Water flow control technology of component air-cooled heat pump unit

Introduce the water flow guarantee method of modular air-cooled heat pump unit and the application of pressure difference water flow switch in modular water system. Key words: Modular unit air-cooled heat pump differential pressure switch flow control target switch flow detection
   0 Overview
  Modular air-cooled heat pump unit is a combination of independent air-cooled heat pump units. At present, modular units are very well sold in the commercial air-conditioning market. Models. Its advantage is that the number of modular unit units can be changed according to the customer’s load conditions or the customer can add units during use, which is convenient for customers to flexibly purchase and use units according to their own funds and usage. In the structure of the modular air-cooled heat pump unit, each module has the same diameter water inlet and outlet pipes. Only the water pipes need to be connected between the modules. The installation is very convenient. Due to the special form of the modular machine, the inlet and outlet of the module unit The flow rate of the water pipe is uncertain, which brings a certain degree of difficulty to the water flow control of the modular unit. Ensuring proper water flow for each module is a necessary guarantee for the reliable operation of the module unit. Improper water flow may cause icing of the evaporator of a module unit, high condensing pressure, compressor “cylinder biting” and other failures. Generally, the cooling capacity of modular units is relatively large. Once the above failure occurs, the reputation of the brand will be greatly affected. Therefore, the water flow control and protection of the modular unit is very important. Appropriate water flow detection methods and detection components can ensure that the unit only works when the system water flow is greater than the allowable minimum water flow, and to a large extent, it can avoid the failure of the air-conditioning host. .
  A module unit connected to the module unit water 1.1 water main layout design
  module unit between the modules due to the common access to water, when the water connection between the modules, if the relatively small number of modules, the main pipe layout can be different programs; if If the number of modules is large, it is better to use the same program layout, so as to ensure that the water flow between each module is consistent.
  1.2 The water flow rate control of the main water pipe in the modular unit
  The diameter of the common inlet and outlet pipes between the modules of the modular unit depends not only on the cooling capacity of each module, but also on the maximum number of modules in the entire modular unit. Usually the flow rate of the main water pipe of the modular machine is controlled within 0.5~4m/s. Because if the flow rate is relatively large, the water pipes between the modules have greater resistance along the way, which is likely to cause uneven water flow distribution between the modules. An appropriate main water pipe flow rate can neglect the pressure drop generated in the main water pipe. If the pressure drop of the heat exchanger of the unit module is 30kPa under the rated flow, the pressure drop of the main water pipe inside the module is within 2kPa, and the water flow between each module is almost the same, but if the pressure drop of the water pipe inside the module exceeds 5kPa, The deviation of the water flow between the modular units may exceed 10%. For the heat exchanger, the greater the pressure drop under the rated flow, the greater the allowable pressure drop of the main water pipe inside the unit and the greater the head of the water pump.
  1.3 Selection and installation module unit water filter system
  module unit pipe diameter is generally larger, are generally not difficult to select the corrosive material like brass, often choose general seamless steel pipe as the pipe material from a consideration of the project cost. The entire water pipeline is connected by electric welding of seamless steel pipes. If the large amount of welding slag generated during the electric welding process cannot be effectively removed. Once the welding slag enters the heat exchanger, it is very harmful and may cause partial blockage of the heat exchanger. If the heat exchanger of the unit uses a brazed plate heat exchanger or a double-pipe heat exchanger, it will be difficult to remove, because these two heat exchangers cannot be mechanically cleaned. If a shell and tube heat exchanger is used, it will be easier to remove Dross. If the welding slag in the heat exchanger cannot be effectively removed, the local water flow of the water-side heat exchanger will decrease. At this time, the refrigerant flow on the refrigerant side remains basically unchanged, which may cause the water temperature to gradually drop or even lower than the freezing point of water, due to blockage The decrease in the flow rate on the water side causes the adjacent refrigerant side to evaporate incompletely, and frost or even icing occurs in the return pipe, which is likely to cause the compressor to produce liquid hammer. If the degree of partial blockage of the heat exchanger is relatively light, the entire water flow of the system has not changed much at this time, and the water flow protection switch does not work. The antifreeze protection sensor (inserted into the heat exchanger to detect the water temperature) also fails to detect the lowest temperature point, and the antifreeze protection does not work. A terrible thing has happened-the heat exchanger freezes until the entire refrigeration system is scrapped. In order to prevent welding slag and other impurities from blocking the heat exchanger, in addition to standard construction during the construction process, this situation can also be avoided during the design of the manufacturer’s chiller.
