Water Flow Control Technology for Small Air-cooled Hot and Cold Water Units

The household air-cooled heat pump unit is a form of household central air-conditioning that has been widely recognized by users of high-end apartments and villas. It consists of components such as an air-cooled condenser, a compressor, and a water-refrigerant heat exchanger. The water-refrigerant heat exchanger exchanges heat between the refrigerant and water to generate chilled water or hot water. Then, a water pump supplies the chilled water or hot water to the fan coil units. The fans inside the fan coil units blow the cooling or heating capacity of the chilled water or hot water in the coils into the rooms to meet the requirements of cooling or heating the rooms. In this system, the energy is transferred from the refrigerant to the water (refrigerant carrier), and then the water transfers the energy to the indoor air. Compared with the variable refrigerant system, if there is a small amount of water leakage in a certain part of its piping system or a slight uneven distribution of water flow, it will not affect the operation of the unit. However, for units with variable refrigerant flow, it's different. When there are piping problems such as refrigerant leakage, uneven refrigerant distribution, pipeline blockage, and poor oil return in the system, the entire system will break down.

The main reason for the freezing damage of the water heat exchanger is that the water flow through the heat exchanger does not reach the minimum flow required for the reliable operation of the unit. Ensuring that the heat exchanger has an appropriate water flow is a necessary guarantee for the reliable operation of the chiller. Inappropriate water flow may lead to problems such as icing of the evaporator of the chiller (which may be damaged by freezing in severe cases), high condensation pressure, and "cylinder seizure" of the compressor.

The water flow control and protection of household chillers are very important. Appropriate water flow detection methods and detection components can ensure that the unit only operates when the system water flow is greater than the allowed minimum water flow, which can largely avoid failures of the air-conditioning main unit.

1.2 Principle for selecting the material of the water pipes outside the household heat pump units
The design of the water pipes outside the household heat pump units should also select appropriate water pipe materials according to the design concept of maintenance-free and low-maintenance. Otherwise, although appropriate water pipe materials are selected inside the unit, if water pipes that are prone to rusting, scaling, and corrosion are still used outside the unit, the entire water piping system cannot achieve maintenance-free and low-maintenance. I think that manufacturers should clearly stipulate the materials of the water piping in the product installation instructions of the household heat pump units, specifying which materials are not allowed to be used and which materials are recommended for use. And the technical personnel of the manufacturers should conduct full-staff training for dealers and installers to completely avoid the blockage of the water system caused by the influence of the water piping materials, thereby reducing the occurrence of the phenomenon that the water flow decreases due to pipeline blockage and the heat exchanger is damaged by freezing.

1.3 Selection and installation of the water filter for the household heat pump units
The diameter of the water pipes of the household heat pump units is generally relatively small (not exceeding DN65), and most of the pipe connections are threaded connections. If materials such as reinforced plastics are used, the connections are mostly hot-melt or glued. The purpose of selecting a water filter is to filter out impurities generated during the installation of the water pipeline and impurities peeled off due to pipeline corrosion and scaling during long-term operation, and to prevent impurities from entering the heat exchanger. Once impurities enter the heat exchanger, the harm is very great and may cause local 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 very difficult to remove the impurities because these two types of heat exchangers cannot be mechanically cleaned and can only be tried to be removed by backwashing. If the impurities in the heat exchanger cannot be effectively removed, the local water flow on the water side of the heat exchanger will decrease, while the refrigerant flow on the refrigerant side basically remains unchanged at this time, which may lead to a gradual decrease in the water temperature and even below the freezing point of water. Due to the decrease in the blocked water side flow, the adjacent refrigerant side will not evaporate completely, resulting in frost or even ice formation on the return pipe, which is likely to cause liquid slugging in the compressor. If the degree of local blockage of the heat exchanger is relatively light, the entire water flow of the system does not change much at this time, and the water flow protection switch will not work. And the anti-freezing protection sensor (inserted into the heat exchanger to detect the water temperature) also cannot detect this minimum temperature point, and the anti-freezing protection will not work. Then the terrible thing happens - the heat exchanger is damaged by freezing until the entire refrigeration system is scrapped. In order to prevent impurities such as Teflon tape and hemp rope used in the construction of galvanized pipes from blocking the heat exchanger, in addition to standardizing the construction process, this situation can also be avoided during the design process of the chiller by the manufacturer.

The water filter is one of the two most critical components in the water system of the heat pump unit. Appropriate selection of the water filter can effectively prevent impurities generated during the construction of the water pipe from entering the heat exchanger. Appropriate mesh number and form of the water filter are very crucial. A valve that bypasses the water inlet pipe of the unit should be reserved during installation. Before starting up and debugging after installation, the heat exchanger of the modular unit should be bypassed to clean the water pipeline. At this time, it is best to use a 100-mesh stainless steel stamping filter screen in the water inlet filter, and the mesh number of the water filter at the end should be greater than 100 mesh (used temporarily, and the bypass valve is opened after debugging). If there is a standby pump in the system, turn on the main and standby pumps at the same time to maximize the flow velocity in the water pipeline. After running for 24 hours, clean the filter screen. In this way, almost all large-particle impurities can be accumulated in the water filter, and then the 60-mesh water filter can be replaced and the unit can be started up for debugging. In addition, to facilitate users to clean the filter by themselves, it is recommended to install a ball valve on the filter. Considering future backwashing to clean the heat exchanger, it is sufficient to install three-way ball valves on the inlet and outlet water pipes outside the heat pump unit. The schematic diagram is as follows:

2.2 Installation and usage of the target flow switch
The target flow switch installs the target plate in the water pipe. The flowing water in the water pipe impacts the target plate to make it bend and deform, thereby driving the micro-switch to output a control signal to the controller of the modular unit to inform that there is water flow and the unit can be started.

