ba-Cold Source System--one-time flow control scheme (reprint)

Control scheme of water flow in air conditioning system

Zhejiang Big Network new fast Granville Technology Huang Yilin

First, Introduction

Building central air-conditioning system of chilled water pump, traditionally used fixed speed water pump. The variable one-time flow control system for air-conditioning water (Vpf:variable-primary-flow, also known as: Chilled water primary pump variable frequency speed control system) is an advanced control scheme that has only begun to appear in recent years. The variable frequency speed regulating refrigeration pump can adjust the chilled water flow to achieve the target of fine control. Although the load side is variable water control, but the frequency conversion speed control and the traditional fixed speed of the primary pump system is different, it is much higher than the traditional method of control requirements. The engineering service of the building automation system is required to design a reasonable variable-time flow control solution to provide the control function to meet the requirements. This paper discusses the solution of a large-scale building with variable-time flow control engineering scheme.
Variable one-time flow control, in recent years in the air-conditioning industry developed advanced refrigeration water control system, which is characterized by the use of the latest measurement and control technology and refrigeration machine in the low water flow capacity of the improvement, in the evaporator pipeline configuration variable frequency speed water pump, so that the freezing water flow through the freezer can be adjusted to the needs of the load, It has less configuration than the traditional two-stage pump system, but has a better energy-saving effect. The variable-time flow control system can track the dynamic change of the system load well, satisfy the load demand of the system, and control the energy consumption at a low level. However, such a system needs to provide a strict control scheme, the design and its matching high-level monitoring and control system, can really play an effective role.
A large building chilled water system discussed here has a total of 3 equal capacity centrifugal chiller units, with a capacity of 500 cold tons per unit. The system is a typical one-pump system from the point of view of the device layout. Using frequency conversion speed control refrigeration pump side-by-side unit, because of its working ability can be continuously adjusted, there is no need to configure a low-capacity parallel unit.
The control scheme of the air-conditioning chilled water system, which changes the flow rate, is closely related to the selection and layout of refrigerating unit equipment, and in order to achieve the predetermined control target, some requirements should be put forward for the technical elements of the related equipment, in a word, the system design is optimized. This is the responsibility of architectural design and building automation system designers.