  The water filter is a common component of the modular unit and one of the two most critical components of the water system. The proper choice of water filter can effectively prevent impurities generated during the construction of the water pipe from entering the heat exchanger. The proper mesh number and form of the water filter are very critical. The valve for bypassing the water inlet pipe of the unit should be reserved during installation. After the installation is completed, the heat exchanger of the module unit should be bypassed for water pipe cleaning before commissioning. At this time, it is best to use a 100 mesh stainless steel punched filter in the inlet water filter, and the mesh number of the water filter to the end should be greater than 100 mesh (for temporary use, the bypass valve will be opened after debugging). If the system has a backup pump, turn on the main backup pump at the same time to maximize the flow rate in the water pipeline. Clean the filter after 24 hours. In this way, almost all large particles of impurities can be accumulated in the water filter, and then the 60-mesh water filter can be replaced to start the commissioning. The schematic diagram is as follows:

  1.4 Selection of water pipe material for
  modular unit    Usually galvanized seamless steel pipe is used in the main plant of the water piping system of the modular unit. Because the steel pipe cannot be galvanized at the construction site, the water pipe is prone to rust and corrosion for a long time. If the steel pipe is lined with corrosion-resistant plastic, there will be no pipeline corrosion and a large amount of electric welding slag will block the water pipeline. Although the installation cost is increased, the maintenance and maintenance costs of the air conditioning unit are greatly reduced. The overall cost during the life cycle will be decrease very much.
  2 Modular unit water flow detection method 2.1 Modular unit water flow detection
  In the control of the module unit, there are currently two categories: each module requires a water flow signal and only one main module requires a water flow signal. These two methods are in the module unit. It is used in production plants. The water flow control method in which each module has a water flow signal can more truly reflect the water flow of the module; if only the main module receives the water flow signal, it can’t really reflect the water flow of each module, this method can reduce the cost for the production plant , But this control method is not ideal for each module.
  As a modular machine for water flow detection, there are currently two simple, practical and low-cost detection methods. One is a target flow switch and the other is a differential pressure flow switch. The installation and use characteristics of the two detection methods are compared below. .
  2.2 Installation and use of
  target flow switch The target flow switch is to install the target in the water pipe, and the water flowing in the water pipe impacts the target stream to bend and deform, thereby driving the micro switch to output control signals to the module unit controller. Inform that there is water flow to start the unit.
  Because the water flow speed of the main water pipe of the modular unit is changing (the number of ends used is increased or decreased), it brings certain difficulties to the installation of the target flow switch. Generally, there are three conditions for the installation of the target tape: one is not moving, the other is stuck in the upper part of the tube and cannot be recovered, and the third is normal. The very good installation situation is very related to the pipe diameter of the water pipe and the experience of the installer. The larger the pipe diameter, the higher the accuracy of one-time installation of the target water flow switch.
  Usually the target streamer does not move because the target streamer installation depth is not enough, and the target streamer needs to be screwed in again or replaced. Many installation workers encounter this situation, if it can not be solved quickly, they often short-circuit the water flow switch or adjust the action adjustment screw, so that the module unit loses the water flow protection.
  If it is stuck in the tube and cannot be recovered, it is often because the target sheet is too wide. It is stuck on the upper part of the tube during the first action (this situation occurs more frequently in small-diameter refrigerant water pipes), which cannot be found by the installer. Yes, the flow switch also lost its function at this time. If the installation clearance is not enough, even if it can work at that time, the actual diameter of the pipe will become smaller due to the rust or fouling of the pipe, which may cause the water flow switch to be stuck in the pipe and cannot operate effectively.
