Phenomenon: The air conditioning effect of an office building is not good.  Room temperature is high, can't drop down, determination:

1) when the outdoor temperature is 29.5℃, the indoor temperature is 29℃;

When the outdoor temperature is 29.4℃, the indoor temperature is 28.4℃.  That is to say, when the outdoor parameters do not meet the design conditions, the room temperature of many southbound rooms has exceeded the design parameters by 27℃.

2) The actual cooling capacity of LF30 cold air unit in North Cooling Plant is lower than its rated value.  The first measured inlet air temperature is 20.25℃, and the cooling capacity is 33.2kW (rated as 35.8kw);  The wet bulb temperature of the inlet air is determined to be 17.7℃ for the second time, and the cold output is 23kW (rated as 30.2kW).

3) After calculation, the cooling load required by the design is 89.2kW, and the cooling capacity of the selected two LF30 units should be 2×38.4=76.8kW, which is about 14% less.

According to the above analysis of the same measurement number, compared with the actual cooling capacity of the air conditioner, the designed cooling load of the unit is about 20% less in total.  Under normal circumstances, this will not affect the use effect.  There must be other reasons for the poor cooling effect of the air conditioner in this office building, which should be thoroughly analyzed from the aspects of design, construction and operation.

Cause and analysis

1) The return air cannot be returned to the engine room according to the designed air volume, and the fresh air leaks into the engine room everywhere, resulting in an increase in the inlet air temperature of the air conditioner.

The return air pipe in the machine room is not connected to the unit, but the negative pressure in the machine room is sucked into the air conditioner together with the fresh air.  The measured return air volume only accounts for about 60%.  There are doors and windows in the computer room, which are not tight.

The ceiling on the first floor is connected with the outdoor.  The return air pipe on the second floor passes through the outer wall of the outdoor ceiling to the indoor ceiling to suck the return air of the previous layer and return it to the machine room.  Because the return air branch pipe is too close to the roof, the outer wall hole can't be plugged tightly after passing through the air duct, and the upper wall of the return air drying pipe is less than 150mm from the roof.  The flange screws on the upper part of the air duct are not screwed, and a large amount of outdoor air is sucked into the return air duct.

The ground return air outlet on the second floor floats on the concrete floor hole, while the air duct under the floor is not connected with the hole edge.

The return air outlet of the air-conditioned room on the first floor is located in the ceiling, and the ceiling is a return air static pressure box.  However, the interior walls of this project are light partition walls, which do not reach the top, so that the air-conditioned rooms communicate with the ceilings of other non-air-conditioned rooms, resulting in almost no return air in the air-conditioned rooms on the first floor.  The return air volume on the second floor is also very small, and the wind speed at the return air outlet is 0.1~0.5m/s (the design value is 1.37m/s).

2) The arrangement of air-cooled condensation by split air supply is unfavorable, it is just placed under the fresh air inlet, and the distance between the condenser and the wall is too small, and brick walls are built around it.  In this way, the temperature of the fresh air is increased, and secondly, the air circulation of the condenser itself is not smooth, which makes the cooling capacity of the fan coil air conditioning unit decrease.

Transformation method

1) Leakage plugging treatment will be carried out one by one for the return air outlets on the second floor to minimize the unorganized fresh air infiltration.

2) Connect the return air pipe of the system to the unit, so as to reduce the temperature rise of return air caused by the heat dissipation of equipment and the cooling load of the machine room itself.

3) Add the air return pipe of the first-floor air-conditioned room.

4) Between the main air supply and return air pipes, an adjustable side ventilation pipe is set to adjust the air volume, so as to avoid excessive noise at the return air outlet.

5) Replace two LF30 units with two LH48 units.  The rated refrigerating capacity is 2×57.6=115.2kW, which is 30% more than calculated.

Results after modification

1) the room temperature has reached the requirements.  After determination, when the outdoor parameters are 29.6℃, 78.1%, and the indoor parameters (second floor) are 23.6, 59.7% (south), 24.9℃, 66.7% (north), when the outdoor parameters are 34.2℃ on average and 37.7℃ on the highest, the indoor temperature is 24.4℃ and relative humidity in the north and south.

2) The actual cold output of 2)LH48 reaches the famous brand performance as follows

9600×1.2(13.1-8.9)=48384kcal/h=56.3kW

1. 50mm polyurethane foam plastic is pasted in the main return air pipe, and the muffler elbow is installed on the air delivery pipe.  Results The telephone switchboard room reached 52dB(A) and the office on the second floor reached 30dB(A).

2. Close all the new air outlets, and about 30% fresh air enters the return air system unorganized.  It can't be solved, only increase cooling load and energy consumption.

Lessons to be learned

A lesson from the design and construction of this project is that outdoor air leaks into the return air system, resulting in increased cooling load and waste of energy.

1) The return air system of the second-floor air conditioner should not be built in the first-floor ceiling, and should not pass through the outer wall.  It is a system layout error that the outdoor ceiling is connected to the air return outlet on the second floor.  Should rely on the inner wall to return air, can achieve the ideal effect.  But it can't be redeemed, so we have to use it as a lesson.  It is the root of fresh air leakage.

2) When there is a window in the air-conditioning room and the building is not sealed, generally do not return air from the room, but connect the return air pipe to the unit.  If the engine room returns air, the doors and windows of the building must be tight, and all kinds of holes on the wall must be blocked, otherwise the return air ratio cannot be reached.

3) When designing air conditioning, we must consider the possibility of construction and installation, not just drawing.