What Are the Types of Chilled Water Systems for Refrigeration and Air Conditioning?
As a core component of building central air conditioning systems, reasonable design and precise control of chilled water systems can significantly enhance energy efficiency.
With the development of energy-saving technologies, chilled water systems have evolved from traditional constant flow modes to intelligent variable flow systems, achieving energy savings while meeting dynamic load demands.
The design of central air conditioning chilled water systems always centers on two core goals: load matching and energy consumption optimization.
Currently, three mainstream schemes dominate the central air conditioning water system field: primary pump constant flow, secondary pump variable flow, and primary pump variable flow. Their core differences lie in flow regulation methods and energy utilization efficiency.
1. Primary Pump Constant Flow System
The primary pump constant flow system is the earliest popularized form of chilled water system. Its core feature is maintaining a constant flow rate on the evaporator side through fixed-speed pumps, adapting to terminal load fluctuations solely by changes in supply and return water temperature difference.
The system consists of refrigeration units, fixed-speed circulating pumps, and bypass pipes. Chilled water units are in one-to-one correspondence with pumps, and a differential pressure bypass valve between the main supply and return pipes balances flow fluctuations. When the terminal load decreases, excess flow returns through the bypass pipe to ensure a constant evaporator flow rate and avoid freezing risks.
A key design point of this system is the parameter matching between the bypass pipe and the differential pressure bypass valve. The design flow rate must equal the rated flow rate of a single chilled water unit to cope with extreme load changes.
Unit start-up logic is based on supply water temperature deviation (exceeding the set value for 10~15 consecutive minutes), while unit shutdown is triggered when the bypass flow reaches 110%~120% of a single unit's flow rate for 10~20 consecutive minutes.
This type of system has significant limitations: pumps operate at rated flow rate at all times. Even when the terminal load drops to 40%, energy consumption remains at full load, resulting in energy waste characterized by "large flow rate and small temperature difference." Therefore, it is only suitable for small air conditioning systems or scenarios with minimal load fluctuations.
2. Secondary Pump Variable Flow System
To address the energy consumption issues of the primary pump constant flow system, the secondary pump variable flow system achieves energy savings through a segmented design of "constant flow on the cold source side + variable flow on the load side."
The system is divided into a primary loop (cold source side) and a secondary loop (load side). Primary pumps maintain a constant evaporator flow rate, while secondary pumps adopt frequency conversion control and are connected to both loops via a balance pipe. When the load side flow rate changes, the water flow direction in the balance pipe adjusts in reverse to ensure stable operation of the cold source side.
In design, the head of primary pumps must overcome the resistance from the evaporator to the balance pipe, while secondary pumps need to cover the resistance of the most unfavorable loop on the load side.
Both unit start-up and shutdown calculate the total load through supply and return water temperature and flow sensors. When the total load exceeds the current unit capacity, start-up is initiated; otherwise, shutdown is performed based on the principle that "remaining capacity still meets demand." Secondary pumps use constant differential pressure control to match terminal flow demands by adjusting rotational speed.
Compared with the primary pump constant flow system, the secondary side energy consumption of the secondary pump system is reduced by 30%~50%. However, it still has the problem of unoptimized energy consumption on the cold source side, along with a complex structure, large floor space, and high control difficulty. It is suitable for district cooling scenarios in large building complexes.
3. Primary Pump Variable Flow System
The primary pump variable flow system is a new-generation design following breakthroughs in chilled water unit manufacturing technology, and it is also an air conditioning chilled water system suitable for high-efficiency refrigeration rooms. Its core innovation lies in achieving synchronous variable flow rates on both the evaporator side and the load side.
The system replaces fixed-speed pumps with variable-frequency pumps and is paired with variable-flow chilled water units, allowing the evaporator flow rate to be infinitely adjusted within the range of 15%~100%, fundamentally eliminating energy waste caused by "large flow rates."
1) The system comprises three key components:
Variable-flow chilled water units: The allowable flow rate change rate and range of their evaporators directly determine system performance. High-quality units can maintain stable outlet water temperature during flow fluctuations.
Bypass device: When the terminal flow rate is lower than the minimum allowable flow rate of the unit, the bypass valve opens to ensure evaporator safety.
Variable-frequency pumps: No one-to-one correspondence with units is required; rotational speed is adjusted through differential pressure signals from the most unfavorable loop.
In design, pump selection must match the total resistance of the system's most unfavorable loop, and the bypass pipe flow rate is set according to the minimum allowable flow rate of a single unit.
Unit start-up logic is based on compressor operating current (exceeding 90% of the rated value for 10~15 consecutive minutes), while shutdown is triggered by calculating the average current (average value lower than 80% of the rated value).
By eliminating secondary pumps, the system structure can be simplified by 30%, floor space reduced by 20%, and comprehensive energy consumption lowered by 40%~60% compared with the primary pump constant flow system, making it the preferred solution for large public buildings currently.
Core Control Technologies of the Primary Pump Variable Flow System
The energy-saving advantages of the primary pump variable flow system rely on feasible control strategies. Current mainstream control technologies focus on two core parameters: temperature difference and differential pressure, achieving load matching and system stability through dynamic adjustment.
2) Temperature Difference Control
Temperature difference control achieves matching between flow rate and load by maintaining a stable supply and return water temperature difference. The theoretical energy-saving effect is superior to differential pressure control, but it has higher requirements for the uniformity of pipe network load distribution.
Studies have shown that when on-off control valves are installed at the terminals, the applicability of temperature difference control depends on two points: first, uniform distribution of pipe network load, and second, similar load change patterns among various users.
In scenarios with uniform load distribution (such as data rooms), the hydraulic imbalance degree of each branch can be controlled within ±10%, and indoor temperature and humidity fluctuations meet comfort requirements (dry bulb temperature change ≤1℃, relative humidity fluctuation ≤0.5%).
However, in cases of concentrated load distribution, overcurrent may occur in low-load branches, leading to significant hydraulic imbalance.
Variable temperature difference control is a further optimization direction. The traditional method of setting temperature differences in segments according to total load rate has drawbacks: at low loads, the dehumidification capacity of the surface cooler decreases, resulting in excessive indoor humidity fluctuations. The improved scheme suggests taking "prioritizing dehumidification capacity and dry bulb temperature" as the principle, using the dehumidification coefficient ξ as a known parameter, and allowing appropriate humidity fluctuations to ensure temperature stability.
3) Differential Pressure Control
The supply and return water differential pressure is a core parameter ensuring the hydraulic balance of the system. The primary pump variable flow system uses a PID (Proportional-Integral-Derivative) controller for regulation, which adjusts system errors through three control methods (proportional, integral, and derivative) to achieve precise control.
Suzhou Pharma Machinery Co.,Ltd.
2026/03/04
Gino
