What is relative humidity?
Outside air WITHOUT humidification during winter time
If 1 m3 outside air with an average temperature of –5° C and 80 % relative humidity is influxed into a production plant and heated up to 20° C, the relative humidity will drop down from 80 % to approx. 14 % RH. The water content remains the same.
To preserve a relative humidity of 55 % RH at 20° C each cubicmeter fresh air of –5° C must be added with 6.97 g water after heating up to 20° C.
To humidify or dehumidify explain?
What should my humidity level be?
50%rh is a neutral state where equilibrium is generally kept. At this condition hydroscopic material such as people, paper, textile, wood etc shall not take on or give up moisture so are kept in a good condition.
This value will vary with different processes, storage conditions or temperatures but the majority of applications require 50%rh as there set point to keep products, production and people at the best level.
How do we calculate humidity?
Humidity is calculated by taking the lowest natural moisture level which occurs externally taken from the humidity level required (40 -50%rh) at the temperature you require.
50%rh humidity at 20C has a far lower moisture content then 50%rh at 40C. For this reason we use the Psychrometric chart shown oposite to determine the start and finish moisture contents required and then multiplying this by the total air volume going into the space and what is known as the specific volume of the air.
Again by providing the figures detailed below a calculation can be made.
-Start temperature (assume 10C)
-Temperature required (22C)
-Humidity start condition (22C, 25%rh – humidity at 20C if no moisture is added when the ambient air is natural dry such as on a cold crisp winters morning) 0.0028kg/kg
-Humidity set point (normally 40 -50%rh)
-Total air volume (m3/s)
-Fresh air volume (m3/s)
-Number of windows
-Number of doors – frequency of opening (more unconditioned air entering the space)
What do the experts say?
The main guidance from the Chartered Instuitute of Building Service Engineers (CIBSE), recommends humidity levels of 40-60 % rH for general comfort conditions. There is also the EU Directive on Display Screen Equipment which recommends humidity control for people working at computer workstations.
What is the humidity level for a digital printer?
Humidity is one of the many environmental variables that can impact on the print process and the acceptable relative humidity (RH) range for commercial workplaces is 40-60% as this suits both human and machine occupants.
In print environments the most common problem is that the RH is too low and this can have a detrimental effect on paper stock as well as causing a build-up of static electricity that hinders the print process and increases downtime.
Humidity and Human comfort?
Low humidity has a negative impact on the health of the workforce, causing dehydration and drying out of respiratory surfaces so that people are more susceptible to infections.
Studies indicate people are generally most comfortable when relative humidity is maintained between 35% and 55%. When air is dry, moisture evaporates more readily from the skin, producing a feeling of chilliness even with temperatures of 24°C or more.
If the RH is too low (below 40%), people will feel colder than is justified by the actual temperature and will often turn the heating up to compensate – once again wasting energy. Ironically, raising the temperature also lowers the RH even further, thus exacerbating the problem.
Stabilising humidity at the optimum level, therefore, ensures the protection and well-being of the workforce.
Why is Humidification Important?
Humidification is simply the addition of water to air. However, humidity exerts a powerful influence on environmental and physiological factors. Improper humidity levels (either too high or too low) can cause discomfort for people, and can damage many kinds of equipment and materials.
Problems with dry air
Dry air can cause a variety of costly, troublesome, and sometimes dangerous problems. Static electricity can accumulate in dry atmospheric conditions and interfere with efficient operation of production machinery or electronic office machines. Where static-prone materials such as paper, films, computer disks, and other plastics are handled, dry air intensely aggravates the static problem. In potentially explosive atmospheres, dry air and its resultant static electricity accumulations can be extremely dangerous.