Problems and solutions in HVAC design

1、 Problems in implementing HVAC design specifications and standards

1. Indoor and outdoor air calculation parameters do not meet the specification requirements

According to the code for design, the indoor air calculation parameters in winter should not be lower than 12 ℃ for toilets and 25 ℃ for bathrooms. However, some public buildings' toilets, washrooms (with external windows and external walls) and toilets of residential buildings (with bath hot water supply in winter should be regarded as bathrooms) are not equipped with radiators, so it is difficult to meet the requirements of room temperature not lower than 12 ℃ and 25 ℃. Some residential buildings do not have radiators in their kitchens. The author thinks that it is not appropriate to install radiators in kitchens according to the requirements of not less than 12 ℃.

According to the code for design, the outdoor meteorological parameters of some major cities should be adopted according to Appendix II of the code. According to the Appendix 2, the outdoor temperature for heating in winter in Beijing should be - 9 ℃ except Yanqing and Miyun. It is obviously inappropriate to use - 12 ℃ for some projects located in the suburbs of Beijing.

2. There are omissions and errors in heating load calculation

According to the code for design, the heat load of heating system in winter should include the heat consumption of heating the cold air which seeps into the room through the gap between doors and windows. However, some projects did not calculate this part of the heat consumption when calculating the heating load, resulting in a large difference in heating heat load; the code for design has made clear provisions on the correction rate of each direction in the calculation of heat consumption of enclosure structure, which is 0-10% in the north, -5% in the East and West, and - 15% - 30% in the south, while in some projects, the correction rate of each orientation is changed to 20% in the north, 15% in the East and 15% in the west, and - 5% in the south, which is contrary to the requirements of the code.

3. Improper selection of radiator type in toilet

According to the code for design, cast iron radiator should be used in rooms with high relative humidity. However, steel radiators are used in toilets of many projects, and anti-corrosion measures are not strengthened. The author has seen that some office building toilets use steel closed radiator, but after a few years of use, the radiator string pieces were corroded, and the remaining two bare tubes were also seriously rusted. Practice has proved that cast iron radiator or aluminum radiator is the best choice for this kind of place.

4. The vertical and branch pipes of radiator in staircase are not separately configured

According to the code for design, the radiator in staircases or other places with freezing risk shall be supplied by separate vertical and branch pipes, and the regulating valve shall not be installed. However, in some projects, the stairwell radiator and the adjacent room radiator share one riser, which is connected by both sides. One side is connected to the stairwell radiator, the other side is connected to the adjacent room radiator, and valves are set on the radiator branch pipe. In this way, because it is difficult to ensure the tightness of the stairwell, once the heating failure occurs, the heating effect of the adjacent room may be affected, and even the radiator will be cracked.

5. The laying slope of heating pipe does not meet the requirements of the specification

According to the code for design, the laying of heating pipe should have a certain slope. For hot water pipe, the slope should be 0.003, not less than 0.002. However, the slope of water supply and return pipe is only 0.001 ~ 0.001 5 in some projects. Of course, the hot water pipeline can even be laid without slope due to the limitation of conditions, but the water flow velocity in the pipe shall not be less than 0.25 m / s.

6. Problems in ventilation of kitchen operation room

It is specified in the design specification that the exhaust velocity from the exhaust duct of the kitchen should not be less than 10.5% of the exhaust air from the air intake hood of the kitchen room, which should not be less than 10.5% of the exhaust air volume from the kitchen hood (3) the make-up air volume of the hot working room should be about 70% of the exhaust air volume, and the negative pressure value of the room should not be greater than 5 PA. However, some kitchens are not equipped with exhaust hoods, only a few exhaust fans are installed on the external walls; some of them are equipped with exhaust hoods, but the suction speed of the hoods is far less than 0.5 m / s, and the air volume of the selected exhaust fans is insufficient. Most of the projects are not equipped with comprehensive ventilation device and air supplement device, so it is difficult to meet the requirements of indoor sanitary environment and negative pressure value

7. The connection between the expansion tank and the hot (cold) water system does not meet the specification requirements

Code for design of boiler room (GB 50041-92) stipulates that valves shall not be installed on the connecting pipe between high expansion tank and hot water system. The connecting pipe here refers to expansion pipe and circulation pipe. This article is also applicable to the chilled water system of air conditioning. However, in some air-conditioning chilled water system, the expansion pipe of the high expansion tank is connected to the water collector in the refrigerator room and the valve is installed, which is not allowed. Once the operation is wrong, it will endanger the safety of the system.

8. The installation of fire damper in ventilation and air conditioning system does not meet the requirements of the specification

It is stipulated in the "high-rise regulation" that the air duct should not pass through the firewall or deformation joint. If it is necessary to pass through the firewall, the fire damper should be set at the place passing through the firewall; when passing through the deformation joint, the fire damper should be set on both sides. However, in some high-rise buildings, the fire damper is not installed at the place where the air duct passes through the fire wall, and when the air duct passes through the deformation joint, the fire damper is only installed on one side, but not on the other side. In addition, the location of fire damper in some projects is improper. According to the requirements, the fire damper shall be set tightly to the firewall, and the thickness of the through wall air duct connecting the fire damper shall be greater than or equal to 1.6 mm, and the air duct within 2 m on both sides of the firewall shall be insulated with non combustible materials. But in some projects, the fire damper on the air duct of ventilation and air conditioning is set at random, far away from the firewall, and the air duct between them is not marked with thickening, and no protective measures are taken.

