Determination of air conditioning load and capacity
Air conditioning load includes air conditioning cold load and air conditioning heat load. The cooling (heating) load of air conditioning refers to the amount of cooling (heating) that needs to be provided to the building per unit time in order to maintain the indoor air parameters in the state of design parameters. This is a variable influenced by many factors, such as indoor design parameters, heat and moisture dissipation of indoor personnel and equipment, the nature of envelope, outdoor air environment parameters (including temperature, humidity, airflow speed, etc. ), solar radiation intensity and so on. The fundamental purpose of air-conditioning design is to make the air-conditioning system properly provide the cooling (heating) quantity to meet the needs of the building under the design and calculation state, and adapt to the cooling (heating) load of the building and its changing needs at any time.
In the process of air conditioning system design, air conditioning load calculation is the first step. The calculation of air conditioning load should include the determination of air conditioning design calculation load and the analysis of load in each period; Secondly, the capacity of the equipment must meet the requirements of air conditioning design and cooling (heating) load calculation; In addition, the equipment configuration should adapt to the characteristics of air conditioning load change. In the design of air conditioning system with air-source heat pump type cold and hot water unit as cold source, the capacity of heat pump unit should not only consider the simultaneous utilization coefficient of all parts of the building, but also consider the actual cooling capacity and actual heat supply of heat pump, which will be affected by poor ventilation, short circuit of partial air flow (the output loss of this part accounts for about 5%), surface area ash and scaling of outdoor heat exchanger, equipment attenuation and other factors, so the selected heat pump unit should consider the safety factor.
Expressed by the formula: Q=β 1? β2? QD。
Where: q-refrigeration (heating) capacity of heat pump unit under design conditions, KW.
Qd-design calculation load, KW
β1-coefficient of simultaneous use, determined according to specific projects, generally 0.75 ~ 1.0.
β2- safety factor, generally 1.05 ~ 1. 10.
In addition, the heat pump unit should meet the cooling requirements of the summer system and the heating requirements of the winter system. The parameters of rated cooling capacity and rated heat supply of heat pump units from different suppliers are different, which are not necessarily consistent with the outdoor design parameters of air conditioners in various regions. As far as Nanjing is concerned, the rated refrigeration conditions of heat pump units provided by general suppliers are consistent with or close to reality. The air dry bulb temperature is generally 35℃, and the inlet and outlet temperatures of chilled water of air conditioner are about 65438 02℃ and 7℃ respectively. The rated operating conditions of heating in winter are outdoor temperature of 7 ~ 8℃ and inlet and outlet water temperature of 50 ~ 55℃. This condition is far from the design and calculation temperature of Nanjing winter air conditioning. The climate in Nanjing is characterized by Leng Xia fever in winter. For general office and hotel-based complex buildings, the calculated heat load of air conditioning heating design in winter is about 70-85% of the calculated cold load of air conditioning design in summer. When selecting a heat pump unit, the actual output of the heat pump unit corresponding to the local design and calculation meteorological parameters should be checked. If the cooling capacity of the heat pump unit under the outdoor parameter design and calculation conditions is greater than the design and calculation cooling load, and the heating capacity is equal to the heat load, the heat pump should be selected according to the heat load. On the other hand, if the cooling capacity meets the requirements of cooling load design and calculation, and the heat supply is greater than the required heat, some air-cooled chillers and some air-cooled heat pumps can be considered to reduce investment. Generally speaking, the heat pump selected according to the cooling load in summer can meet the requirements of heating in winter.
Determination of unit type and quantity
Air-cooled heat pump type cold and hot water units can be divided into scroll heat pump units, piston heat pump units and screw heat pump units according to different compressors; Heat pump units can be divided into integral heat pump units and modular heat pump units according to the structural size and combination scale of the units. There is no essential difference between integral heat pump unit and modular heat pump unit. The so-called modular heat pump means that a heat pump unit is composed of multiple heat pump units (with independent refrigeration circuit, independent evaporation and condensation, independent rack and even independent control panel) connected in parallel. The combination of each unit is flexible and convenient, and the failure of any unit will not affect the work of other units.
Domestic heat pump manufacturers mostly produce modular heat pump units, while integral heat pump units are combined units and integrated frames in appearance. Although there may be multiple compressors or even more than two refrigeration circuits inside, they are generally inseparable. The main advantages of modular heat pump unit are low noise and small vibration. Because there are many refrigeration circuits in the system, it has little effect on the water temperature of the system when defrosting in winter. System compatibility is also good. In addition, heat pump units are generally placed on the roof, and modular heat pump units have the advantages of flexible combination, small size and light weight, and are convenient to transport, hoist and install. If the project is large, the modular heat pump unit may be slightly less efficient than the integral unit under rated working conditions because of the large number of refrigeration units, many fault points and large maintenance. In addition, because modular heat pumps generally use plate heat exchangers, the requirements for water quality and hydraulic balance between units are higher. To sum up, modular heat pump has its advantages for smaller systems or occasions with special requirements on size, weight and hoisting.
As for piston heat pump unit and screw heat pump unit, theoretically speaking, screw heat pump unit has less moving parts, less maintenance, high efficiency and low noise. However, the noise of screw heat pump is slightly lower than that of piston heat pump (about 3-5dB(A)) because a large part of the noise of heat pump comes from the fan, and the noise of compressor can be reduced by adding sound insulation cover. In addition, the thermal resistance of the heat pump unit is mainly on the outdoor heat exchanger side, and the efficiency of the heat pump is affected by factors such as the area of the two devices. Therefore, from the engineering point of view, the efficiency difference between screw heat pump and piston heat pump is limited, but the price of screw heat pump is higher than that of piston heat pump.
