【 Weak current College 】 BAS system chiller group control strategy

Abstract: analysis and comparison of several possible group control mode, such as the return water temperature control, flow control method, thermal control method, flow/heat control, differential pressure control, differential pressure/flow control method, and the freezer data interface combined group control law and several special control method
Keywords: BAS chiller control strategy

1 chiller group control of significance
1.1 energy saving
– According to system load size, open the appropriate unit, thus saving energy, and save operating costs.
-The closure of the pump, or lower the water pump motor speed, so as to achieve the purpose of energy saving.
1.2 longevity fate transfer
– Active group control, helps to extend the unit's life, improve equipment utilization efficiency.
1.3 device protection
-Reasonable group control, makes the system more comfortable, and avoid too cold, more easily meet the design requirements
2. several possible group control mode analysis
2.1 return water temperature control method
2.1.1 return water temperature control method
By measuring the air conditioning system of frozen water system water temperature, according to the size of its value and decide on the number of cold water unit, to control the number of chillers.
2.1.2 return water temperature control method of the control flow chart 1
2.1.3 return water temperature control method of analysis
1: return water temperature adaptability, particularly large temperature difference between hour, error, on the negative energy.
2: can be used for low temperature protection and refrigerator.
3: but the device is simple and inexpensive.
4: criterion is not clear.
2.2 flow control method
2.2.1 flow control method control principle
Obtained by measuring the cold water flow signal, and then put this flow value and chiller rated flow, enabling chiller number control.
2.2.2 the flow control method of analysis
Flow control principle is based on three assumptions
1: proportional load and discharge
2: frozen water temperature constant backwater
3: run under the design conditions
But in fact, these three assumes that one could not be established, and there is no chance at the same time.
Flow control method can guarantee system flow, avoiding water chillers evaporator ice, but is not a good adaptation system load changes. Because the coil of heat and flow is not a linear relationship. Experiments and studies show that frozen water flow and building heat load is logarithmic relationship between. This relationship with chilled water inlet temperature, coil size structure and coil surface area and coil surface in contact with the air temperature and wind speed varies, so it not only is nonlinear, or one with a variety of factors change curve. Does not reflect changes in load, and thus cannot be effective energy-saving.
2.2 thermal control method
2.3.1 thermal control method control principle
By measuring the temperature of the chilled water for water and water supply (back) to obtain the temperature and flow signal, and then adds two signals based on thermodynamic formula to calculate the amount of actual need cold, then this cold measure and chiller capacity compared to the cooling capacity, enabling the number of the chiller.
2.3.2 thermal control method control flow chart 2
2.3.3 the thermal control method of analysis
Thermal control method for control regulation and energy-saving effect is good. Cannot guarantee that the system flow to avoid water chillers evaporator ice. 2.4 flow/heat control method
2.4.1 flow/heat control method control principle
Taking into account the flow and thermal control, this is one of the ways we use, we refer to as "flow/heat control method".
2.4.2 flow/heat control method for a control flow chart 3
2.4.3 the flow/heat control method of analysis
Refrigerator groups charged is correct and appropriate solution in the working point within the control zone, the determination of certain points of frozen water and entire building's air conditioning load to an algorithm and the criterion for the decision to start or stop a refrigeration unit. How to determine the unit start stop
Criterion 1: start the new chillers judgment the following points when
1, building on the heat load demands more than just online operation of chiller capacity.
2, building on the frozen water needs more than just online operation of chiller capacity.
3. the above two criteria of any establishment, both effective and sufficient conditions.
Criterion 2: stopping a running water chiller is the judgment the following two points
1, if there are N chillers online run, determine a load of switch point, at this point, the N-1 chillers rated load capacity is exactly equal to the current N table load weight of cold water machine.
2. the judgment of such a work, at this point to stop a running cold water machine online will not lead to a building on freezing water demand is greater than the rest of the running ability of chillers.
3. the above two criteria established is only valid for the necessary and sufficient conditions.
How to determine the start stop which units
Control objectives and control refrigerator in load effectively section 40%-100% range.
Unit selection alternative power condition (opening a chiller can press):
1. current outage time maximum priority
2, the total running time of at least the priority of
3, or alternate Queuing
Alternative downtime conditions (outage a chiller can press):
1. currently the longest running priority
2.Total running time up to the priority of
3, or turn in line, etc.
2.5 pressure control method
2.5.1 pressure control method control principle
Catchment and water separator between bypass pipeline with pressure electric regulating valve. Backwater duct between the description of the user load increases, and the load side flow reduction, adjustable bypass valve to open degree. But only under pressure for number control is very difficult. Pressure differential signals can consist of a pressure difference of two pressure sensor gets calculated after the signal, or directly by the pressure sensor.
2.5.2 for pressure control method of analysis
Each item of pressure differential is not the same, because each item's water system is not the same.
Some papers and abroad proposes to remove the pressure control, that criterion is not clear.
2.6 pressure/flow control method
2.6.1 pressure/flow control method control principle
The pressure differential control method, based on the by-pass pipe with a flow meter and flow switches, you can implement a number of control, we use one method of control, we refer to as "pressure/flow method".
2.6.2 pressure/flow control method for control flow chart 4
2.6.3 for pressure/flow control method of analysis
According to the flow and flow switch criterion explicitly. Requirements for air-conditioning system design reasonable, the water pump flow/head of choice is reasonable.
2.7 and DATAPORT data interface Kalee combination group control
2.7.1 the world's most influential several freezing and air-conditioning manufacturers have claimed to have a set of control systems, such as Carrie's DATAPORT, BA systems require the development of response data interface, such as KMC company Carrier KMD-5540 series of product development.
2.7.2Carrier company DATAPORT, generally only on your own machine control, let the refrigeration unit is responsible for the air conditioning system is a huge mistake.
2.7.3 make full use of the refrigeration unit of data interfaces, can reduce the BA group control system of initial investment, while enhancing the supervisory function BA system.
3. Summary and improve
1: must be personalized according to the specific project and the art of design.
2: more optimized design need expert support system.
3: you must put the theory and practice.
4. bibliography
4.1KMC technical manual