Energy-optimised Actuation/Control of Air Heaters and Air Coolers in Air Handling Units

Introduction

Excellent indoor air quality is essential for our well-being and health, as we spend over 90% of our time indoors. The coronavirus pandemic has heightened awareness of the importance of healthy indoor air. In modern buildings, mechanical ventilation systems, especially air handling units, ensure high air quality by supplying outdoor air, filtering out pollutants, and regulating both air temperature and humidity.

Energy-efficient operation of these devices is essential for achieving the ambitious climate targets of the EU. Nonetheless, many air handling units are inefficient. More than 60% of existing systems in Germany are defective, and even new systems often do not achieve optimum energy efficiency [1]. The main causes often lie in the incorrect design and control of heat exchangers.

In addition to the electrical power for the fans, heat exchangers are the components in an air handling unit responsible for converting the majority of thermal energy [2]. Choosing the right valve-actuator combination to control the heat exchangers, therefore, offers significant potential savings.

The article sheds light on the causes of inefficient operation, shows practical solutions, and provides recommendations for increasing energy efficiency. It should be noted that the statements contained in this article are primarily in reference to air coolers and heaters with throttle and injection circuits. The behaviour of a depressurised mixing circuit may vary slightly, however.

Figure 1. Schematic of an air handling unit with air heater and injection circuit.

Possible causes and effects of inefficient operation of heat exchangers

The transfer behaviour of air-to-water heat exchangers is shown by the transferred thermal output in comparison with the water flow rate (grey dots in Figure 2). As the flow rate increases, the output increases logarithmically until the heat exchanger reaches saturation at higher flow rates. With constant inlet conditions, the transfer behaviour corresponds to the heat exchanger characteristic curve (blue line). In reality, however, the variability of these conditions leads to a cluster of points.

A correctly designed system generally exhibits low dispersion and ideal transmission behaviour, as shown in Figure 2. In practice, however, cases exist that deviate from this behaviour. These do not always have a negative effect on energy efficiency, but unfavourable scenarios can be identified by recording the transmission behaviour.

Figure 2. Heat exchanger characteristic curve with ideal transfer behaviour.

Temperature and pressure fluctuations in hydronic systems

Temperature and pressure fluctuations can appear in hydronic systems. Temperature fluctuations caused, for example, by a distribution network that has been set up to produce potable hot water, influence the output of the heat exchanger and thus the supply air temperature. The temperature control compensates for this by controlling the valve, but usually with a delay, due to the slow reactivity of the heat exchanger, temperature sensor, and controller. These kinds of changes are noticeable and undesirable in sensitive applications. Poor controller settings can even cause the system to oscillate.

Temperature changes lead to a change in the power output at a particular valve position (or flow rate). The operating points deviate accordingly from the ideal heat exchanger characteristic curve, as shown in Figure 3. Additional recording of the supply temperature would provide a direct indication of this situation.

Figure 3. Dispersion of the operating points.

Pressure fluctuations occur when consumers are switched on or off. This changes the flow rate and thus the power output of the heat exchanger without directly influencing the transfer behaviour. These changes must also be compensated for by the temperature control, which can lead to undesirable effects.

Saturation operation of heat exchangers

Operating a heat exchanger in saturation reduces the efficiency of the overall system. Saturation occurs when the transferred power hardly increases with increasing water flow rate (yellow area in Figure 4). An increase in flow rate results in a slight increase in output, but increases the pump flow and reduces the differential temperature between the supply and the return (low differential temperature syndrome). This leads to poor efficiencies in heat generators or chillers and makes the system inefficient.

Causes for operation in saturation can be:

·         Contamination of the heat exchanger on the air side due to missing/defective filters or poor maintenance

·         Undersized heat exchanger

·         Insufficient supply temperature

Figure 4. Transfer behaviour in saturation.

Incorrect sizing of valves

The heat exchanger in an air handling unit is designed by the manufacturer, while the valve is usually determined by the heating engineer, which can lead to non-optimal sizing of the valve. Air handling units are often designed for nominal load, but operate mainly at partial load. Dynamic heating or cooling load conditions and variable air volume flows require precise sizing of the heat exchanger and the control valve. If the valve is determined according to the pipe diameter, then this will often result in oversizing, which in turn causes an inaccurate control characteristic at partial load. Even small valve openings lead to an excessive flow rate, which causes temperature fluctuations in the supply air and a loss of comfort. These fluctuations can also influence neighbouring systems.

Valves that are too small, on the other hand, restrict the flow rate. The heat exchanger could have additional capacity in the event of extensions or changes of use, but the valve does not allow a higher required flow rate, even though it is permanently fully open (see Figure 5).

Figure 5. Limited flow rate with a valve that is too small.

Operating states do not match the design state

The design of the heat exchanger is based on heating or cooling capacity, water temperatures, water flow rate, and air flow rate. Operating states that deviate from the design can cause problems. Higher supply temperatures cause the heat exchanger to be "oversized", whereby small changes in flow rate cause large changes in output. This makes control more difficult and increases the tendency to oscillate.

The transmission behaviour in Figure 6 shows operating points in the lower flow range when the supply temperature is too high.

If the supply temperatures are too low, then the heat exchanger cannot transfer the required output and is operated inefficiently when in saturation. The required output is not achieved, which can also result in diminishing comfort.

Figure 6. Transfer behaviour with a heat exchanger that is too large.

