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HVAC common design misunderstanding

2018-06-04

  The design of the HVAC design plays an extremely important role in the HVAC project. It is directly related to the success of the project. Although the quality of design personnel has been greatly improved in the design drawings since the implementation of the construction plan review system, there are still problems in the design process.

  First, heating

  (I) Problems with entrance and exit devices

  According to the Heating Ventilation and Air Conditioning Design Specification (hereinafter referred to as the Design Code), the hot water heating system should be provided with thermometers, pressure gauges, and decontamination devices on the supply and return headers of the thermal inlet. If necessary, a calorimeter should be installed. Designers often only pay attention to the setting of the household heating device, neglecting the entrance device. Some drawings indicate the standard atlas number adopted by the entrance device, and some drawings are omitted. Therefore, the drawing approval department often gives opinions. In the construction drawing, the designer should not only specify that the above regulations are followed, but also emphasize two points:

  1. The heat inlet water supply pipe is preferably equipped with a two-stage filter. The first stage should be a coarse filter with a pore size of 3.00 mm, the second stage should be a fine filter with a size of 60 mesh, and the filter should be set with a filter size of not less than 60 mesh. filter.

  2. Install a drain cock on the water supply pipe before entering the home. For example, in a residential area, the centralized heating method of household measurement is adopted. After the heating, the radiators of many households are not hot, and the cause is checked. No blockage is found at the filter, and the radiators are all of no cast iron type. After the pipeline is finally dismantled, It was found that the reason was that the elbows at the downturns of the front branch of the pipe in the piped well were filled with a large amount of fine sand. Although there was a water filter, the filtering hole was too large, causing a blockage, and a drain was added later. Several times of water was released after the valve was blocked, and no heat problem occurred again.

  (II) Question of the number of entrances

  With regard to the number of entrances, the practice varies. Some developers require one entrance per unit to facilitate the management and collection of heating bills. Some developers require an entrance to a building because the infrastructure investment can be saved, so there is not one. A unified model, but no matter which method is adopted, designers must consider the rationality of the indoor heating system and consider the rationality of connecting with outdoor pipelines. It is not only possible to design the indoor system to be convenient and save trouble, without regard to the outdoor pipeline system. .

  At the same time, the horizontal distance, heat consumption and pipe diameter of the inlet pipeline from the building axis shall be indicated in the design drawings. If there are multiple inlets, the heat load and pipe diameter of each unit shall be indicated separately, and some designs only indicate the total Heat load, this is not appropriate, because in most cases, the outdoor heat network is designed and installed by the thermal system.

  (Ⅲ) Staircase radiator standpipes should be set separately

  The design code states that in stairwells or other places with danger of freezing, the radiators should be provided with independent standpipes for heating, and no regulating valve must be installed. However, some projects use the radiators in the stairwells in common with the radiators in adjacent rooms. The vertical riser is connected on both sides with one side connecting the stairwell radiator and the other side connecting with the room radiator of the adjacent room. In this way, due to the difficulty of ensuring the airtightness of the stairwell, once the heating fails, the heating effect of the adjacent room may be affected. Cracked radiator.

  (IV) The issue of common riser installation of the retractor

  At present, multi-storey or high-rise residential buildings are designed with common riser systems. In the design, vertical partitioning of heating systems and common risers is generally required based on factors such as system hydraulic balance, cooling equipment, pressure capacity, and chemical pipe properties. , and should consider the issue of pipeline thermal compensation.

  However, some designs consider that indoor laying is buried and neglecting the thermal expansion of the shared riser in the tube well. Therefore, the retractor is not provided; while some compensators are designed, the amount of thermal expansion is not carefully checked to determine the compensator. The position; some designs have fixed brackets installed at the upper and lower positions of the compensator, so that the compensator does not achieve compensation for the expansion and contraction of the pipe due to thermal expansion, resulting in cracking of the branch pipe due to thermal expansion and contraction of the riser. phenomenon.

  (V) Heating problems of houses with ground floor shops

  For residential design specifications with ground floor shops, it is clearly stipulated that a heating system and a heat metering device should be set separately for the public and public spaces in the building. The existing design problem is that the shop does not have a separate heat metering device. Or use a shared system with residential buildings. At present, there are more and more buildings along the street with underground merchants. Designers should strictly follow the specifications to avoid unnecessary trouble after use.

