Frequently asked questions in HVAC engineering design?

Since the implementation of the construction drawing review system, designers' awareness of implementing relevant national design codes, regulations and standards has been greatly strengthened, and the quality of construction drawing design has been greatly improved. However, due to numerous infrastructure projects and various engineering categories, there are some different understandings and practices in implementing design specifications. This paper discusses some common problems and solutions in HVAC design and construction drawing review.

First, heating.

(A) the entrance device problem

According to the Code for Design of Heating, Ventilation and Air Conditioning (hereinafter referred to as the Code for Design), thermometers, pressure gauges and decontamination devices should be installed on the main supply and return pipes at the thermal entrance of hot water heating system, and heat meters should be installed when necessary. Designers often only pay attention to the setting of indoor thermal equipment, but ignore the air inlet equipment. Some drawings are marked with the standard atlas number of the air intake device, while others are omitted, so the drawing review department often puts forward opinions. In the construction drawing, in addition to the above provisions, designers should also pay special attention to two points:

1. It is best to set two-stage filters on the water supply pipe at the hot entrance. The first stage should be a coarse filter with an aperture of 3.00mm, and the second stage should be a fine filter with a mesh number of 60. Return pipe should be equipped with a filter with a mesh of not less than 60 meshes.

2. Before entering the house, install a drain cock on the water supply and backwater branch pipes. For example, in a small shop, central heating is measured by households. After heating, the radiators in many families are not hot. The reason is that the filter element is not blocked, and the radiators are all sand-free cast iron. Finally, after the pipeline was demolished, it was found that the elbow of the branch pipe was filled with a lot of fine sand particles. Although there is a water filter, the aperture of the filter screen is too large, which leads to blockage. After that, install a drain valve to block it and let the water flow several times.

(2) Number of entrances

There are different approaches to the number of entrances. Some developers require an entrance for each unit to facilitate the management and collection of heating fees, and some require an entrance for a building, so there is no unified model because it can save capital investment. However, no matter which way is adopted, designers should consider the rationality of the connection between indoor heating system and outdoor pipeline, not only the convenience and trouble of indoor system design, but also the outdoor pipeline system. At the same time, the horizontal distance, heat consumption and pipe diameter of the inlet pipeline from the building axis should be indicated in the design drawings. If there are multiple entrances, the heat load and pipe diameter of each unit should be indicated separately, while some designs only indicate the total heat load, which is not suitable, because in most cases, the outdoor heating network is designed and installed by the thermal system.

(3) The riser of the radiator in the stairwell should be set separately.

According to the design specification, the radiator in stairwell or other places with freezing danger should be heated by independent riser, and no regulating valve should be installed. However, in some projects, the radiator in the stairwell is connected with the radiator in the adjacent room through a riser, one side is connected with the radiator in the stairwell, and the other side is connected with the radiator in the adjacent room. It is difficult to ensure the air tightness of the stairwell. Once the heating fails, it may affect the heating effect of adjacent rooms and even freeze the radiator.

(4) * * * Install expansion joint with riser.

At present, most multi-storey or high-rise residential buildings are designed with riser systems. In the design, the heating system and the riser system should be vertically divided according to the hydraulic balance, heat dissipation equipment, pressure bearing capacity and the characteristics of chemical pipelines, and the thermal compensation of pipelines should be considered. However, some designs think that indoor laying is underground, but ignore the thermal expansion of the riser in the tube well, so there is no expansion device; Although some people designed the compensator, they didn't carefully check the thermal expansion to determine the position of the compensator. Another design is to install fixed brackets at the upper and lower positions of the compensator, so that the compensator can not compensate the deformation and expansion of the pipeline due to thermal expansion, and as a result, the branch pipe is cracked due to thermal expansion and expansion of the riser.

(e) Heating of residential buildings with shops on the ground floor.

The Code for Design of Residential Buildings with Stores on the Ground Floor clearly stipulates that heating systems and heat metering devices should be set up in public rooms and public spaces in buildings. The problem in the design is that either the shops don't set up separate heat metering devices, or the houses adopt * * * system. At present, there are more and more buildings with shops along the street, so designers should design in strict accordance with the requirements of the code to avoid unnecessary trouble after use.

Second, air conditioning and ventilation

(1) The installed capacity of refrigerator is too large.

