Following on from my previous article, where I explored the importance of integrated control systems in managing fan speed, temperature and humidity to optimise paint booth performance, we now turn our attention to three advanced technologies that further enhance environmental control and energy efficiency.
In this second part, I take a closer look at adiabatic humidification, evaporative cooling and heat recovery examining how their correct integration into the control logic can bring substantial energy savings without compromising process quality.

Adiabatic humidification

Adiabatic humidification lowers the air temperature, and therefore temperature-humidity control needs to consider the link between the two variables: to maintain the set point in paint the booth, as the flow-rate of moisture transferred to the air stream increases, preheating also needs to be increased. To limit swings in temperature due to humidification, the most effective humidifier control method is based on absolute humidity (amount of water vapour present in 1 kg of air, measured in grams per kilogram of dry air). In fact, unlike relative humidity, absolute humidity does not vary as a function of temperature.

At the same time, temperature-humidity control also needs to take into account the fact that the effectiveness of the humidification process is influenced by the conditions of the incoming air stream. Cold air does not have the energy needed to evaporate and absorb moisture. The result is that large amounts of water are needed to transfer small quantities of moisture to the air. The air therefore needs to be preheated, both to offset the drop in temperature and to guarantee high water absorption efficiency. A typical control system uses a temperature probe installed after the heating coil, called a “saturation probe”, and works to bring the temperature to an adequate level to ensure absorption. Sometimes additional heating may be needed downstream of the humidifier to reach the desired temperature value. This occurs with wetted media systems: given the inertia of the evaporating matrix, wetted media humidifiers do not allow humidity production to be modulated quickly, and therefore require the air downstream of the humidifier to be reheated, so as to avoid unwanted temperature swings.

The use of variable-speed adiabatic atomisers allows very precise control of the conditions downstream of the humidifier, based on the saturation temperature reading or, even more effectively, measuring the air temperature and humidity downstream of the air preheating coil, and trying to reach the enthalpy of the desired set point. Adiabatic humidification is an isenthalpic transformation and therefore, starting from a point with the correct preheating temperature and the same enthalpy as the end point, the set point conditions can be reached and maintained with the highest precision, by continuously varying the flow-rate of atomised water.

Example of enthalpy-based preheating control with preheating only

In principle, by having modulating actuators available and setting the controller correctly, preheating can be controlled using the temperature probe downstream of the humidifier: higher humidification demand brings about a drop in temperature that the heating coil needs to offset. In this case, the responsiveness of the controller must be consistent with the inertia of the devices: if the valve on the heating coil takes two minutes to complete its travel but the humidifier can respond in just a few seconds, when the humidifier is activated there will be a sudden increase in relative humidity and drop in temperature, thus failing to reach the set point. It is therefore important to use the PID values, for example, setting a lower proportional constant for the humidifier, and then acting on the integral value to ensure more stable control and avoid swings.

Evaporative cooling

Direct evaporative cooling (DEC) can be used on the incoming air supplied to the paint booth. It is only possible if the incoming relative humidity does not exceed the limit values for the process, usually around 80% RH. In this case control uses a temperature probe, usually together with a humidity limit probe to avoid high values (>80% RH) that could lead to condensation in the ducts or paint defects. The use of modulating humidifiers allows optimised control: humidifier operation is modulated based on the cooling requirements and the humidity limit, reducing production and allowing cooling to be delivered by other devices. In this case, it is very important to ensure that the cooling device does not dehumidify the air: removing the moisture previously added to the air stream is in fact a waste of energy. Therefore, when dehumidification occurs, the controller must stop the evaporative cooling system.

Typically, in temperate climates, cooling is often associated with latent loads, and therefore humidity needs to be lowered rather than increased. In painting processes, where the optimal relative humidity level required is quite high, direct evaporative cooling (DEC) can be advantageous, pushing the humidity to the maximum limit to take advantage of its free cooling effect.

Alternatively, evaporative cooling can be used to cool the exhaust air entering the heat recovery unit, consequently bringing about a drop in the supply temperature without affecting the humidity. This process is called indirect evaporative cooling (IEC).

Adiabatic humidification and evaporative cooling (DEC+IEC) layout using a single humidifier pumping station

Heat recovery

The various heat recovery technologies examined here differ in terms of the possibility of modulating operation and the type of heat recovered, whether sensible or sensible plus latent. From a control point of view, a heat recovery unit behaves just like a heating or cooling device, with possible dehumidification. Modulation is managed with appropriate ON/OFF or modulating outputs that control bypass dampers (modulating or ON/OFF), rotation speed (for thermal wheels), or the fluid flow-rate in a run-around coil system. More specific additional functions are available for heat recovery units, which vary in complexity depending on the devices and the inputs and outputs available.

The criterion for activating a heat recovery unit is normally based on a temperature differential between the outside air and the extracted air. This temperature difference gives an idea of the energy that can be potentially recovered by the heat exchanger, which must then be compared against the energy required to make the heat recovery system work, whether the power consumption of the motor that drives a thermal wheel or the pump on a run-around coil system, as well as the energy consumed by the fans to overcome the extra pressure drop due to the heat exchanger being placed in the air stream. If the temperature difference is not sufficient, the energy recovered is less than that consumed by the fans to overcome the pressure drop, and therefore it is more cost-effective to bypass the heat recovery unit, using the bypass dampers. Normally, the heat recovery unit activation temperature differential is defined during the design phase, and is generally set at around 1-2°C.

The bypass is very useful in winter, when cold temperatures can lead to condensation and consequently ice forming inside the heat recovery unit. The cold air bypass allows only the hot air leaving the paint booth to flow through the heat recovery unit, and this melts any ice that has formed. The bypass can be activated based on temperature or pressure, using, for example: thermostats installed in specific positions in the heat recovery unit, temperature probes that measure heat exchanger efficiency, temperature and humidity probes upstream and downstream of the heat exchanger, and differential pressure probes that check for pressure drop, which change significantly when there is ice blocking the channels. There are various possible control strategies, and these depend on the probes available: the most common are based on time or on measuring the exhaust temperature, which tends to rise once the heat recovery unit has been defrosted. It is clear that sophisticated logic requires microprocessor controllers with the ability to compare the readings of multiple sensors and store data.

Crossflow plate heat exchanger with central bypass section and dampers

Unlike plate heat exchangers, thermal wheels have different auxiliary logic, normally managed by a specific controller. In addition to defrosting as described above, a thermal wheel also needs to manage feedback on rotation of the drum (often via a pulse signal), for example to check for problems with the drive belt. Another additional control function is “rotation due to inactivity”, activated when the heat recovery unit remains off for an extended time, to prevent the wheel from deforming due to its own weight while always remaining in the same position.

Crossflow plate heat exchanger with central bypass section and dampers

The control of heat recovery when indirect evaporative cooling is used is also worth looking at briefly. Evaporative cooling involves using a different activation threshold for the heat recovery unit: heat recovery conditions that are not cost effective due to a temperature differential that is too low, as described previously, could actually become cost-effective if evaporative cooling is activated. Activation of the heat recovery unit and the evaporative cooling system need to be evaluated based on various cost-effectiveness parameters, above all the cost of energy and the cost of water.

The topics covered in this blog post about humidity and temperature in painting booths are explored in detail
in the white paper “Highly-efficient solutions for painting – Humidity, temperature and heat recovery control”.

Download the white paper