Control Logic for Parallel Cold Storage Units | Shift Start Stop, Load Distribution, and Fault Switching Scheme
Multi head parallel units are suitable for large-scale cold chain storage, slaughter and processing cold storage. They rely on controllers to distribute loads and input heads as needed, resulting in lower energy consumption and higher redundancy compared to single large units.
The Xiteluo parallel piston unit commonly uses a combination of 4 heads and 6 heads. The controller increases or decreases the operation of the heads according to the return air pressure signal, with priority given to rotating operation and balancing the operating time of each head.
Core principle of load distribution: During the light load phase, only a portion of the machine heads are activated, and the load increases step by step; The storage temperature approaches the set value and is unloaded step by step to avoid multiple machine heads synchronously starting and stopping and impacting the power grid.
Parallel units must share a common oil distribution pipe and a unified return oil main pipe. Imbalance in return oil is the highest fault for parallel units, and some machine heads continue to lack oil, which can easily cause cylinder failure.
A single machine head failure triggers a protective shutdown, and the control system automatically shields the faulty machine head. The remaining machine heads continue to operate without directly causing the entire warehouse to shut down, making it suitable for food cold storage that cannot be cut off from cooling.
Many engineering companies only perform parallel projects by simply connecting pipelines without rotation control. Two units are fixed for long-term operation, while the other two units are idle for a long time. The probability of refrigerant migration and liquid hammer during start-up is high for idle machine heads.
Each head of the parallel unit is independently equipped with phase sequence, high voltage, low voltage, and oil pressure differential protection. The main controller only performs overall planning and cannot cancel the self-protection of a single machine.
For large-scale slaughter cold chain projects in foreign trade, customers attach great importance to redundant design. When quoting, they will compare the shutdown risk and maintenance cost of single large head scheme and multi head parallel scheme.
The flow rate of the parallel unit's return air main pipe needs to be calculated, and the flow rate of the main pipe should be controlled at 8-12 m/s to ensure that the machine head is shut down at low loads, and the return air pipe still has the ability to carry refrigeration oil.
Key maintenance points for parallel units: Regularly check the operating hours of each machine head, check whether the oil level in the crankcase of each machine head is balanced, and promptly investigate the return oil branch if the oil level deviation is too large.
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FAQ
What signal is used to load and unload parallel unit control? Use the return air pressure as the main control signal.
What is the biggest advantage of parallel units? Load is distributed according to demand, and single machine failures keep the entire warehouse running.
What are the most common problems with parallel units? The return oil is unbalanced, and some machine heads are short of oil.
What is the recommended flow rate range for parallel return air main? 8～12m/s。
5. Can the single machine protection be cancelled and only the main controller protection be used? Not allowed, standalone protection must be retained.
What is the purpose of rotating start stop? Balance the running time and reduce idle machine head liquid return faults.
7. Which scenarios prioritize the use of parallel units? Large scale cold chain, slaughter and processing, and uninterrupted cold storage.