  Second, air conditioning and ventilation

  (I) Large installed capacity of chillers

  At present, in the design process of air-conditioning systems, some designers use load indicators to estimate that the installed capacity of refrigerating machines is generally too large, resulting in a large waste of initial investment and at the same time affecting the efficiency of the cooling machine under partial load.

  The air conditioning and refrigeration technical manuals, heating, air conditioning, and refrigeration manuals give estimates of the summer cooling load for shopping mall buildings ranging from 210W/m2 to 240W/m2, and the hotel office cooling load indicators range from 94W/m2 to 163W/m2.

  In the actual design process, due to the consideration of various safety factors, the installed capacity of the refrigerators per unit area of the air conditioner is mostly larger than the estimate of the cold load in the manual, which is much larger than the peak value of the unit air conditioning area in actual operation, causing air conditioning. The initial increase in investment in the system is large, and from the whole year, the actual load of the building is at a short time, so in fact most of the cold machine will run at a relatively low load rate. The COP is not high, from the Taiyuan area. According to the statistics, the cold load in the mall is between 100W/m2 and 150W/m2, and the office cooling load is between 70W/m2 and 90W/m2 to meet the requirements.

  (II) Improper selection of thermal insulation materials

  The selection of insulation materials should take into consideration the service life and the occasion of use. There are many varieties of insulation materials on the market, but many projects use aluminum foil glass wool insulation because of cost and ease of construction. Aluminum foil glass wool products can also be used as heat insulating materials for air ducts, because the surface temperature of air ducts is high and it is not easy to cause condensation. However, in the construction process, the capacity of materials and the quality of aluminum foil tapes must be controlled to ensure the service life of the materials. Thermal insulation effect.

  Due to the high water absorption of glass wool, it is not suitable for the insulation of chilled water pipes, especially risers. In previous years, due to lack of experience, the use of glass wool insulation in early air-conditioning design has caused condensation. Therefore, it is recommended that the owner should use better insulation materials as far as possible to avoid unnecessary rework and waste.

  () Improper selection of pump head

  Pump head selection varies greatly from design to design. For example, a cooling tower is placed on a 60-meter-high roof, and the cooling water is a closed-loop system. The designer adds the elevation to the pump head when selecting the pump head. Results The selected pump head can be as high as 80 meters.

  Other designs do not consider the difference between winter and summer flow when selecting a chilled water pump. For example, the circulatory flow of chilled water at 7°C to 12°C for a certain project in summer is 600m3/h in winter and 50°C to 60°C in winter. The amount of hot water circulation is 289m3/h. It can be seen that the circulation of chilled water for air conditioning in summer is much larger than that for heating in winter, so it is not appropriate to use one pump in summer and winter. The freezing pump heat pump should be set separately.

  (IV) Problems with the installation of fire prevention and smoke prevention valves for air conditioning and ventilation systems

  Fire dampers are different from smoke fire dampers in that they cannot be used in a mixture of these two valves with different functions. Fire dampers are usually installed in ventilation and air-conditioning pipelines to cross fire partitions or deformation joints. They are usually opened. In fire, when the smoke temperature reaches 70 At °C, the fusible element in the valve body melts, thereby cutting off the smoke and the fire spreading along the ventilation duct to other fireproof areas. High regulations stipulate that the air duct should be equipped with a fire damper through the firewall; when passing through the deformation joint, fire dampers should be provided on both sides.

  However, some designs do not have a fire damper at the wind pipe through the firewall. Some ducts have a fire damper on one side only when the duct passes through the deformation joint, and the other side is not. In addition, some engineering fire dampers are not properly positioned. According to the requirements, the fire damper shall be located close to the fire wall, and the thickness of the wall-through air duct connecting the fire damper shall be δ ≥ 1.6mm. The air duct within 2m on both sides of the firewall shall be insulated with non-combustible materials.

  However, fire dampers on some ventilation and air-conditioning ducts of some projects are arbitrarily set up and away from the fire wall. The air ducts between them are neither thickened nor protected. The smoke fire damper is located on the special exhaust air duct or dual-use air duct, and the 280°C fuse temperature fuse is installed on the exhaust valve body. When the exhaust temperature reaches 280°C, the temperature fuse is actuated and the valve is closed and stopped. Smoke exhaust.

  Concluding remarks: To avoid design mistakes, it is necessary to strengthen the design of professional knowledge and professional norms of the design staff, and at the same time more communication with the construction and installation personnel to understand the use of the project.

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