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The method of water flow assurance of modular air - cooled heat pump unit is introduced. The application of pressure - difference flow switch in water system of module machine is introduced.
Keywords: modular unit air-cooled heat pump pressure switch flow control target switch flow detection
0 Overview
Modular air-cooled heat pump unit is a separate combination of air-cooled heat pump units used together, the current modular air conditioner unit is the market sales of very good models. Its advantage is that it can change the number of modules and units according to the load of customers or allow customers to add more units during the process of using, so that customers can flexibly purchase and use the units according to their own funds and usage. Modular air-cooled heat pump units in the structure, each module has the same diameter of the inlet and outlet pipes, the module can be simply docking between the water pipes, the installation is very convenient, due to the modular machine in this special form, the module unit out The uncertainty of the flow rate of the water pipe brings certain difficulty to the flow control of the module unit. Ensuring proper flow of water to each module is a necessary guarantee of reliable operation of the module. Improper water flow may result in icing, high condensation pressure, and "biting" of the compressor in one module unit. For the general cooling capacity of the module unit is relatively large, in the event of these failures, the brand's reputation will be very large impact. Therefore, the water flow control and protection of the module is very important. The proper water flow detection method and testing components can ensure the unit to work only when the system water flow is greater than the allowable minimum water flow, to a great extent avoid the failure of the air-conditioning main engine .
1 modular unit connected to the design of the water pipe 1.1 module unit within the main water pipe layout design
When the module is connected to the water pipe between the modules, if the number of the modules is relatively small, the main water pipe can adopt the same-style layout; if the module number is more, it is better to adopt the same program layout so as to ensure that every The flow of water between the modules is the same.
1.2 The main unit within the module water flow control
The pipe diameter of the common inlet and outlet pipes between the modules of the module unit depends not only on the cooling capacity of each module but also on the maximum module quantity of the entire module unit. Usually the main flow control module machine in 0.5 ~ 4m / s is more appropriate. Because if the flow rate is relatively large, the resistance of the water pipelines between the modules is relatively large, it is easy to cause uneven distribution of water flow between the modules. The appropriate main pipe flow rate can ignore the pressure drop generated in the main water pipe. If the unit module's heat exchanger pressure drop at nominal flow rate is 30 kPa, the pressure drop across the main water pipe within the module is within 2 kPa and the water flow between each module is almost the same, but if the pressure drop across the module's internal water pipe exceeds 5 kPa, Water flow deviation between module units may exceed 10%. The larger the pressure drop at the rated flow rate of the heat exchanger, the greater the pressure drop allowed by the main water pipe inside the unit and the larger the lift of the pump.
1.3 module unit system water filter selection and installation
Module unit of the water pipe diameter is generally larger, considering the cost of the project will not generally choose copper pipes and other non-corrosive materials, often choose the general seamless pipe as a water pipe. The entire water pipe using seamless steel pipe welding connection, welding process generated a lot of welding slag if you can not effectively remove. Once the welding slag into the heat exchanger is very harmful, it may cause partial blockage of the heat exchanger. If the unit heat exchanger is brazed plate heat exchanger or casing heat exchanger will be difficult to remove, because these two heat exchangers can not be mechanically cleaned, if the use of shell and tube heat exchanger will be relatively easy to remove Dross. If the welding slag in the heat exchanger can not be effectively removed, the local water flow of the heat exchanger on the water side will decrease. At this time, the flow of the refrigerant on the refrigerant side remains basically unchanged, which may cause the water temperature to gradually drop even lower than the freezing point of water. Of the water side of the flow caused by the decline in the adjacent side of the refrigerant is not completely evaporated, the return pipe frosting or even ice, it is easy to produce liquid hammer compressor. If the heat exchanger is partially blocked to a lesser extent, the entire water flow of the system does not change much at this time and the water flow protection switch does not work. The frost protection sensor (inserted into the heat exchanger to detect the water temperature) also can not detect the minimum temperature, frost protection does not work, a terrible thing happened - heat exchanger frozen until the entire refrigeration system scrapped. In order to prevent impurities such as welding slag clogging heat exchanger in addition to standard construction during construction, the manufacturer of the chiller design process can also avoid this situation.
