. Effect on the Selection of Cold and Heat Source Equipment

The air-cooled heat pump unit is used as the cold and heat source of air-conditioning in this project, with a coincidence factor of 0.85, a cooling and heat load of 3376 kW and 2641 kW. The runner energy recovery unit is used to pretreat fresh air with exhaust air. The recovered cooling capacity is 436 kW and the heat is 532 kW. The required refrigeration capacity and heating capacity is 2940 Kw and 2109 kW. According to the meteorological parameters of Nanjing in winter, heat pump units are required to provide: 7℃ /12℃ cold water at 35℃ outdoor ambient temperature in summer and 45℃ /40℃ hot water at -6℃ outdoor ambient temperature in winter. If the same type of air-cooled heat pump unit is used, the refrigeration capacity and heat capacity is 876 kW and 525 kW. If the runner energy recovery unit is not used, five units are needed; if used, four units are designed and selected. The total cooling capacity and heating capacity of the system is 3504 kW and 2100 kW.

If the runner energy recovery unit is not used, the initial investment of water pump and heat pump unit is about 4.9 million RMB; if used, the initial investment of water pump and heat pump unit is about 3.92 million RMB, runner energy recovery unit about 150,000 RMB, totaling about 4.07 million RMB.

After the runner energy recovery unit is adopted, the initial investment of the cold and heat sources is saved about 830,000 RMB.

5.3. Distribution comparison

Before and after the runner energy recovery unit is used, the total distribution of air conditioning system is compared as follows:

After the runner energy recovery unit is used, the total power distribution saves about 269 kW, about 15% of the total HVAC power distribution. HVAC power distribution accounts for 58% of the total power distribution. After the runner energy recovery unit is used, the saved power distribution makes up 10.35% of the total power distribution.

Based on the design parameters of GB50189-2005, the annual energy consumption is reduced to 50%. By using the runner energy recovery unit, the Energy conservation contribution rate of air conditioning system is increased by about 5.17%, from 20% to 25%. 

5.4. Operation Cost Analysis

Under normal climatic conditions in Nanjing, air conditioning operates four months in summer and three months in winter. It runs nine hours every day and 22 working days every month. The full load rate is calculated by 70%, and the electricity price of office air conditioning is calculated by 0.88 RMB/kWh. The total electricity consumption and operating cost of air conditioning system are compared in the following table:

If the runner energy recovery unit is used, the annual operating cost of air conditioning can be saved about 222,000 RMB, and the annual operating cost of the runner recovery unit is 75,000 RMB, so the initial investment of the runner recovery unit can be recovered in one year.

6. Conclusion

In this project design, by using exhaust air to pretreat fresh air through the runner energy recovery unit, the initial investment of heat and cold sources is saved, the total distribution of air conditioning system is reduced by about 15%, the total distribution of the project is decreased by 10.35%, the contribution rate of Energy conservation is increased by 5%, indicating a good energy conservation effect.

In the design, fresh air volume should be calculated according to the maximum number of personnel. It is suggested that new air frequency conversion device should be used to adjust fresh air volume according to the indoor CO2 concentration detection value. The exhaust air frequency conversion device should be used synchronously to maintain the positive pressure of the room and reduce energy consumption.