As for refrigerants, if possible, choose refrigerants with little impact on the environment, such as R 134a and R407C, of which R407C is the first choice, followed by R134a; Considering the price of refrigerant, R22 is the cheapest one at present.
Location of heat pump unit
The positions of the heat pump unit are as follows: first, it is placed at the top of the podium; second, it is placed at the top of the tower; third, it is placed on the window sill; fourth, it is placed indoors with high clear height. Considering the reasons of hoisting and replacement in the future, heat pump units are often placed on the roof of the podium. When the heat pump unit is placed at the top of the podium, its impact on the main building and the surrounding environment should be evaluated. For a large heat pump unit (≥200RT), the noise of a single unit is about 75 ~ 85 dB (A). When necessary, you can add a sound barrier, or avoid opening the door near the main building of the unit, and make double-layer windows or high-quality insulating glass instead of ordinary single-layer glass windows. The heat pump unit arranged on the windowsill is often the place where each floor needs independent configuration and measurement. Only for small-capacity heat pump units, it is advisable to adopt the form of side-in and side-out. When selecting the upper exhaust heat pump unit, the duct should be installed and the side exhaust should be changed. Even if there is a high clearance indoors, it is not appropriate to place the heat pump unit indoors. If it must be set indoors due to conditions, there should be available through-flow in the room and sufficient air intake area, and the exhausted air should be discharged to the outside in an organized way through the air duct to prevent short circuit of air flow. When connecting the exhaust pipe, the fan should be adjusted accordingly to avoid reducing the ventilation due to the increase of resistance. The ideal method is to put the heat pump unit at the top of the tower, so that the heat pump unit has good ventilation conditions and the noise influence surface is the smallest.
However, it should be noted that heat pump units should not be arranged near houses or other rooms with high noise requirements, and heat pump units and pumps should not be arranged above or below houses (rooms). The spring shock absorber should be used for vibration isolation of heat pump units, and the model and arrangement point of the shock absorber should be determined by calculation. The distance between heat pump unit and parapet is more than 3m, and the distance between units should not be less than 3m. When conditions permit, the distance should be increased. The layout of heat pump unit should not only consider the small impact on the surrounding environment and good ventilation, but also consider the factors such as pipeline layout, equipment hoisting and subsequent replacement. If conditions permit, reserve 1 ~ 2 sets, leaving room for future development, and considering sufficient load conditions, it is convenient for equipment installation and replacement.
Selection and arrangement of water pump
The number of water pumps should correspond to the number of heat pump units. The connection mode of heat pump unit and water pump should be one-to-one series, and the heat pump unit and water pump should be linked. When the number of heat pump units is large, the water pump can be arranged next to the heat pump units, and the water pump should be waterproof and equipped with a rain-proof sound-absorbing cover; When the number of heat pump units is small, the pump should be arranged indoors. The standby water pump can be temporarily replaced without installing it first. If the pumps are connected in parallel first and then in series with parallel heat pump units, the number of parallel heat pump units should not exceed 6, and reliable hydraulic balance measures should be taken. In this connection mode, the water pump should be arranged indoors near the heat pump or basement, and the number of water pumps should be 1 ~ 2 spare pumps. When choosing the specifications of the pump, try to choose a low-speed pump to reduce noise. The flow rate of the pump can be selected according to 1. 1 times of the flow rate required by the system, and the lift of the pump should be equal to the total resistance that the system needs to overcome. The power consumption of water pump should be controlled within 1/30 of the output of heat pump. The arrangement of pumps should have a certain spacing, and the installation position of 1 ~ 2 pumps should be reserved when conditions permit for development. The pump should also have reliable vibration isolation measures.
Selection of Terminal Equipment (Level 1)
Under summer working conditions, the rated supply and return water temperatures of heat pump units are 7℃ and 12℃ respectively, which are consistent with the rated working conditions of general air conditioners, and the selection calculation of air conditioners is consistent with other forms of air conditioning systems. In winter, under the rated condition of heat pump air conditioning system (outdoor air is 8℃), the rated supply and return water temperatures of heat pump units are generally 47℃ and 42℃ respectively. When the outdoor temperature is low, the water supply temperature of heat pump air conditioning system is generally maintained at 39 ~ 40℃. The water temperature condition is obviously lower than the rated supply and return water temperature of the boiler heating system (60℃ and 50℃ respectively), that is, lower than the rated inlet and outlet water temperatures given in the general air conditioning performance parameter table (60℃ and 50℃ respectively). Because of the different water temperature, the heat dissipation of air conditioners is also obviously different. Therefore, some scholars believe that the terminal equipment of heat pump air conditioning system should be enlarged on the basis of the calculation and selection results of summer working conditions. However, according to our calculation, the terminal of heat pump air conditioning system in Nanjing area can adopt the calculation and selection results under summer cooling conditions. On the one hand, the heating load of general buildings in Nanjing is less than the cooling load in summer. In addition, the heating capacity of the same air conditioner at 60℃ water inlet temperature is obviously greater than the cooling capacity at 7℃ water inlet temperature. When the inlet water temperature drops to 39 ~ 40℃ in winter, the heat dissipation of air conditioner can meet the requirements of indoor heating. In addition, it is customary to choose air conditioners according to the mid-range parameters, and there is a certain margin. If the heat pump air conditioning system has more than four refrigeration circuits, defrosting will not cause obvious fluctuation of water temperature, so it will not affect the fluctuation of indoor temperature. However, when the heat pump system only has 1 ~ 2 loop, in order to reduce the influence of defrosting on indoor temperature, the start-stop control of air conditioner can be synchronized with the water temperature when conditions permit. For example, when the water temperature is lower than 35℃, the air conditioning fan stops running, and when the water temperature is higher than 35℃, the fan resumes running. This can effectively improve indoor comfort.
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