9. There are some problems in the determination of the air volume of the air supply outlet in the front room of the smoke proof staircase

According to the high-rise building code, the pressure air supply volume of the front room of the smoke proof staircase in high-rise buildings is specified, and the specific air volume value is given according to the situation. It is stated in the note that the wind speed through the door should not be less than 0.7 m / s when opening the door; the article description specifies the number of doors to be opened, 2 below 20 floors and 3 above 20 floors. "High code" also stipulates that the pressurized air supply outlet in the front room of smoke proof staircase should be set up on each floor. According to these regulations, it can be calculated that the air volume of the air supply outlet in the front room of each floor should be L / 2 (below 20 floors) or L / 3 (above 20 floors, l is the total pressurized air supply volume of the front room). However, in some projects, the air volume of the air supply outlet in the front room of the smoke proof staircase is marked as L / N (n is the number of floors of the building), which is obviously much smaller. For example, for a 12 storey building, the total pressurized air supply volume of the front room of the smoke proof staircase is set as 16 000 m3 / h, but the air volume of the air supply outlet in the front room of each floor is marked as 16 000 / 12 ≈ 1 300 (m3 / h), which is obviously smaller. The correct mark should be 16 000 / 2 = 8 000 (m3 / h), and the outlet size should be configured according to this.

10. Mistakenly mix the calculation of smoke control area exhaust air volume with that of smoke exhaust fan

The "high-rise planning" has made clear provisions on the air volume of smoke exhaust fan: when discharging smoke in one smoke control zone, it shall be calculated according to the area of the partition not less than 60 m3 / h; when it is responsible for the smoke exhaust of two or more smoke control zones, it shall be calculated according to the maximum smoke control area of no less than 120 m3 / h per m2. Please note that this refers to the selection of the air volume of the smoke exhaust fan, not to double the exhaust air volume of the smoke control zone (the exhaust air volume of each smoke control zone is still not less than 60% per m2 of the smoke control area) When the smoke exhaust fan is responsible for two or more smoke control zones in horizontal or vertical direction, the air volume of the smoke exhaust fan is determined only according to the smoke exhaust of two or more smoke control zones at the same time. However, in some projects, the horizontal direction of the smoke exhaust fan bears the smoke exhaust of 2-3 smoke control zones with different sizes. In the design, the air volume of the smoke exhaust fan is calculated according to the total area of the 2-3 smoke prevention zones not less than 60 m3 / h per m2, instead of the maximum smoke control area per m2 When M2 is not less than 120m3 / h, the air volume of smoke exhaust fan is too small to meet the fire protection requirements. The other exhaust fans (Systems) are responsible for the smoke exhaust of more than two smoke control zones (inner corridor) in the vertical direction. In the design, the exhaust air volume of each smoke control zone (inner corridor) is calculated according to the respective area of not less than 120 m3 / h, rather than the respective area per m2 of not less than 60 In the calculation of m3 / h, the exhaust air volume of each smoke prevention zone (inner corridor) in the vertical direction is increased by one time, resulting in the air duct and tuyere configuration on each floor too large.

11. Improper selection of smoke outlet of smoke exhaust system in high-rise buildings

According to the code of the people's Republic of China, fire damper should be set at the partition wall where the air duct passes through the fire compartment. The author thinks that it is not suitable for the smoke exhaust duct to pass through the firewall. If it is necessary to pass through the firewall, a fire damper which can automatically close when the flue gas temperature exceeds 280 ℃ should be set at the place where the smoke exhaust duct passes through, and interlock with the smoke exhaust fan. However, some projects are negligent in design. For example, a smoke exhaust system in the basement of a project is responsible for the smoke exhaust of three rooms and one inner corridor (the doors between each room and the inner corridor are all fire doors), and a smoke exhaust fire damper is set on the smoke exhaust main pipe, and the smoke exhaust outlets of each room and corridor are all single-layer louver tuyeres, and there is no smoke exhaust fire damper at the smoke exhaust pipe passing through each firewall. The problem is: the fire door of each room is in vain. Once a fire breaks out in one room, it will spread to other rooms through smoke exhaust pipe. The correct way is to add smoke fire damper (automatically closed at 280 ℃) behind the single-layer louver exhaust outlet (where the smoke exhaust duct passes through the firewall) or change the single-layer louver tuyere into a special smoke exhaust outlet (normally closed at ordinary times, automatically open the smoke exhaust in case of fire, and close it again at 280 ℃).