Due to the relatively small diameter of the water pipes of the household heat pump units, it brings certain difficulties to the installation of the target flow switch. Usually, there are three situations for the installation of the target plate: one is that it does not move, the second is that it is stuck at the upper part of the pipe and cannot return, and the third is normal. A very good installation situation is very much related to the diameter of the water pipe and the experience of the installer. The larger the diameter of the pipe, the higher the one-time installation accuracy rate of the target water flow switch. When household air-conditioning installers install household heat pump units, the one-time installation failure rate of the target water flow switch is as high as 70%.

Usually, the target plate does not move because the installation depth of the target plate is not enough, and it needs to be screwed in again or the target plate needs to be replaced. Many installers, when encountering this situation and unable to solve it quickly, often short-circuit the water flow switch or adjust the action adjustment screw, causing the modular unit to lose water flow protection.

If it is stuck in the pipe and cannot return, it is often because the target plate is too wide and is stuck at the upper part of the pipe when it moves for the first time (this situation occurs more frequently in small-diameter refrigerant water pipes). Installers cannot detect this, and at this time, the flow switch also loses its function. If the installation clearance is not enough, even if it can work at that time, due to the reduction in the actual diameter of the pipe caused by rusting or scaling of the pipe, it is also possible that the water flow switch is stuck in the pipe and cannot operate effectively.

In addition, when the water system is mixed with air, the air in the water impacts the target plate, causing the flow velocity to decrease, and the target plate instantaneously resets and sends incorrect flow information to the unit (usually, the unit controller processes the flow information with a delay to prevent this error from occurring). At the same time, the target plate of the target water flow switch is in a bent and deformed state under the pressure of water flow during normal use and is prone to fatigue damage. Especially when installed in a vertical pipe and when the flow velocity exceeds 3 m/s, its service life will be greatly shortened. Generally, the service life of a correctly installed target flow switch is one year, which is the reason why large and medium-sized chillers basically replace the target flow switch every year.

2.3 Installation and usage of the differential pressure flow switch
The differential pressure water flow switch is designed according to the resistance and flow curves of HVAC equipment. We know that devices such as heat exchangers, water filters, water pumps, and valves of HVAC equipment all have their resistance and flow performance curves. By detecting the pressure difference between the inlet and outlet at both ends and comparing it with the preset value of the device, the flow can be accurately controlled.

Compared with the target flow switch, the differential pressure flow switch is an accurate flow control method with accurate flow control values. It is directly installed inside the unit, avoiding incorrect installation by users. If it is installed inside the unit and two copper pipes are connected from the differential pressure switch to the inlet and outlet of the heat exchanger to measure the pressure difference between the inlet and outlet, the flow can be reflected. And users do not need to install and wire on site, thus avoiding the hidden danger of unit failure caused by inaccurate installation of the target water flow switch.

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 interference from water flow disturbance. It can replace any form of target flow switch for flow control of the HVAC water system.

Compared with the target flow switch, it can avoid false flow caused by pump cavitation (the actual flow velocity is large, but the actual flow is not large because the water is mixed with air), so it can be widely used in the flow control of the modular air-cooled heat pump units using brazed plate heat exchangers, double-pipe heat exchangers, and shell-and-tube heat exchangers and also has the function of partial anti-freezing protection.

3.2 Exposing installation and design problems of the exposed water pipeline
The differential pressure flow switch with appropriate action and reset values configured in the factory can be used as an important basis for checking whether the design and installation of the water system outside the chiller are correct. Because for a certain chiller, regardless of your installation situation, if the water flow through the heat exchanger does not reach the reset flow of the differential pressure switch (the data tested during factory configuration, and the reset flow remains unchanged as long as the model of the differential pressure switch remains unchanged), the differential pressure switch will not close. When encountering a situation where it does not close on site, there must be a problem with the water system outside the unit, and the reset flow has not been reached.

Usually, the resistance of the system water pipeline is too large, some valves are not opened, the water pump has not released air, the filter is blocked, and the pipeline has not removed air, etc. This prompts installers to check and solve these problems one by one, which has a guiding role. Because installers usually do not have flow meters and cannot test the flow of the water pipeline they install. If the target flow switch is used, it is very difficult to determine the problem of the water pipeline because the flow of the water pipeline cannot be determined.

3.3 For the reduction of water flow caused by filter blockage to the flow protection value, the differential pressure flow switch can provide very effective protection. If the terminal fan coil units of the installation unit use two-way valves, that is, a variable water volume system is wrongly adopted. When the number of two-way valves that are gradually closed, once the flow of the open two-way valves is less than the disconnection flow of the differential pressure switch, the differential pressure switch will accurately output a disconnection signal within 3 seconds.