Skip to content
Home » Making sense of sensors in the HVAC&R industries

Making sense of sensors in the HVAC&R industries

  • marimac 

By Christo du Toit (B. Eng.), Metraclark Engineering

This article covers technical aspects of how sensors of this nature tie into an overall system and how each would function.

Sensors are key to providing optimised control, data logging, monitoring, systems management, and efficiency with information on the status of any refrigerated or air-conditioned room. Sensor functionality in refrigeration systems mainly contribute to control, equipment protection, and safety regulation in the system, and around the installation site.

Pressure and temperature sensor

Pressure and temperature sensor information plays a fundamental role in HVAC and refrigeration systems to control the balance between evaporating and condensing phases of refrigerants in the respective low- and high-pressure zones in the system.

In a refrigeration system, these two groups of sensors work hand-in-hand in the evaporating stage to ensure the correct evaporating pressure and temperature is maintained. This is achieved through constant adjustment of the expansion valve opening percentage based on how much heat is absorbed into the evaporating refrigerant.

Register for free to gain access the digital library for Cold Link Africa publications

All the liquid is evaporated into a superheated gas with a set temperature difference above the boiling point of the refrigerant at that pressure (superheat). The superheat balance point is set so that there is always just enough liquid refrigerant being let through by the expansion valve to ensure that no un-evaporated liquid refrigerant returns to the compressor unit, yet maintaining optimum efficiency and performance of heat transfer through the system.

Certain pressure and temperature sensors are designed to be fundamentally simple in design and operation to improve their reliability in the field. They function as equipment protection safeties so prevent damages to equipment due to abnormal pressures, temperatures and even electrical supply surges. SANS-10147 regulates that each refrigerating system that has a prime mover part that exceeds a rating of 10[kW] or a displacement of 0.024[m3/s] shall be protected by pressure relief devices.

Humidity, Indoor Air Quality (IAQ), and occupancy sensors

These sensors are all installed in the air conditioned or refrigerated rooms. They do not directly control the refrigeration systems as in the case of pressure and temperature sensors, but rather focus on the air quality and moisture content of the air inside the respective space. Air-conditioned public spaces like airports and commercial aircraft as well as clean rooms such as electronic manufacturing facilities or medical operating theatres will require IAQ sensors to manage the desired air quality.

Occupancy sensors

Occupancy sensors would switch on air conditioning units or lights in a room when an occupant is detected inside the room. Similarly, a refrigerated room like blast freezers might have door and occupancy sensors which switches off the refrigeration unit once the room door is opened or an occupant is detected inside.

Humidity sensors

Humidity sensors in refrigeration are typically used in fruit ripening rooms to prevent the refrigeration unit from drying out the air too much as to dehydrate the product inside. The humidity sensor would then activate a humidifier or cut off the refrigeration plant.

Indoor air quality

Indoor air quality monitors utilize various types of sensors all at once to get an overall measurement of different properties of the air inside a room. Properties like temperature, humidity, CO2 level, and other harmful chemicals or particles can be detected. A detailed warning of deteriorating air quality would allow one to address the issue for example by switching on an air humidifier, purifier, heating/cooling unit or opening a window.

Register for free to gain access the digital library for Cold Link Africa publications

Gas sensors

Gas sensors are installed in the refrigeration condensing unit or inside the compressor unit plant room to detect any refrigerant gas leakage that might occur. An alarm would notify the contractor or operator of a gas leak to prevent refrigerant loss into the atmosphere. Detecting a gas leak early is critical in preventing refrigerant loss and harmful gasses being released into the atmosphere.

Green refrigerants like CO2-(R744), Ammonia-(R717) or Propane-(R290) offer an environmentally friendly alternative to freon. The usage of green refrigerants does however include severe health risks and safety hazards. Early warning mechanisms such as gas leak detectors are especially important on green refrigerant installation sites to ensure the safety of the public and personnel in close vicinity of the refrigeration system.

SANS-10147 regulates that each ammonia refrigeration machine shall have at least two ammonia detection sensors to set off an alarm at a value not higher than 25 ppm and trip the complete installation at a value not higher than 50 ppm. A serious ammonia gas leak with a value higher than 200 ppm would isolate all power supply to the plant room except for emergency lighting and ventilation fans.

How technology/accuracy/system efficiency has improved over the years

Sensor technology has developed over the years to provide more reliable and cost-effective sensing equipment to such an important industry. The introduction of resistance temperature detectors (RTD’s), such as PT1000 probes, ushered in a new standard in reliability, cost effectiveness, and accuracy compared to pressure temperature sensors (PTDs).

RTDs are made to have a certain electrical resistance at a given temperature. There are no moving parts or electronics involved. The sensor constantly outputs an electrical signal based on its input voltage without the use of moving parts, or the need for any maintenance.

PTDs are made to physically measure a pressure difference of an enclosed gas chamber typically referred to as a feeler bulb. A change in temperature causes gas to expand or contract which translates to mechanical movement. The precise movement is then translated to a needle on a display gauge or pressure sensitive relay switch in thermostats as output. No electronics are involved at any stage during temperature measuring.

The importance of sensors related to IoT and system efficiency today

The conclusion I can make is that new era sensors allow for better electronic sensor data logging, monitoring, safety and control of refrigeration and air- conditioning systems.

Electrical wires are more versatile and allow for easier installations compared to pressurized tube pigtail probes of which the length from probe to controller cannot be altered. Even though RTDs have to be calibrated upon commissioning to compensate for line resistance in probe wires, electronic signals have the benefit of real time sensor information to be transmitted over data networks across the globe and be monitored from anywhere on earth.

Real time sensor information enables constant optimal control through the use of electronic controllers. This means that refrigeration systems constantly run at optimum efficiency and performance as they adapt to environmental changes throughout the day, all while doing so safely.

Register for free to gain access the digital library for Cold Link Africa publications