At present, in the process of air conditioning system design, some designers use load index estimation, which leads to a large installed capacity of refrigerator, which causes a great waste of initial investment and affects the efficiency of refrigerator under partial load.

The cooling load of shopping mall buildings in summer is estimated to be 2 10w/m2 ~ 240w/m2, and that of hotel office buildings is 94w/m2 ~ 163W/m2. In the actual design process, due to various safety factors, the installed capacity of the refrigerator per air conditioning area is mostly larger than the cooling load estimation in the manual, which is much larger than the peak cooling capacity per air conditioning area in actual operation, resulting in a substantial increase in the initial investment of the air conditioning system. Moreover, from the perspective of the whole year, the actual load of the building is at the peak for a short time, so in fact, the refrigerator will run at a relatively small load rate most of the time, and the COP is not high. According to the statistical results of Taiyuan, the cooling load of shopping malls is between 100 W/m2 ~ 150 W/m2, and the cooling load of offices is between 70 w/m2 and 90 w/m2, which can meet the use requirements.

(B) improper selection of insulation materials

The selection of thermal insulation materials should consider the service life and use occasions. There are many kinds of thermal insulation materials in the market, but due to the cost and convenient construction, many projects use aluminum foil glass wool for thermal insulation. Aluminum foil glass wool products can be used as heat insulation materials for air ducts, because the surface temperature of air ducts is high and it is not easy to condense. However, in the construction process, the capacity of materials and the quality of aluminum foil tape should be strengthened to ensure the service life and heat insulation effect of materials. Glass wool has strong water absorption and is not suitable for insulation of chilled water pipes, especially risers. A few years ago, due to lack of experience, the water pipe system with glass wool insulation in early air conditioning design had already appeared dew condensation. Therefore, it is suggested that the owners try to use better insulation materials, such as Owens Kenning, Forrest and other closed-cell insulation materials, or "rubber and plastic" materials commonly used in the market at present, so as to avoid unnecessary rework and waste in the future.

(C) improper selection of pump head

In different designs, the head selection of water system is very different. For example, the cooling tower of a project is placed on the roof of more than 60 meters high, and the cooling water is a closed circulation system. However, when designers choose the pump head, they actually add the elevation to the pump head. Therefore, the head of the selected pump is as high as 80 meters. Other designs do not consider the difference of flow in winter and summer when choosing chilled water pump. For example, the circulating flow of chilled water at 7℃ ~ 12℃ required by air conditioners in summer is 600m3/h, while the hot water circulating flow at 50℃ ~ 60℃ required in winter is 289m3/h, indicating that the circulating flow of chilled water in air conditioners in summer is much larger than that of hot water for heating in winter, so it is not appropriate to share a pump in winter and summer. Chilled water pump and hot water pump should be set separately.

(d) Installation of fire prevention and smoke prevention valves in air conditioning and ventilation systems.

Fire dampers are different from smoke exhaust fire dampers and cannot be used together. Fire dampers are generally located where ventilation and air conditioning pipes pass through fire zones or deformation joints, and are generally open. In case of fire, when the flue gas temperature reaches 70℃, the fusible piece in the valve body melts, thus cutting off the spread of smoke and fire to other fire areas along the ventilation pipeline. According to the high specification, the air duct should be equipped with a fire damper where it passes through the firewall; When crossing the deformation joint, fire dampers should be set on both sides. However, some designs have no fire damper at the place where the air duct passes through the firewall, and some air ducts pass through the deformation joint only with fire damper on one side and no fire damper on the other side. In addition, the location of fire dampers in some projects is unreasonable. According to the requirements, the fire damper should be close to the firewall, the thickness of the through-wall air duct connecting the fire damper should be δ≥ 1.6mm, and the air ducts within 2m on both sides of the firewall should be insulated with incombustible materials. However, in some projects, the fire dampers on the air ducts of ventilation and air conditioning are set at will, far away from the firewall, and the air ducts in the middle are neither thickened nor protected. The smoke exhaust fire damper is installed on the special smoke exhaust pipeline or dual-purpose pipeline, and the smoke exhaust valve body is equipped with a temperature fuse that blows 280℃. When the smoke exhaust temperature reaches 280℃, the temperature fuse acts and the valve closes to stop smoke exhaust.

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