3、 Problems in design drawings

1. The design description is incomplete

"Design depth regulation" specifies the contents of HVAC design specification. The design description shall include indoor and outdoor design parameters; heat source and cold source conditions; heat medium and refrigerant parameters; heating load and heat consumption index, total resistance of the system; radiator model; cooling and heating load of air conditioning; system form and control method; noise elimination, vibration isolation, fire prevention, anti-corrosion and temperature protection; material selection and installation requirements of air duct and pipeline; system pressure test requirements. However, some engineering design specifications are not complete.

2. The depth of the plan is not enough, and some contents that should be drawn are omitted

"Design depth regulation" has made detailed provisions on the contents of HVAC plan. However, quite a number of engineering designs have not been drawn in accordance with the regulations. The main problems are: some of the heating plans are not marked with the diameter and positioning size of the horizontal main pipe; some of the risers are not numbered; some are marked with the number of risers, but the riser is omitted; some are combined to draw a plan from the second floor to the top floor, and the number of radiators is also marked by layers, but the corresponding level is not indicated Some have only the first floor heating plan, but not the second floor to the top floor heating plan. In the ventilation and air conditioning plan, some did not indicate the equipment number and location size; some did not indicate the diameter and location size of chilled water pipe. There are also public building design, the kitchen part of the heating, ventilation, air conditioning and other content left to the kitchen equipment manufacturers to do, this is very inappropriate.

3. The depth of the system diagram is not enough

"Design depth regulation" has clear requirements for HVAC system drawing. However, some engineering designs are not carried out according to the regulations. The main problems are as follows: in the heating system diagram, some risers are not numbered, but replaced by building axis number; some pipeline numbers indicate the slope and aspect, but do not indicate the elevation of the beginning or end of the pipeline; some of the elevation at the change (turning point) of the pipeline is omitted; some even do not draw the heating system diagram or riser diagram. For air conditioning and ventilation design, some projects do not draw air conditioning chilled water system diagram and air system diagram (if the plan is fully explained, the system diagram can not be drawn, but for some more complex ventilation and air conditioning design, it is difficult to express it clearly by single * plan).

4. The design of boiler room is too simplified

"Design depth regulation" has made detailed provisions on the boiler room construction drawing design. However, in some boiler room designs, only one plan is drawn, without any section and system diagram. Many contents that should be explained are not explained, which is far from the design depth requirements.

5. The content of the calculation sheet is incomplete or even blank

The design depth regulation specifies the contents that should be included in HVAC design calculation sheet. However, quite a number of engineering designs do not have HVAC design calculations. Some heating and air conditioning design has calculation sheet, but the content is incomplete. In some heating design, only the heat consumption calculation is carried out, but not the hydraulic balance calculation and radiator selection calculation; in some high-rise buildings, the central air conditioning and smoke control design only have the calculation of summer cooling load, and there is no hydraulic calculation of air conditioning system and water system, no selection calculation of refrigeration and air conditioning equipment, and no calculation of smoke control and exhaust. In some air conditioning designs, the cooling load in summer and the heat load in winter are estimated according to the same index regardless of the room size, orientation, level and location (middle or end), which is not appropriate to configure air conditioning equipment.

6. The numbered list of HVAC equipment shows that the pictures are complicated and unclear

"Drawing standard" stipulates that heating, ventilation and air conditioning equipment, components, parts should be numbered list, and their models and performance should be filled in the table completely and clearly, and only the number should be indicated in the drawing. However, some HVAC designs are not carried out according to this regulation, but the names, models and even performances of various equipment and components are written on the drawings. The words on the drawings are complicated and time-consuming, and the notes are incomplete and unclear.

7. The plan, section and system drawings are inconsistent

In HVAC design, the equipment, size and other contents in the plan and section drawings and system drawings should be completely consistent, otherwise it will bring trouble to the construction, installation and use management. However, in some heating designs, the number of radiators and the plan are not consistent with the system diagram; the diameter of the main water supply and return pipes is not consistent with the system diagram; the plan is not consistent with the system diagram for pipe connection. In some air conditioning and ventilation design, the size of air duct, the plan and the system drawing are not consistent; the location and size of equipment and components, the plan and the section are inconsistent; the number, quantity, and drawing of equipment are inconsistent with the equipment list; and there are some air conditioning refrigeration equipment models selected in the air conditioning design, and the notes on the plan, system diagram and equipment list are inconsistent, which makes people confused.

8. The design drawings are inconsistent with the calculation sheet

In HVAC design, the selection of all equipment, pipes and components is determined by calculation. In a sense, the design drawing is the embodiment of calculation sheet, so the design drawing and calculation sheet should be completely consistent. However, some heating design drawings, such as the number of radiators and the diameter of riser pipe, are inconsistent with the calculation sheet, or even quite different. If the calculation sheet is not available, it appears on the drawing. If the calculation sheet is small, it is enlarged on the drawing; if the calculation sheet is large, the drawing is reduced. When the calculation is finished, if unreasonable points are found during drawing, adjustment is allowed, but adjustment calculation sheet or adjustment description shall be provided to unify the design drawing and calculation sheet.