Air humidity in rooms – the new reference variable?
Introduction
If condensation is non-desirable when cooling with an air conditioner or an HVAC system, it is essential to keep above the dewpoint of indoor air. This will save energy. But does it also make sense and is the constant measurement of temperature and relative humidity in rooms a suitable reference variable?
Depending on the location, season and application, the supply air for rooms, production or storage facilities needs to be humidified or dehumidified in different ways. For example, the ideal value for people is a near as possible constant 40 percent relative humidity in indoor air. Paper, on the other hand, is best stored at 50 to 55 percent relative humidity and at an ambient air temperature of between 20 and 23 °C.
Technical humidification is carried out isothermally with steam or adiabatically by evaporation, misting and atomisation of water. Dehumidification is achieved by means of adsorption and condensation processes. This almost always requires additional energy input, usually electrical, but occasionally with gas. However, air humidity in rooms is actually rarely the significant reference variable when planning a ventilation or air conditioning system. But would it perhaps be better to work with the operator to determine what the requirements will be and how the air humidity in rooms can be ensured on this basis before designing air conditioning and ventilation systems? It may even make sense to recover humidity. And, if relative humidity is taken into account with foresight and continuously measured, can energy, money and above all CO₂ emissions be saved to a significant extent if the relative humidity in rooms is introduced as a reference variable for air conditioning and ventilation technology?
Control circuit principle: The controlled variable (x) of the system (controlled system) is continuously recorded by the sensor and the feedback value (r) is compared with the reference variable (w). In the event of a control difference (e), the controller reacts with the manipulated variable (y) so that the controlled variable and target variable match again.
What is a reference variable?
The term “reference variable” is one of the fundamentals of control engineering, a technical discipline of many engineering sciences. The DIN IEC 60050-351 “International Electrotechnical Vocabulary” provides the following definition: “Closed-loop control is a process whereby one variable quantity, namely the controlled variable is continuously or sequentially measured, compared with another variable quantity, namely the reference variable, and influenced in such a manner as to adjust to the reference variable. Characteristic for closed-loop control is the closed action in which the controlled variable continuously or sequentially influences itself in the action path of the closed loop.”
The control of a single value (for example, the temperature, the CO2 content or the relative humidity in rooms) is very common in practice. If disturbance variables occur in rooms, the controller responds. With the reference variable air humidity in rooms, disturbance variables can be people, plants, aquariums, kitchens, bathrooms or the dry supply air via an open window. If the “disturbance” is compensated, a control circuit will function. Incidentally, temperature control was first used in the 17th century. Back then, the Dutch scientist Cornelis Jacobszoon Drebbel invented a self-regulating thermostat, which he used for an incubator for chicks.
Are there specifications for a control system?
Paper 2 of this expert series on “Air humidity in rooms” provides comprehensive information on its significance in international regulations. But what about the control of air humidity? Is it a specified reference variable in a HVAC system or does its monitoring play a subordinate role?
Preliminary information is provided by a note in Part 6 of DIN 1946-6 “Ventilation for residential buildings - General requirements, requirements for design, construction, commissioning and handover as well as maintenance”. The wording of this provision states that air volume flows can also be adapted to a room by means of a suitable reference variable (e.g. room air humidity, carbon dioxide or mixed gas content of the room air), i.e. independently of the user or depending on the user's habits or the presence of people. The formulation reveals that control is a voluntary measure. No further information on the specific design of a ventilation concept according to one of these selected reference variables can be found in the DIN.
Another source of information is the German Energy Saving Ordinance (EnEV), which is still valid. This is because its incorporation into the German ‘Energy Saving and Use of Renewable Energies for Heating and Cooling in Buildings Act’ (GEG) is imminent (the Federal Cabinet approved its draft at the end of October 2019, and its adoption in the Bundesrat is considered a formality) and will be completed in 2020. However, irrespective of the EnEV or GEG, the following applies to the regulation of indoor air humidity in both ordinances: If the humidity in a room is changed by the supply air of a ventilation and air conditioning system, the system must be equipped with an automatic control device when it is installed in a building and also in the event of renovation. Its task is to set separate setpoints for humidification and dehumidification.
At a minimum, the directly measured supply or exhaust air humidity serves as the reference variable. If such control devices are not available in an existing system, the operator must retrofit them within six months. According to this definition, the air humidity in rooms always becomes a reference variable when specialist planners or operators decide to install equipment that humidifies or dehumidifies in the supply or exhaust air. Practice shows that there are essentially three reasons for doing this:
- If the focus is on processes or goods, there are precise recommendations for optimum values of relative room air humidity all year round. For this reason, humidification and dehumidification systems are very often used in large ventilation and air conditioning systems. The relative humidity in production facilities, clean rooms, swimming pools, computer centres, museums, libraries, laboratories or storage facilities, together with the temperature, is therefore the most important reference variable, whereby the humidity of the supply air is controlled to the exact setpoint. In the private sector, the same happens on a smaller scale, for example, when luxury houses or villas and the assets they contain (furniture, paintings, antiques or wooden floors) are to be protected and perhaps contain a swimming pool. In all these cases, the focus is on maintaining consistent indoor humidity throughout the year. This is followed by energy-optimised operation in second place.
- Energy-saving measures lead to ever tighter residential building envelopes in new buildings and in renovations. If temperatures fall below the condensation point, the humidity in the room will condense on cold surfaces if there is no mechanical ventilation. This can result in spores, mould and structural damage, usually caused in the winter months. For this reason, it is essential that buildings constructed in accordance with EU Building Directive 2010/31 EU with requirements for overall energy efficiency monitor and, if necessary, regulate the relative humidity of the supply air and of rooms.