Second, refrigeration unit monitoring Program

According to the equipment configuration of the building air-conditioning chilled water system, its monitoring and control system can be divided into several aspects, the following are described below:
1 monitoring and control of all equipment operating conditions:
(1) General monitoring contents of chilled water pump, cooling water pump and cooling tower fan:
Monitor operation status, fault status and manual/automatic form, cumulative run time, count of running times, and BAS can start and stop control of these devices.
(2) Variable frequency speed control of chilled water pump:
The pressure difference between the supply and return mains is monitored, and the pressure difference is used as the process variable to control the frequency conversion speed of the chilled water pump.
(3) Cooling tower Fan control: monitoring the cooling tower supply, return to the main temperature, the cooling tower water supply temperature setting value as the target, the cooling tower fan for the frequency control of the variable speed.
(4) Electric butterfly valve monitoring content: The corresponding interlock switch control.
(5) The control system should also be monitored: chilled water supply, return to the mains temperature, chilled water supply mains flow;
(6) Set up data communication interface for refrigerating unit
The process parameters of the following chillers are transmitted (or received) between BAS and chiller through this interface:
Host Run status
Host fault alarm status (can be encoded to represent a variety of fault information)
Host load level Absolute or percentage, or host current or current percentage
Current water supply Temperature setting value
Measured value of inlet and outlet temperature of evaporator
Measured value of inlet and outlet temperature of condenser
Measured value of evaporator (frozen) water flow
Evaporator refrigerant pipe Pressure measurement value
Condenser (cooling) water flow measured value
This run time and cumulative run time
Cumulative number of launches
Lubricant temperature and supply level
Receiving bas-given chilled water supply temperature re-SetPoint accepts Bas-issued start-stop control commands
Through communication, the control system obtains the corresponding key working condition parameter, it is the optimal scheme. However, BAS and chiller host can exchange what data, also need to see the freezer brand, model and specifications to determine, it is necessary to the Chiller Master Controller for further understanding of the signal can be transmitted to confirm. The parameters listed above, which are required by BAS to complete the control, do not imply that any chiller host will provide these process parameters. So from the system configuration and system integration requirements, the monitoring point of BAS must be added to the data communication parameters. Especially because this system is variable once flow (variable frequency speed control refrigeration pump) control, the evaporator pipeline water flow monitoring is essential, need to attach great importance. Similarly, for parameters that can be obtained by communication, BAS does not have to configure the monitoring element to avoid repeated setting of the monitoring point, resulting in waste. According to experience, the general refrigeration machine will provide: the current water supply (evaporator effluent) temperature measured value, evaporator and condenser pipe flow status parameters. Therefore, the monitoring point of the refrigeration unit of BAS should be set according to the actual situation, can not generalize. If the water flow parameters of evaporator pipeline can be obtained by data communication, it is necessary to set up flow monitoring.
2 Start-up and stop chain control for refrigeration units:
The host's start-stop and the related equipment interlock control is the important condition which guarantees the equipment normal, the safe operation.
(1) Chain boot sequence:
Cooling water tower fan → Cooling water tower electric Butterfly valve → condenser electric butterfly valve of refrigerating machine → cooling water pump → condenser flow switch signal indication (as the return signal of the chain condition) → Chiller evaporator electric butterfly valve → chilled water pump → chilled flow switch signal indication (as the return signal of the chain condition) → Chiller Main unit
(2) Chain stop order:
Freezer → (delay for several minutes, depending on equipment requirements and operating conditions to determine the delay time) → refrigeration water pump → evaporator flow switch signal indication (as the return signal of the chain condition) → evaporator electric butterfly valve → cooling water pump → condenser flow switch signal indication (as the return signal of the chain condition) → Condenser electric butterfly valve of freezer → cooling water tower electric butterfly valve → cooling water tower fan
Dynamic start-stop control requires special attention: electric butterfly valve Open or close the full travel time of about 30 seconds, in the start, stop switching process, should be avoided as far as possible the sharing of the water caused by the manifold phenomenon. This system adopts frequency conversion speed regulating water pump, if the opening degree of electric valve and the water of the pump and the start of the freezer can be properly coordinated control, it will be better to overcome this problem. This kind of synchronization control and the time interval of chain control need to be determined by certain debugging method during commissioning. Before this kind of fine control is realized, the phenomenon of water-string in this finite time period is generally tolerable.
Variable primary water flow control of 3 refrigerating unit
In order to save energy, according to the total load level of the cold source system to control the number of chillers, the purpose is to make the number of running refrigeration machine can meet the needs of the system load, and make the opening of the freezer as far as possible in its most efficient operating area operation. The following combination of variable frequency speed control refrigeration pump, the control scheme for the introduction of the outline.
