Humidification system

An improved humidification system for jet-powered commercial aircraft comprising an evaporator adapted to add moisture to a gas flow, a gas flow control system for applying to the evaporator a gas flow having an internal energy sufficient to vaporize the moisture and a temperature measuring system for measuring the temperature of the gas flow exiting from the evaporator for regulating the amount of moisture added to the gas flow so as to maintain the evaporator exit temperature constant is disclosed. In a particular embodiment, the humidification system consists of a tank having an outlet duct adapted to deliver moisturized air to the passenger compartment of the aircraft, a water injection system adapted to flow water into the tank upon the temperature within the outlet duct rising above a preselected value and to terminate water flow upon the temperature falling below the preselected value, and a gas flow control system coupled to the tank and adapted to control the flow of air into the tank at a rate such that the air has sufficient internal energy to vaporize the water. An evaporator plate system is incorporated within the tank to ensure that all the injected water is vaporized before exiting the tank. Additionally, a demisting filter is mounted between the evaporator plate system and the outlet duct to ensure that all particulate matter is removed from the moisturized air before leaving the tank.

TECHNICAL FIELD 
This invention relates to humidification system and in particular to a 
system suitable for use on jet-powered aircraft which cruise at high 
altitudes. 
BACKGROUND OF PRIOR ART 
Controlling the relative humidity level, whether in an aircraft or in a 
building, is desirable in order to maintain a comfortable environment for 
the people within. If the relative humidity in an aircraft is too low, 
generally below 10%, passengers become uncomfortable due to the occurrence 
in a long duration flight of, for example, dry, itchy skin, nasal 
irritation, and gritty eyes. This condition of low relative humidity 
naturally occurs in modern, jet-engine powered commercial aircraft which 
typically use bleed air from the engines and/or the auxiliary power unit 
(APU) as the source of air for pressurization. The bleed air from the 
various compressor stages of the engines are interconnected by bleed air 
ducts and control valves and fed to as many as three separate air 
conditioning systems. The air exiting the air conditioning systems is 
mixed in a plenum chamber and distributed to the passage compartment by 
ducting mounted above the passenger compartment. The air circulates 
through the passenger compartment, down through the below-deck cargo 
compartment walls and then out flow control valves mounted in the bottom 
of the fuselage. The use of such a system at altitudes above 25,000 ft., 
however, reduces the relative humidity in the passenger compartment to a 
value of about 5-7%, causing the uncomfortable conditions mentioned above. 
In order to rectify this condition, moisture must be introduced into the 
above air conditioning system. While a 50% relative humidity level is 
considered to be ideal for passenger comfort, levels of 15-30%, still 
adequate to ensure passenger comfort even on flights as long as 9 to 11 
hours, are desirable to reduce the amount of water that must be carried on 
board the aircraft to raise the humidity, as such additional water adds 
weight and therefore increases fuel consumption and reduces payload. 
Additionally, maintaining the relative humidity at between 15-30% reduces 
the possibility of condensation of moisture on cold surfaces, thus 
reducing the possibility of corrosion of the aircraft structure, and 
condensation of moisture on the overhead structure of the passenger 
compartment, thus reducing the possibility of water droplets forming which 
can fall on the passengers and crew. 
The prior art humidification systems designed for use in aircraft, such as 
mechanical atomizers or centrifugal "slingers", inject droplets of water 
into the temperature-conditioned air. These systems do not, however, 
ensure that the water is completely vaporized. Therefore, water tends to 
collect on ducting surfaces within the aircraft with the results 
previously mentioned. If the injection nozzle apparatus of these devices 
is made small enough to more completely atomize the water, calcium and 
other mineral deposits rapidly build up and clog the nozzle. They thus 
must be cleaned often, causing higher maintenance costs. These mineral 
deposits are also carried into the flight station and electronic bays 
causing corrosion. While such contamination can be avoided by using 
distilled water, this increases operating costs and is not practical for 
commercial aircraft. 
Most modern humidification systems, furthermore, as shown, for example, in 
U.S. Pat. No. 3,642,201, "Humidifier Control", by P. E. Potchen, require 
humidity level sensors in order to control the humidity level. Airlines 
are reluctant, however, to install such complicated devices on board their 
aircraft and prefer open-loop systems. 
Accordingly, it is a general object of the present invention to provide a 
humidification system for humidifying a compartment of an aircraft. 
It is another object of the present invention to provide a humidification 
system for an aircraft that is capable of maintaining a substantially 
constant level of humidity using a source of air that varies in 
temperature and pressure, such as bleed air from the engines. 
It is a further object of the present invention to provide a humidification 
system for an aircraft that minimizes the usage of bleed air. 
