Standard air-conditioning units
Carel Cod. +030221421 – Rel. 1.2 – April, 11, 2003
10
2.3 HUMIDITY AND STEAM PRODUCTION CONTROL
The steam production of the humidifier is controlled according to:
• the humidity
• the production set on the screen (value between 30% and 100% of rated production)
Humidity control is performed by the program based on the reading of the humidity probe, the humidity set point and the humidity differential.
The program calculates the proportional humidity error, ERP:
The graph of humidifier production control is based on the rated production, set production and proportional error (ERP):
ERP = proportional humidity error
Set production: A = 100% rated output
B = 75% rated output
C = 45% rated output
The humidifier has a minimum production equal to 20% of the rated output (for technical reasons) when ERP is between 0% and 20%, and
increases as the ERP increases until reaching the set production when ERP=100%.
Below is a brief description of the algorithm embedded in the bios for the management of a humidifier with 1 or 2 immersed electrode cylinders.
In this type of humidifier, the steam is produced by boiling the water contained inside the cylinder. This occurs by simply
filling the cylinder with water and applying a voltage to the electrodes. According to the Joule effect, the current will tend
to heat the water until it boils.
The current that runs through the electrodes in the cylinder depends essentially on the voltage applied to the electrodes, the
conductivity of the water inside the cylinder and the level of the water.
The aim of the algorithm is to maintain the current that runs through the electrodes at a reference value so as to ensure the
percentage of steam production required, according to the readings of the humidity probes and the parameters set by the
user.
During evaporation, the level of the water falls, and as the current is directly proportional to the quantity of water present
in the cylinder, to keep it constant the cylinder would need to be constantly filled with minute quantities of water.
To avoid this, the current is maintained within a certain range around the reference value, by repeated “water
fill/evaporation” cycles.
As well as the level of water in the cylinder, the other factor that determines the current level is the conductivity of the water inside the cylinder. In fact,
during the fill/evaporation cycles, the conductivity of the water will tend to increase, due to the increase in the concentration of salts in the water. The
conductivity of the water inside the cylinder is measured indirectly, by calculating the time required for a complete evaporation cycle. This time is then
compared against a reference (typical for each cylinder) and, if lower, a certain quantity of water is drained and then the cylinder is topped up with less
conductive mains water.
The humidifier also features a conductivity meter that measures the conductivity of the mains water entering the appliance during the filling
cycles. In the case of high conductivity of the supply water, the control algorithm first signals a pre-alarm (that doesn't stop operation) and
then, if necessary, an alarm (that stops operation). This is essential to avoid the introduction of excessively conductive water into the cylinder,
which may compromise the correct operation of the humidifier.
Another fundamental element, installed at the top of the cylinder, is the high level sensor, used to detect any water or foam.
The high level electrodes may be activated for one of the following reasons:
- over-filling of water in the boiler – when the unit is off – due to a leak in the fill electrovalve;
- high water level when first filling the cylinder;
- high water level following the depletion of the cylinder due to fouling on the plates;
- formation of foam.
In the first case, when the high level sensor is activated, the algorithm stops operation and signals a cylinder full alarm, while in the other three
cases the humidifier responds by draining the water so as to decrease the level.
In the event of repeated activations of the high level sensor, the algorithm evaluates the possibility that the causes may be due to the presence of
foam. In this case, if after having performed a complete washing cycle (complete emptying-complete refill-complete emptying) the high level
sensor continues to be activated, the controller signals a foam alarm (that does not stop operation).
A crucial point in the operation of the humidifier is the control of any excess current levels.
In fact, whenever voltage is applied to the electrodes in the cylinder, after a period of inactivity, there may be short but very intense peaks in
current.
In the current is excessive in this initial period, the algorithm responds by immediately switching off the electrodes and performing a drain
cycle. If the excess current continues, the operation of the humidifier is stopped and a high current alarm is signalled.
The algorithm also controls the drain cycles, signalling a drain alarm if there is no appreciable decrease in current when the drain cycle starts.
Vice-versa, a no water alarm will be signalled if there is no appreciable increase in current when the humidifier is being filled with water.
ERP
SETPOINT
DIFFERENTIAL
20%
100
%
HUMIDITY % r.H
0% ERP
100% ERP
10% ERP
20% P_NOM
100% P_NOM
A
B
C
0% P_NOM
ON
I
V
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