The optimization strategy for the control of the number of running stations is: first of all, the supply of chilled water in the control system (total flow) is always kept at the level that meets the system load requirements, and the pressure difference of the system for the most unfavorable load is stable at a certain preset value, which needs to be completed by high quality variable frequency speed control refrigeration pump; According to the difference of the system load and the supply capacity of the current chiller, the time of the next start or stop is decided, the principle is to choose the working mode which can meet the demand of the load and the closest to the system requirement value. The key to this control is to find the best start-stop switching point in the commissioning, and the program that runs the number control should keep the freezer around the optimal efficiency point. The efficiency distribution curve of the chiller is provided by the supplier.
System configuration of different capacity of the cooler, can reduce the number of system load capacity changes in the stage, so that the operating capacity closer to the requirements of the system, there is better energy-saving effect. But the requirement for control is much more complex than the parallel capacity units. Now the use of all frequency conversion speed of the refrigeration pump, the system water supply flow (cooling capacity) can be continuously adjusted according to load needs, so there is no need to configure different specifications of the capacity of the parallel unit.
Traditional one-pump system, only in the load side of the variable flow control, and in the cold source one side is a constant flow (frozen water pump fixed speed), the difference between the flow and the system pressure, by the pressure difference by-pass control to balance, in the normal operation of the bypass tube will inevitably have bypass flow, That is, the full-speed operation of the refrigeration pump part of the power consumption in the bypass cycle, while the pump head produced by the return of the higher pressure between the chief, so that a large amount of water flow can be consumed in overcoming the resistance of the pipeline. The most valuable function of the control system of variable primary flow (chilled water pump) of air conditioning system is to avoid most unnecessary bypass water flow and to maintain low system pressure, which greatly reduces the power consumption of the pump.
The variable-time flow control system can track the dynamic change of the system load, meet the load demand of the system properly, and control the energy consumption at a lower level. However, such a system needs to provide a strict control scheme, in order to truly benefit. The general method of changing the flow control: The design pressure difference at the most unfavorable condition point (which should be selected at the far end of the supply back) as the control preset setting value, with the point pressure difference measurement as the PID adjusting process variable, and the variable frequency speed regulating refrigeration pump as the actuator of the control system, The control goal of the chilled water water supply is to approximate the predetermined value of the process variable; when the system load flow of chilled water is greater than the minimum allowable flow value of a chiller evaporator, the flow bypass control valve is closed; when only one unit is running, and the system load flow of chilled water is lower than the minimum allowable flow value of a chiller evaporator, it is necessary to control the bypass flow between the sub-collector and the water collecting device, and the control goal is to make the water flow rate of the evaporator of the running freezer greater than or equal to the minimum allowable flow value. This bypass valve is different from the conventional differential pressure bypass control valve and can be referred to as a low-flow bypass control valve.
It is a very important technical measure to ensure the minimum water flow of the evaporator, otherwise it may damage the normal working condition of the chiller or even damage the freezer. The bypass control that changes the flow control is actually set for this protection. For a variable flow system operating mode, this bypass valve does not need to be very large, its circulation capacity as long as the largest chiller evaporator to ensure the minimum water flow. However, at present, the design changes once the flow system, often also to retain the traditional mode (frozen pump all constant speed) to work the possibility, considering this, this flow bypass valve may also assume the role of traditional pressure bypass control, so its caliber should meet such needs. For this reason, the bypass valve of this program does not select a smaller flow capacity. This design is designed to enhance the fault tolerance of chilled water system, and to improve the reliability of system operation to take necessary measures.
The control logic should ensure that the minimum flow of the evaporator running the freezer is guaranteed at any time (this minimum water flow is indicated on the use of the chiller); synchronous variable frequency speed control of parallel running unit is the most commonly used control mode; the refrigeration pump for starting the refrigeration unit should adopt soft start mode to avoid the large current impact of the pump motor. Chilled water Regulation control when switching from one to another, ensure a smooth and seamless transition.
Reasonable change of the flow control, can avoid the freezing water pump to work hard phenomenon, can get better energy-saving effect, and real-time tracking of the load side of the water volume changes for the freezing water control and regulation, while ensuring that the system for the return manifold pressure stability. However, the change of the flow control system, the need to arrange more monitoring points, the control and maintenance requirements are very high. This fine process control requires the engineering service to have an accurate and comprehensive grasp of the system control.
The configuration and layout of the chiller, the chilled water pump and the electric butterfly valve have an important influence on the control mode, investment and operation efficiency, and the control Plan designer should communicate with the building HVAC professional designers in order to obtain a reasonable system plan. Variable one-time flow chilled water control system
To better illustrate the problem, here are three typical air conditioning chilled water systems, as shown in Figure 1. They are: traditional chilled water pump control (fixed speed) system, secondary pump control system and variable one-time flow control (primary pump variable speed) system. The water system diagram is simplified in order to be concise and not represented in the discussion of components and monitoring points that are irrelevant to the discussions.
From the diagram shows that only a change in flow control, all the pipeline is a variable flow, and its equipment and traditional fixed speed once the pump control as simple.