It is another object of the present invention to provide a humidification 
system that completely vaporizes injected water and is tolerant of mineral 
deposits so that tap water can be used. 
It is still another object of the present invention to provide a 
humidification system that will control the humidity level within desired 
limits without the use of a humidity sensor. 
SUMMARY OF THE INVENTION 
A humidification system suitable for jet-power aircraft is provided. The 
humidification system comprises an evaporator adapted to add moisture to a 
gas flow, a gas flow control system for applying to the evaporator a gas 
flow having an internal energy sufficient to vaporize the moisture and a 
temperature measuring system for measuring the temperature of the gas flow 
exiting from the evaporator for regulating the amount of moisture added to 
the gas flow in order to maintain the temperature at a constant value. 
In a particular embodiment, the humidification system consists of a tank 
having an outlet duct adapted to deliver moisturized air to the air 
conditioning ducts leading to the passenger compartment, a water injection 
system, connected to a source of pressurized water, adapted to inject 
water into the tank when the temperature within the outlet duct of the 
tank rises above a specific preselected value and to terminate the flow of 
water upon the temperature falling below the preselected value and a gas 
flow control system coupled to the tank and adapted to regulate air flow 
into the tank at a rate such that the air has sufficient internal energy 
to vaporize the injected water. Additionally, a baffle-type evaporator 
plate system is incorporated into the tank to ensure that all the moisture 
is vaporized prior to exiting the evaporator. A demisting filter is also 
incorporated into the tank to ensure that any particulate matter is 
retained within the tank. 
The novel features which are believed to be characteristic of the 
invention, both as to its organization and its method of operation, 
together with further objects and advantages thereof, will be better 
understood from the following description in connection with the 
accompanying drawings in which a presently preferred embodiment of the 
invention is illustrated by way of example. It is to be expressly 
understood, however, that the drawings are for purposes of illustration 
and description only, and are not intended as a definition of the limits 
of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, and in particular to FIG. 1, the humidification 
system is shown designated by the numeral 1. The system consists of a 
generally cylindrical, pressurizable tank 2. Although the tank can be of 
one-piece construction, it is preferably manufactured in sections, 
designated by numerals 2A, 2B, 2C and 2D, which are joined together by 
conventional "V" band couplings 4 to provide for easier handling during 
overhaul and/or repair. At the top of the tank 2 is an outlet duct 6 which 
connects to the air conditioning ducts 8 which carry fresh, temperature 
and pressure-regulated air from the air conditioning systems (not shown) 
and distribute the air throughout the aircraft passenger compartment and 
flight station. The tank 2 is also provided with a port 10, sealed by a 
plug 11, located at the bottom of tank section 2D to provide for the 
drainage of any accumulated water in the interior 12 of the tank 2 prior 
to dismantling. 
A water injection system, designated by numeral 14, is coupled to the tank 
2. The system 14 consists of a pipe 16 which extends into the interior 12 
of the tank 2 and terminates in a water injection nozzle 18 located near 
the bottom of the tank 2. The pipe 16 is connected at its opposite end to 
a source of pressurized water, such as water reservoir 20. Typically, the 
drinking water reservoir of the aircraft, which is pneumatically 
pressurized, is used for the water reservoir 20. Alternatively, a separate 
pressurized tank, or an unpressurized tank with a suitable water pump, 
could be used as an adequate source. Of these three systems, it is 
preferable to utilize the drinking water reservoir since such a use 
requires a slight enlargement of the reservoir, reduces complexity of 
installation of the humidification system 1 and keeps the overall increase 
in weight of the aircraft to a minimum. 
To control the flow of water into the tank 2, a solenoid operated valve 22 
is installed in the pipe 16 between the reservoir 20 and the tank 2. The 
valve 22 is controlled, as explained below, to open and allow water to 
flow into the tank 2 when the temperature in the outlet duct 6 rises above 
a preselected value and to close when the temperature falls below the 
preselected value. For example, on a 250 passenger airbus type aircraft 
when the passenger compartment is maintained between 70.degree.-80.degree. 
F., a preselected temperature in the outlet duct 6 of around 155.degree. 
F. would be used. This preselected temperature determines the desired 
degree of humidity maintained in the passenger compartment since a higher 
temperature level in the duct 6 would cause less water to be injected into 
the tank 6 and thus lower the humidity level in the passenger compartment 
while a lower temperature level in the duct 6 would cause more water to be 
injected into the tank 6 and thus increase the humidity level in the 
passenger compartment. 
The temperature in the outlet duct 6 is sensed by thermistor 24, such as, 
for example, model No. 0-109-UUA-3503, manufactured by Omega Engineering 
Company, Stamford, Connecticut, which, when placed in an electrical 
circuit, is capable of varying the resistance of the circuit as a function 
of temperature. A resistance-sensing relay 26 is electrically coupled to 
the thermistor 24 and the valve 22 to actuate the valve 22. The relay 26 
may be a solid-state type, such as, for example, model EP-376 manufactured 
by the Leach Relay Company, Los Angeles, California. 