- If the focus is on people, it is mainly a matter of occupational health and safety. Yet the active regulation of room air humidity has so far been an optional regulation. It is therefore left to the employer to follow existing recommendations – usually from insurance companies or on the part of employees. In the private sector or in rented housing, recommendations simply state that the relative indoor air humidity should be maintained at values of around 40 percent all year round. For this reason, architects or planners still pay too little attention to the correct humidity level in rooms. As a result, air-tight buildings often lead to health problems, especially in the private sector in winter, because they provide an ideal environment for viruses and bacteria to thrive in dry indoor air. Little by little, residential ventilation systems with integrated moisture recovery or decentralised individual room solutions with humidity control are being used, although so far they have been the exception rather than the rule. Incidentally, one relatively unknown use of the reference variable “indoor air humidity” is for indoor air conditioning systems that “gently dehumidify”. In recirculation cooling mode, this function ensures that the ambient air does not dry out too much. This is achieved by controlling the cooling capacity in terms of both temperature and humidity via the compressor speed. This is designed to reduce condensation on the cold evaporator surface at an unchanged setpoint temperature and to increase the relative humidity by up to 10 percent compared to uncontrolled cooling mode. At the same time, the compressor requires less condensation energy, which ultimately saves electricity.
The table provides information on suitable air conditions for humidification. These are subject to constant change as substances change in their composition. Machines and devices change in the same way. It is therefore important to discuss this in detail with the operator's specialists before planning.
Reference variables for control
If the supply air is controlled, the CO2 content and the indoor air humidity can be used as reference variables in addition to the temperature. However, CO2 has the disadvantage of only occurring when there are people in a room. If there is no one present, ventilation is quickly reduced, and an incorrect humidity or temperature is ignored. Depending on the season, this can result in odours or moisture damage if rooms are not used for a long period of time. This is because sources of moisture such as plants, aquariums and bathrooms are always present in apartments or houses.
Hence, indoor air humidity makes sense as a reference variable. It is a means of averting any negative impact on the fabric of a building or the promotion of health-endangering air pollutants; on cold winter days, it can even have a positive effect on ventilation heat losses and thus energy consumption, because dry outside air is incorporated into the control process. This triggers an input for minimum volume flows, which not only saves on heating and electrical drive energy, but also reduces CO2 emissions without falling below a minimum air exchange rate. Consequently, residential ventilation systems with controlled moisture recovery benefit from a bonus in their efficiency rating. The same principle is currently being sought as a new correction factor in European regulation law for HVAC systems that can support the stipulated heat recovery via moisture recovery (see paper 2).
In the meantime, there are also fans available with integrated humidity sensors. In small decentralised ventilation systems as well as in large ones with several 1,000 m³/h delivery volumes, they can have a direct effect on the relative indoor air humidity in the supply air flow by increasing or decreasing the fan speed until the setpoint in the home, in the room or in the supply air duct is reached. This means that condensation or damage due to excessive humidity can be prevented, but not the shortfall in minimum air humidity level. This requires an additional humidification system in the HVAC unit or directly in the room. Ideally, the two systems are connected and controlled relative to each other.
The right control strategy
Air humidification and dehumidification changes the physical state of water. This is an effect that can be utilised with a suitable control strategy. Isothermal and adiabatic solutions (see paper 1) can be used for air humidification. Isothermal humidification means that the supply air is largely humidified with steam so that no significant temperature increase occurs in the comfort zone. Therefore, only the humidity values are usually taken into account as reference variables in a control system. Adiabatic systems, on the other hand, humidify with cold water, which is added to the air either by evaporation or misting. This cools the air to be humidified. This means that, in addition to the humidity value, the air temperature must also be measured by a sensor and monitored by a controller to prevent condensation. Adiabatic humidifiers can also be installed in the exhaust air for evaporative cooling. By means of heat recovery, the pre-cooled exhaust air with high relative humidity (up to 96%) enters the warm supply air, extracts heat from it and cools it down. In the best case scenario, temperature differences of between 8 and 10 K can be achieved. In summer, this relieves the load on a refrigeration unit or, depending on the installation location and application, even makes it superfluous. This not only saves energy and reduces CO2 emissions for mechanical cooling, but also investment costs. The table shows the available control strategies for the various humidification and dehumidification options.
Source: “Hygiene Criteria – Planning Guidelines for Humidification”; Condair GmbH
The air we consume
People feel comfortable with as little air movement as possible, pleasant thermal radiation, room temperatures between 20 and 24 °C and a relative humidity of between 40 and 60 percent. These two parameters are physically interdependent and vary according to the season and location in relation to the air pressure. For testing and documentation purposes, the testo 400 universal IAQ instrument measures temperature, humidity, pressure, light, degree of turbulence, CO₂ and CO. It offers specialist planners and plant engineers smart measuring menus with intelligent calibration using touch screens. The measurement results can be used in planning to determine if and how the humidity in rooms is to be actively influenced, controlled and regulated in the next project. The measuring instrument is also used for constant monitoring and testing of the system inventory as well as predefined reference and control variables. At the same time, people's comfort level and health must be taken into account - on an equal footing with goods, production processes or the fabric of a building, which very often requires that the humidity in rooms is kept at a constant level to ensure their protection and safety. An investment in the reference variable indoor air humidity is not only an investment in technology. It also regulates the quality of our comfort levels, our health and the air we consume, which is, after all, around 10,000 litres or 14 kilograms a day.
Are you looking for a reliable way to measure indoor air humidity and moisture?
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