  Due to the variable number of modules in the module unit on site, the flow rate of the main water pipe cannot 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 swing action of the target flow switch to reset frequently; When the water system is mixed with air, the air in the water impacts the target streamer and causes the flow rate to drop. The target streamer is reset instantly and sends the wrong flow information to the unit (to prevent this kind of error, the unit controller usually delays the processing of the flow information). At the same time, the target sheet of the target water flow switch is in a bending deformation state under the pressure of the water flow for a long time during normal use, and it is easy to be fatigued and damaged, especially when the flow rate exceeds 3m/s, its life will be greatly shortened.
  If each module needs a flow signal, it is even less suitable to install a target flow switch. At present, the heat exchanger of the module machine of each module machine manufacturer is close to the main water pipe of the module unit. It is impossible to provide the minimum straight pipe distance between the front and rear sections required to install the target flow switch. Some manufacturers have to give up the need for a water flow for each module. Signal is an ideal method of water flow protection.
  2.3 Installation and use of
  differential pressure flow switch    differential pressure flow switch is designed according to the resistance and flow curve of HVAC equipment. We know that the heat exchangers, water filters, water pumps and valves of HVAC equipment have their resistance and flow performance curves. By detecting the pressure difference between the inlet and outlet water at both ends, and comparing with the preset value of the device, Then the flow can be accurately controlled.
  Compared with the target type flow switch, the differential pressure type is an accurate flow control method, and it has an accurate flow control value. It can be installed directly in the unit or on-site. If two copper pipes are connected from the differential pressure switch to the inlet and outlet of the heat exchanger installed in the unit, and the pressure difference between the inlet and outlet is measured, the flow rate is reflected. However, the user does not need to install and wire on site, which avoids the hidden danger of unit failure caused by inaccurate installation of the target water flow switch. If it is installed on site, a differential pressure switch with adjustable double set points can be used. Just connect the copper pipe Æ6.35 from the main inlet and outlet pipes of the module to the differential pressure switch with a pipe nut. If the site has a portable ultrasonic flowmeter to measure the flow of the main water pipe, it is better to adjust the adjustment screw of the differential pressure switch to set the protection flow value you need. If the site does not have this condition, you can also combine it according to the different modules provided by the factory The flow and pressure difference curve of the main inlet and outlet pipes are set to an appropriate pressure difference value to achieve precise control of the flow rate.
  The other set point of the differential pressure switch can be adjusted on site to indicate that the differential pressure of the module unit is too large (that is, the internal scale or blockage of the module unit). The specific method is to adjust the set point of the differential pressure switch under the normal water flow to make it closed or the output indicator light is on, then slowly screw in to make the differential pressure switch open or the output indicator light is off and then 1 to 2 turns That’s it. That is, if the pressure difference between the inlet and outlet pipes of the modular unit and the heat exchanger is greater than the set value, an alarm message will be issued to remind the user to clean the heat exchanger and pipeline.
  The differential pressure flow switch has the characteristics of accurate flow control, no additional resistance to the system, no requirement for the diameter of the water pipe, and no disturbance of water flow. It can replace any form of target flow switch as the flow control of the HVAC water system. Compared with the target type flow switch, it can avoid false flow caused by pump cavitation (the actual flow rate is very large, but the water is mixed with air and the actual flow rate is not large), so it can be widely used in the use of brazed plate heat exchangers and casing pipes. The modular air-cooled heat pump unit of the heat exchanger and the shell and tube heat exchanger has the water flow control and part of the antifreeze protection function.
  3 Application of differential pressure water flow switch in modular unit 3.1 Water flow protection and blockage indication
  For the brazed plate heat exchanger, sleeve heat exchanger or shell and tube heat exchanger used in the module machine, the pressure difference and flow rate The curve may not be the same,<

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