What is a pass box? The size categories of the pass box

To distinguish, compare AHU and RTU how different, people are based on 3 criteria: structure, function, and operating principle. Along INTECH find out through the article below.

How are AHU and RTU different?

Distinguish, compare AHU and RTU

Comparison of structure:

AHU has basic structure is:

Air mixing box

Air filter

Heat exchanger

Fan level


– Shaped aluminum frame: Designed rounded with the effect of preventing dust, is the insulation shell.

– Heat-proof hoses: Two-layer structure, when insulated hoses are installed outdoors, it will be equipped with insulation tape and anti-corrosion tape.

– Panel PU:

Sandwich structure, has two layers of outer metal layer and inner galvanized metal layer, arranged alternately to create solid hardness.

Shaped aluminum frame, fixed by bolts to enhance insulation, anti-rust, ensure the structure of the panel is light but still very strong

– Raw filter, filter bag, Hepa filter:

This is a dust filter, keeping the dust with different levels of dust filtration. Has the effect of removing pollutants from the air, helping to improve air quality.

The filter has an important role in the structure of the AHU, which works to provide clean air, keeping the remaining parts always in a clean state.

The filters are arranged in different levels of filtering, from the lowest to the outermost, and the highest to the last. Depending on each AHU, the filters are individually configured.

Regular monitoring to replace filters helps to ensure they are always supplied at the cleanest level.


RTU (Remote Terminal Unit / Gateway) is a device placed at a power station or a power plant to serve the collection and transmission of data into the SCADA system of the electrical dispatching center (Control Center). RTU can collect signals from other smart devices (IEDs).

RTU Remote Terminal Unit

RTU – Remote Terminal Unit

RTU is composed of:

Information portal on the control center.

I / O channels (IO): Measurement. sensor, status indicator, control

The structure of the RTU consists of a flexible structure of modules, each module has a processing block and input and output signal block, which makes it convenient to collect existing and extended signals.

The operating system of the processing module is real-time multitasking that allows it to work independently, the modules are distributed in equipment cabinets in the power supply station (distributed RTU), or there will be many. Modules can be linked together to integrate into a centralized RTU.

RTU has communication port using serial communication method via RS232, RS485 or RS422 interfaces.

Functional distinctions:


Air-conditioning and air-frequency recycling, with heating, ventilation and air-conditioning effects.


Collect information about the electrical system and then send that information to the CC control center according to the channels from the CC request.

Receive control information, synchronize time from CC, execute commands and send results to CC (Control center).

Comparison of operating principles:


Cold water will run from the Chiller system to lead to the AHU. The heat exchanger has the function of exchanging heat between cold water when passing through Ahu’s copper pipes and the passing air.

When the room temperature is larger than the valve set temperature, the 3-way valve will open so that cold water flows through the entire heat exchanger so that when the temperature is lower than the set temperature, the valve The 3 door closed, cold water flowed through the bypass to the center.

The air will be filtered through filter and filter bag before going into AHU, if it is necessary to have high cleanliness, use Hepa filter bag.

The filtered air at the filter will contact the internal heat exchanger of AHU to create cold air. Cold air is brought into the circulation chamber to be blown through the ventilation ducts and then to the air-conditioned room.

Currently, AHU is equipped with smooth control actuators instead of installing the 3-way valve technology as before, which saves more energy, and this new technology has an accurate control of each% of the valve opening. cold, bring higher efficiency.

The working principle of AHU

The working principle of AHU


During operation, RTU continuously scans for closed and open states in 10 – 20 ms period.

When there is a contact of a breaker, isolator, earth knife at the power station that changes the status from CLOSE to OPEN state, or vice versa, or is INVALID, the signal will immediately be recognized and notified. Change of state ”along with the address of the device in the current state at the time of occurrence is sent via MODEM to the CC control center.

On the moderator’s console screen and on the printer, the status “8BS572 OPEN” will appear and on the diagram of the console screen corresponding to the 572 circuit breaker will turn on “open”.

How does RTU work

How does RTU work?

AHU and RTU are quite different devices. Therefore, to distinguish and compare how AHU and RTU are different, we only need to know about the 3 structural elements, functions and operating principles.


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