Possible solutions for improving energy efficiency

The following solutions, which are in reference to the problems outlined above, can contribute to improving energy efficiency and room comfort.

Creating transparency

The availability of measured values is crucial for recognising inefficient operation of the heat exchanger or the hydronic system, especially in existing systems where the design values are often no longer correct due to modifications. Transparency also helps to identify faults in new systems at an early stage.

Important measured values such as valve position, temperatures, and water flow rate must be recorded in order to calculate the transferred output. The transmission output in connection with the flow rate represents the transmission behaviour, which in turn provides an indication of the efficiency of the system.

This data is useful only if it is processed. Digital communication (e.g. Modbus, BACnet) enables evaluation and optimisation by the air handling unit or the building management system.

b. Measuring and controlling the flow rate

Pressure fluctuations in a hydronic system must be compensated for by the air handling unit. This is usually done via the supply air temperature control, but this leads to slow reactivity and possible oscillations if the control is not well-tuned.

Pressure-independent valves that control the flow rate offer a more direct solution, as they compensate for pressure fluctuations at the water side and guarantee stable transmission conditions. In addition, their use enables simple hydronic balancing by setting the maximum flow rate, which saves both time and money.

Both mechanical and electronic pressure-independent valves are available. Electronic pressure‑independent valves offer the advantage that the flow rate is known as a measured value and can be used for evaluation. They furthermore offer a large dynamically adjustable control range, which allows the flow rate to be adapted and incorrect sizing to be avoided.

Measuring and controlling output

The transferred output of a heat exchanger can be measured and calculated on the basis of the flow rate and the differential temperature between the supply and return. This creates transparency in the system. If this measurement is combined with a valve, then the output can also be controlled. This offers some decisive advantages in practice.

The power control enables both pressure and temperature fluctuations in the supply to be recognised and compensated for. It also ensures perfect linearization of the system and facilitates stable operation of the temperature controller, particularly in the partial load range. The performance and flow rate can be analysed over a longer period of time, which provides valuable insights for improving the system as described above.

Electronic pressure-independent valves with energy monitoring, which are referred to as energy valves, combine a flow sensor with temperature sensors and a valve. They control the flow rate and output autonomously and make all measured values available to the higher-level system via communication interfaces.

Valve as part of the air handling unit manufacturer's scope of delivery

Air handling units (AHUs) are increasingly being offered "ready-to-connect". This means that the control and field devices (actuators and sensors) are being supplied directly by the manufacturer of the AHUs as part of the scope of delivery. Often, the appropriate valve-actuator combination for controlling the heat exchangers is not supplied by the manufacturer and is then planned and installed by other trades.

Problems with the control of the heat exchanger could be avoided if the manufacturer of the AHU also supplied the valve or at least designed it. The manufacturer responsible for the sizing of the heat exchanger could thus define the appropriate valve size and avoid oversizing the valve. The integrated control of the AHU could be optimised to the valve, which improves the control quality. In addition, a communicative interface to the valve actuator could be designed so that all important data is available in the AHU. With analogue integration, important information from the valve actuator is not available to the control system or building management system.

Discussion

Possible solutions are only as good as their implementation in practice. There are often some stumbling blocks when implementing such solutions.

Creating transparency

Recording measured values initially means higher costs, but these often pay for themselves several times over during an operating period of 15 to 20 years. The recording of important values such as valve position, temperatures, and flow rate enables the performance to be evaluated transparently. Electronic pressure-independent valves with energy monitoring can automatically measure and visualise these values. The challenge often lies in integrating the valve-actuator combination into the device control system of the air handling unit. In contrast to analogue integration, with digital integration, all information is available to a higher-level system. If analogue control is unavoidable, then hybrid integration could at least be used to read out the available information.

Measuring and controlling the flow rate

Pressure-independent valves are fully developed but are not yet used everywhere as they are more expensive. The use of electronic pressure-independent valves can, however, save costs in the long term, for example by reducing the effort required for hydronic balancing and subsequent optimisation of operation.

Measuring and controlling performance

The EPBD Directive (Energy Performance of Buildings Directive) requires continuous monitoring of energy consumption. In the case of heat exchangers, it is complex to measure energy flows via the air. A simpler solution is calculation on the water side, which is reliably possible by using energy valves. The initial investment costs for energy valves are usually quickly amortised by the gains in efficiency and are economically justifiable.

Concrete recommendations for action

The following recommendations optimise the operation of heat exchangers in air handling units and contribute to achieving the EU's climate targets by reducing CO₂ emissions and saving costs at the same time:

1. Use pressure-independent valves

Pressure-independent valves should always be used, as they are considered state-of-the-art. Electronic pressure-independent valves offer additional advantages by providing necessary system transparency.

2. Use electronic pressure-independent valves with energy monitoring

These valves record the current transferred output at the heat exchanger and provide important system transparency. Their integrated functions make a significant contribution to reducing energy consumption.

3. Digital interface for control

Valves must be integrated into the control system or the building management system via a digital interface. Only in this way can the recorded data be used for evaluations that lead to further energy savings.

References

[1]     cci Zeitung August 2021, Bauherren-Newsletter, Baurechtsreport 2021. Ergebnisse der Prüfungen gebäudetechnischer Anlagen (Building Law Report 2021. Results of the inspections of building technology systems).

[2]     Eurovent Energy Classification (ECC) for Air Handling Units – Case Study – Energy Consumption by Energy Class – January 2025.