Thus, when the temperature within the outlet duct 6 rises above the 
preselected value, the changed resistance level of the thermistor 24 is 
sensed by the relay 26 which applies electrical power to the valve 22 to 
cause valve 22 to open and allow water to be injected into the tank 2 via 
nozzle 18. When the temperature falls below the preselected value, relay 
26 cuts off electrical power to the valve 22, causing the valve 22 to 
close and stop the injection of water into the tank 2. While the 
thermistor-relay combination has been found to provide the most accurate 
and repeatable results, a less accurate temperature switch can be mounted 
in the outlet duct 6 as a substitute for the thermistor 24 and relay 26 if 
less control of the humidity level within the passenger compartment can be 
tolerated. 
Since it is desirable that the water injection system 14 be self-draining 
in order to reduce scale buildup and to prevent water from leaking out 
upon disassembly, settling in the fuselage and possible initiating 
corrosion, the reservoir 20 is placed at a level below the tank 2 with the 
pipe 16 slanted at an upward angle. A bypass line 30 is connected at its 
end to pipe 16 on both the upstream and downstream sides of valve 22. A 
check valve 32 adapted to allow flow only in the direction toward the 
reservoir 20 is incorporated in line 30. Thus, when the reservoir 20 is 
depressurized, water trapped in pipe 16 downstream of the valve 22 will 
drain into line 30 through check valve 32, back into pipe 16 and into the 
reservoir 20. 
Still referring to FIG. 1, the gas flow control system of the present 
invention, designated by numeral 34, is illustrated. The gas flow control 
system 34 includes a duct 36 which is connected at end 36A to one or more 
of the jet engine compressor bleed air ducts 38 and at end 36B to an air 
injection nozzle 40 located within the interior 12 of the tank 2 in 
proximity to the water injection nozzle 18. 
While bleed air is the most convenient source of high temperature air on 
jet-engine powered commercial aircraft and is used for deicing the wings 
and engine inlets as well as a source for pressurizing and air 
conditioning for the flight station and passengers' compartment, the 
simple use of bleed air as the source of air for the humidification system 
presents problems because the bleed air is taken from the compressor 
stages of the engine and thus the pressure and temperature levels of the 
bleed air will vary with the power settings of the engine and the altitude 
of the aircraft. For example, bleed air temperatures on an aircraft 
typically can vary from a low value of 220.degree. F. to a high of 
450.degree. F. while bleed air pressures can vary from 10 psi to 40 psi. 
Furthermore, while it is desirable that the usage of bleed air be kept at a 
minimum since any bleed air taken from the engines increases fuel costs, 
it is important, as stated previously, that there always be a sufficient 
amount of air flowing into the tank 2 at a sufficient temperature to 
ensure that the air has sufficient internal energy or enthalpy to 
evaporate all the injected water to prevent condensation, corrosion and 
water droplets. However, it is also important to maintain the airflow rate 
at a minimum in order to minimize passenger compartment temperature 
changes. 
In order to satisfy the above requirements, the gas flow control system 34 
also includes a temperature-biased pressure regulator 42 incorporated into 
the duct 36 between the tank 2 and bleed air ducts 38 to regulate the 
downstream pressure as a function of upstream temperature so that the 
value of the weight flow times the temperature of the air exiting the 
regulator 42 and entering the tank 2 is substantially constant, i.e., the 
airflow has constant internal energy or enthalpy, and is sufficient to 
evaporate all the injected water. A suitable temperature-biased pressure 
regulator is model 2770120 manufactured by the Air and Fuel Division of 
Parker-Hannifin Corporation, Irvine, California, and is an 
electrical/pneumatic-type using the bleed air as the control fluid. A 
suitable alternate temperature-biased regulator is manufactured by 
Sundstrand Advanced Technology Group, Rockford, Illinois. While the 
particular temperature and pressure specifications of the regulator are 
generally fixed in accordance with the requirements of the humidification 
system, the regulator may be made adjustable to accommodate changes in the 
requirements of the humidification system. Sources of gas other than bleed 
air may, of course, be utilized in the above injected system. 
There may be aircraft designs which may not have sufficient bleed air 
capacity to supply the required airflow to tank 2. In such situations, an 
alternate gas flow control system 34' shown in FIG. 2 may be used. A 
compressor 43 is shown having its inlet side connected to a source of air, 
such as cabin air or air diverted from the air conditioners, and its 
outlet side connected to tank 2 via duct 44. An electrical heater 45 is 
installed in the duct 44 between the compressor 43 and the tank 2. Such a 
gas flow control system 34' would eliminate the need for the regulator 42 
because the heater 45 and compressor 43 can be sized or adjusted to supply 
an airflow with the required internal energy, as specified above. 