A water filter is a common component of a module unit and is one of the two most critical components of a water system. The proper choice of water filter can effectively prevent impurities generated during the construction of water pipes into the heat exchanger. Proper water filter mesh number and form is very crucial, should be set aside to install the bypass unit inlet pipe valve, the installation is completed before commissioning bypass module unit heat exchanger for water pipeline cleaning. In this case, it is best to use a 100-mesh stainless steel press filter in the inlet water filter. The number of water filter heads to the end should be larger than 100 mesh (temporary use, after commissioning, the bypass valve is opened). If the system has a standby pump at the same time open the main backup pump, so that the maximum flow in the water pipe, open the filter after 24 hours cleaning. This will almost all of the large particles of impurities accumulated in the water filter, and then replace the 60-mesh water filter to start debugging. Schematic diagram is as follows:
1.4 module unit pipe selection
Usually in the modular unit of the water pipeline system host plant galvanized seamless steel pipe, steel pipe in the construction site can not be implemented galvanized process, eternal rusty pipe corrosion. If the use of corrosion-resistant steel lined with plastic pipe there will be no pipeline corrosion and a large number of welding slag to block the water pipeline, although the increase in installation costs, but greatly reduce the cost of air conditioning unit maintenance and maintenance costs over the life of the meeting will be decrease very much.
2 module unit water flow detection method 2.1 module unit water flow detection
There are currently two types of flow control signals required for each module in the control of the module unit and one for the main module, both of which are used in the module plant. Each module has a water flow signal flow control method can more accurately reflect the water flow of the module; if only the main module connected to the water flow signal can not really reflect the water flow of each module, this method can reduce costs for the manufacturer , But this method of control is not ideal for each module.
At present, there are mainly two kinds of simple and practical low-cost test modes for the water flow detection of the module machine. One is the target type flow switch, and the other is the differential pressure type flow switch. The installation and use characteristics of the two detection modes are compared as follows .
2.2 target flow switch installation and use
The target flow switch is to install the target in the water pipe. The water flowing in the water pipe impinges on the target chip to make it bend and deform, so as to drive the micro switch to output the control signal to the module controller, informing that the water flow can start the unit.
As the flow rate of the main water pipe of the module unit is changing (increasing or decreasing the number of ends used), it is difficult to install the target type flow switch. Target flow is usually installed in three situations: one is not action, the second is stuck in the upper tube can not be restored, the third is normal. The installation is very good situation and the pipe diameter and installation personnel experience is very relevant, the larger the diameter of the target flow switch installed once the higher the accuracy.
In general, the target slice does not move because the depth of the target slice is not sufficient and the target slice needs to be screwed in again or replaced. Many installers encounter this situation if they can not quickly solve the problem of often shorting the water flow switch or adjusting the action adjusting screw, thus the module unit loses water flow protection.
If the card can not be recovered in the pipe is often the target is too wide, so the first action was stuck in the upper part of the pipe (in this case in the small diameter coolant pipes appear more), this is the installer can not be found At this point, the flow switch also lost its effect. If the installation gap is not enough even if you can work at that time, due to the tube rust or scaling caused by the actual use of smaller diameter, may also make the water switch stuck in the pipe can not be effective action.
Due to the variable number of module units in the field, the flow rate of the main water pipe can not be determined, which brings great difficulty to the installation of the target flow switch. The change of the flow rate on the main water pipe causes the reset of the swing of the target flow switch frequently. When mixed with air in the water system, the air in the water impinges on the target vane causing the flow rate to drop, and the target vane instantly resets the error flow information sent to the unit (to prevent this error, the unit controller delays processing of the flow information). At the same time, the target flow slice of the target flow switch is subjected to long-term compression by the water flow in the normal state of use and is in a state of bending deformation and is easy to be damaged by fatigue, especially when the flow velocity exceeds 3 m / s or above, its life will be greatly shortened.
If each module requires a flow signal, it is more improper to install a target flow switch. At present, the heat exchangers of the module machines of each module manufacturer are close to the main water pipes of the module units, and it is impossible to provide the minimum straight pipe distance required for installing the target flow switch so that some manufacturers have to give up one water flow for each module Signal This ideal water flow protection method.
2.3 pressure-type flow switch installation and use
Differential pressure water flow switch is based on the HVAC equipment resistance and flow curve designed. We know HVAC equipment, heat exchangers, water filters, pumps and valves and other devices have their resistance and flow curve, by measuring the water pressure at both ends of its access, and with the device's pre-set value for comparison, You can accurately control the traffic.
Differential pressure is a precise flow control method compared to target flow switches, with accurate flow control values. It can be installed directly in the unit or on site. If installed in the unit from the pressure switch to connect the two brass to the import and export of heat exchangers, measuring the pressure difference between import and export, that reflects the flow. The user site does not require installation and wiring, it also avoids the installation of the target flow switch is not accurate lead to the hidden dangers of unit failure. If installed at the site, a double setpoint adjustable differential pressure switch can be used to pull the Æ6.35 brass tubing from the main inlet and outlet of the module to the differential pressure switch. If the portable ultrasonic flowmeter is used to measure the flow of the main water pipe better, adjust the adjustment screw of the differential pressure switch to set the protection flow value that you need. If the condition is not available in the field, it can also be set according to the different module combinations provided by the factory The main inlet and outlet pipe flow and pressure difference curve, set to the appropriate pressure value to achieve precise control of flow.