4 cooling water and cooling tower control

The basic requirement for cooling water control is to ensure that the cooling water temperature (i.e., the cooling water temperature that enters the condenser of the chiller, or the water temperature of the cooling tower) meets the requirements of the chiller, which is provided by the operating instructions of the chiller.
According to the cooling water supply temperature to the cooling tower fan operating table number control, when cooling water supply water temperature higher than the required value of high wind volume (or increase the number of cooling tower fan operating table), conversely, reduce the air volume, in order to reduce energy consumption.
For multi-blower (or multi-speed) cooling tower, if all the fans open (or at the highest speed), the cooling water supply temperature still can not meet the process requirements, then through the BAS program to open another cooling tower to increase the cooling effect.
Therefore, the necessary conditions for cooling tower control are: monitoring the cooling tower water supply temperature, the cooling tower fan and the corresponding electric butterfly valve control.
The variable frequency speed control of cooling water is also the goal of saving energy. When the system's demand for chilled water decreases, the demand for cooling water will be reduced accordingly. This load change reflected in the cooling water, its performance is the condenser outlet temperature than the rated value, cooling tower water temperature is low, can reduce the number of cooling tower running fan, at the same time can also reduce the cooling water pump operation level, in order to save the cooling water pump power consumption.
The variable frequency speed regulation of cooling water and the frequency-conversion speed control of the chilled waters have no direct logic relation and only indirect influence. However, as with the minimum water flow limit of the evaporator, the condenser also has a minimum water flow limit, so the adjustment of the speed of the cooling pump, there is a minimum limit requirements.
　　

III. technical measures to improve fault tolerance and reliability of the system

Air-conditioning chilled water control system, including cold source side and load side, involving a lot of equipment, control complex, the normal or not the air conditioning device, directly affect the comprehensive function of building equipment, the relationship is significant. The control scheme adopts variable one-time flow and has strict technical requirements. Therefore, the control system design should have the technical measures to improve the fault tolerance and reliability of the system.
(1) In addition to the main mode of operation, also consider the backup mode of work
The main operating mode of the system is to change the flow control as the design goal, and also retains the possibility of working in the traditional mode (as a standby mode). When the equipment is all normal, the variable frequency speed control refrigeration water pump closed-loop controls the operation, the system makes the change once the flow control; In case of the frequency conversion speed regulation closed-loop control can not be put into operation, the system can also be very convenient to switch to the traditional mode of operation, that is, the constant speed of the refrigeration pump, while the But the latter's energy-saving effect is worse than the former.
In order to achieve this backup capability, only the specifications of the low-flow bypass valve can be selected as a specification for the differential-bypass control. These two control modes are set up in the corresponding controller and enable the operator to make mode selections. Because the two modes of operation differ little in equipment configuration, the configuration and selection of control components can meet the needs of these two working conditions.
(2) System operation can not rely entirely on the BAS and the refrigeration machine communication
It has been introduced that the cooling water supply backwater temperature (i.e.: condenser inlet and outlet temperature) can be obtained through the data communication of the freezer, and it appears that the temperature of cooling water supply and return is not necessarily measured on the surface. In fact, to increase the temperature of cooling water supply and return to the mains, can make the monitoring of the cooling water system is not dependent on the data communication of BAS and freezer, BAS can carry out cooling water supply temperature control, so that the fault-tolerant ability of the system is greatly improved. The cost is not high, but the flexibility and reliability can be effective for a long time. The system can only operate in standby mode without relying on BAS and freezer communication, and manual operation is performed on the start-stop control of the refrigeration unit.

Iv. on the low water flow performance of the evaporator of refrigerating machine

In order to enable the system according to the design requirements to achieve the predetermined control function, the performance of all electromechanical equipment, specifications should be matched with the system. In particular, the low water performance and water supply temperature control performance of the evaporator is the key to the success of a variable flow control system.
At present, the low water capacity of the chiller on the market is very different, and their minimum permissible flow is roughly between the 20~90% of the rated value. If the minimum permissible flow of the freezer evaporator is low, the chilled water can be adjusted to a large range, using variable frequency speed control of chilled water flow, can work in a larger dynamic range, can achieve a good energy-saving effect.
If the minimum permissible flow and rating of the freezer evaporator is close, the adjustable range of chilled water is small, and how to meet the requirement of load side in actual operation is difficult, the frequency conversion speed range is narrow, and the efficiency of variable once flow control is not high. Therefore, the minimum allowable flow value of the evaporator of the freezer the selection of the technical index of the chiller is of critical importance for the variable flow control. In short, the refrigeration pump using frequency conversion speed, it should be configured with low water flow capacity of the chiller, in order to and frozen water change once flow control package.
In the past, the water flow in the evaporator of the freezer was not allowed to be below the rated value, which was clearly stated in most of the product manuals previously. At that time, the freezer evaporator is too low water flow, it is possible to freeze the evaporator, causing serious equipment accidents. In recent years, the development of measurement and control technology has greatly improved the comprehensive control level of refrigeration machine. With the advent of one-time flow technology in air conditioning chilled water, the minimum allowable water flow rate of the evaporator of the freezer has been decreasing. For example, the minimum flow of a centrifugal compressor with a double head can be close to 20%, which creates conditions for a good variable-time flow control.