The other setpoint of the differential pressure switch can be field adjusted for excessive pressure in the module unit (that is, internal fouling or blockage of the module unit). The specific method is under normal water flow, adjust the differential pressure switch set point adjustment screw to make it close or output indicator light, and then slowly precession pressure switch to disconnect or output indicator off after 1 to 2 laps Can be. That is, if the pressure difference between the inlet and outlet of the module and the heat exchanger is greater than the set value, an alarm message may be issued to prompt the user to clean the heat exchanger and the pipeline.
Pressure-flow switch with accurate flow control, no additional resistance to the system, but also on the diameter of the water pipe does not require interference and interference characteristics such as water flow can replace any form of target flow switch as HVAC water flow control system, Compared with the target flow switch, it can avoid the fake flow caused by the pump cavitation (the actual flow rate is very big, but the air is mixed with the air and the actual flow rate is not big), so it can be widely used in the brazed plate heat exchanger, Type heat exchanger and shell and tube heat exchanger modular air-cooled heat pump water flow control unit with both partial frost protection.
3 differential pressure water flow switch in the module unit 3.1 water flow protection and blockage instructions
For the brazed plate heat exchanger, bushing heat exchanger or shell-and-tube heat exchanger used in the module machine, the curve of pressure difference and flow rate may be different. If each module machine uses a fixed set point differential pressure The switch, which can be tested in the factory, ensures that the configured differential pressure switch has a suitable protection flow value. It is recommended to disconnect the flow greater than 50% of the rated flow and reset flow less than 80% of the nominal flow.
If only the main module of the module unit receives the water flow signal, adjustable double set point differential pressure switch can be used. According to the module unit inlet and outlet pipe pressure drop and flow curve to adjust the pressure set point, the other set point can be used as a heat exchanger blockage or scaling instructions, the specific installation location refer to the figure.
3.2 pressure bypass control
In the modular unit of the water system, the module side of the water system is usually used to set the water system to ensure the stability of the module unit operation, the water system at the end of the water system to reduce the water pump to reduce energy consumption and transfer between the ends of the cold, for End of the temperature control and reduce the initial investment of equipment and reduce operating costs are very favorable.
In order to satisfy the constant water flow control on the module side and the variable water flow control on the terminal side, a pressure differential bypass control method is generally adopted, in which the change of the water flow at the terminal side (that is, the change of the load due to the terminal capacity coinciding with the actual load) Road pressure changes with change. According to the performance curve of the pump, if the pressure difference between the water supply and return of the main water pipe is guaranteed, the flow of the water pump will be guaranteed, and the water flow into the unit will not change.
The Delta-PFS â„¢ Dual Setpoint Differential Pressure Switch can be used to control the differential pressure between the return pipe for the variable volume system. That is, when the differential pressure between the supply and return exceeds the high pressure setpoint, the high Limit the output of the NO and the C terminal is closed, so that floating-point control of the electric bypass valve to open the action to reduce the pressure difference between supply and return pipes; when the supply and return pipe pressure drop below the lower limit of the differential pressure switch Point, the low pressure switch output NO and C-side closed, so that the floating-point control of the electric bypass valve to perform the closing action. When the pressure difference between the supply and return pipes is between the high and low set point of the pressure switch, the electric bypass valve controlled by the floating point does not operate and the constant flow rate is maintained to keep the pressure difference constant. Dual set point differential pressure switch can be a floating point differential pressure bypass controller, by adjusting the height of the differential pressure limit set point, you can adjust the floating zone, much better than the previous differential pressure controller control method .
4 Conclusion Each module in the module unit using the pressure switch or the main module using the pressure switch, have been very good results, especially in the module changes caused by changes in the main pipe flow and water pipe when there is a small amount of air , The pressure switch work very stable, there will be no floating similar to the target flow switch, after repeated tests and reset the flow rate has a clear flow value, with better than the target flow performance.
References [1] Zhang Yadong "central air-conditioning water flow control of the precise control of" HVAC equipment in June 2004.
[2] Shanghai Anchao Online Control Technology Co., Ltd. "Differential pressure water flow switch" sample.
[3] Shanghai Anchao Online Control Technology Co., Ltd. "Pressure-difference flow switch" installation instructions.
* About the Author:
Shu Weimin Commercial Air Conditioner Equipment Co., Ltd. of Guangdong Midea Commercial Air Conditioning Test and Evaluation Engineer, Committed to the Reliability Research of Commercial Air Conditioners Zhang Yadong Senior Engineer of HVAC System, Shanghai Anchao Online Control Technology Co., Ltd., Devotes to Developing Innovative Solutions for HVAC System Tel: 021 -62119010