V. Demand-based control-current development trend of chilled water system control

In recent years, the Advanced Area HVAC control technology has a great development. The performance of chilled water system control is to provide the appropriate chilled water flow according to the actual demand of the load side. This is the demand-based control strategy. Here is a brief introduction to these latest developments.
The previous discussion takes the process variable of the variable frequency speed control of the chilled water pump-the most unfavorable pressure difference between the system supply and return controller as a fixed monitoring point, and the preset value of the pressure difference is fixed. In fact, the most unfavorable pressure between the system for the return of the controller is nearly the same as the load changes throughout the building. Although the distal end of the return to the main side is more likely to be at the most unfavorable state, when the near end of an air processor load is very large, the remote air processor load is very small, the most unfavorable pressure is not necessarily at the far side of the pipeline. Therefore, the practice of fixing the monitoring point of the process variable to a certain point is flawed.
In order to overcome this problem, the control of the system pressure difference is set up. The pressure difference setting value is called "re-set value" is because this setting value is not fixed, but according to the load side of the water flow needs to constantly adjust, it is called "re-set". The system needs to measure the opening degree and the load rate level of all air handling units on the load side of the air conditioning water system, and give a reasonable pressure difference and set the value, the control goal is to keep the system pressure to meet the load demand level; it is always constantly adjusting the preset value of the system pressure difference, The control valve of the air processor with the highest relative load rate in the system is just in full open.
Such a control algorithm can ensure that the load side flow rate is the largest (most unfavorable) branch to get enough pressure, while the system pressure to the lowest possible. Due to the different load levels required by the system pressure difference is very large, so the pressure difference of the re-set and control algorithm will decide to change the flow control system energy efficiency. To achieve this measurement and control, it is necessary to rely on BAS control network, because the air conditioning water system control valve distribution in all parts of the building, only through the control network, it is possible to transfer the percentage of valve opening to the controller as a variable once flow policy control.
Described here is the application of the control strategy based on demand in the control of water flow in the air conditioning. This control requires monitoring the opening of many air handlers on the load side and their operating conditions, so a variable-time flow control system is inseparable from the control network and DDC control technology. Further deepening this control strategy, under the DDC control network and certain software support, can even abandon the traditional PID control algorithm, the formation of a new control method, this is Thomas Hartman (Thomas B. Hartman) advocated by the global optimization of HVAC system control strategy.
This advanced, new control algorithm and strategy, the requirements of HVAC equipment and its control components, the design of building equipment control system and system integration requirements are very different from before. So building an advanced and sophisticated building automation system is not just a matter of controlling engineers. This requires the designers of HVAC technology, building owners, experts, control system Designers and all concerned about the development of the industry to pay close attention to the development and progress.

Reference documents:

1. Thomas Hartmann (Thomas B. Hartman): Optimizing all-variable speed Systems with Demand Based Control, the Automator, automatedbu Ildings.com May 2002
2. Thomas Hartmann (Thomas B. Hartman): all-variable speed centrifugal chiller Plants:can We make our Plants more efficient? The Automator, automatedbuildings.com March 2002
3. Thomas Hartmann (Thomas B. Hartman): Global optimization Strategies for high-performance Controls, ASHRAE Transactions Vol. 101, Pt. 2; June, 1995
4. Thomas Hartmann (Thomas B. Hartman): PID control:may It Rest in peace, HPAC Engineering July 2002 1

(from the city, building intelligent system, phase tenth)

ba-Cold Source System--one-time flow control scheme (reprint)