In order to ensure that all the injected water is evaporated by the 
ariflow, an evaporator system 50, illustrated in FIG. 1, is incorporated 
within the tank 2 between the nozzles 18 and 40 and the outlet duct 6 of 
the tank 2 to ensure that all the water passes through the evaporator 
system 50. The evaporator system 50 consists of a series of plates 52 
having substantially centrally located apertures 54 alternating with a 
series of solid plates 56 which cooperate with the tank wall sections 2B 
and 2D to form circumferential apertures 58. The plates 52 and 56 are 
coupled to the tank wall sections 2B and 2D by brackets 59A and 59B. The 
system 50 ensures evaporation of substantially all the injected water 
because any water not initially evaporated by mixing with the hot gas flow 
is deposited uniformly on the plates 52 and 56 by the gas flow and is 
evaporated off the plates 52 and 56 by the gas flow, which flow also heats 
the plates 52 and 56 to the temperature of the hot gas. The baffle 
configuration illustrated which creates a labyrinth of alternating inward 
and outward radial flow paths has been found to maximize evaporation of 
the water and provide uniform scale buildup on the plates. While stainless 
steel has been used for the evaporator plate material because of its 
excellent corrosion resistance, other corrosion-resistant materials would 
be satisfactory for use in the above configuration. An alternate baffle 
configuration is shown in FIG. 3 in which the plates 60 and 62 are 
alternately staggered to the left and right creating a lateral 
"crisscross" flow path from one side of the tank 2 to the other. 
As stated above, if tap water is used in the humidification system, any 
impurities in the water will tend to deposit as scale on the tank walls 
and the evaporator plates. Such particular matter as may break off the 
evaporator plates, or even particles in the water itself, can act as 
points of condensation for the evaporated water and should be prevented 
from reaching the outlet duct 6 and the passenger compartment. Therefore, 
a demisting filter 66, made for example of polyurethane foam, is shown in 
FIG. 1 mounted between the evaporator system 50 and the outlet duct 6 to 
trap such particles. 
Also shown in FIG. 1 are two safety systems to shut down the humidification 
system should there be an out-of-tolerance performance or a failure. The 
first safety system acts to shut off the flow of air to the tank 2 by 
closing the temperature-biased regulator 42. A normally closed temperature 
switch 70 is mounted in the outlet duct 6 and is coupled to the circuit 
(not shonw) which provides electrical power to regulator 42. The switch 70 
is set to open should the temperature in the outlet duct 6 reach a 
temperature level that indicates an out-of-tolerance performance and 
thereby cut off electrical power to the regulator 42 causing it to close. 
For example, if the valve 22 is set to open when the temperature within 
the outlet duct 6 reaches 155.degree. F., the switch 70 would be set to 
open at 200.degree. F. Upon the temperature dropping below 200.degree. F. 
due to the lack of airflow, the temperature switch 70 would close and 
electrical power would again be supplied to the regulator 42 allowing 
airflow to resume into tank 2. The switch 70 can also be utilized to shut 
off the compressor 43 in the gas flow control system 34' shown in FIG. 2. 
The second safety system consists of a normally open temperature switch 72 
mounted in the outlet duct 6 and electrically coupled to a normally open, 
motor-operated shutoff valve 74 mounted in the duct 6 upstream of the 
regulator 42. When the temperature in the outlet duct 6 reaches a higher 
level than that required to initiate regulator shutdown by the switch 70, 
such as, for example, 250.degree. F. which is indicative of a serious 
failure, the switch 72 closes and electric power is applied to the valve 
74 causing it to close off airflow upstream of the regulator 42. The 
switch 72 can also be utilized to shut off the valve 74' in the gas flow 
control system 34' shown in FIG. 2. 
In order to determine water and airflow rate requirements and outlet duct 
temperature settings, one need only know the required flow rate of 
conditioned air into and out of the passenger compartment, which depends 
upon the size of the aircraft and the number of passengers that will be 
carried, the desired temperature to be maintained within the passenger 
compartment and the existing and desired humidity levels. For example, on 
a typical 250 passenger wide-bodied aircraft, the required air flow rate 
necessary to pressurize the passenger compartment and also supply 
sufficient fresh air is around 220 lbs./min. The temperature within the 
passenger compartment is normally maintained at a comfortable 
70.degree.-80.degree. F. As previously discussed, the desired humidity 
level is 15-30%, while the humidity normally present at a high altitude is 
5-7%. From these parameters, a water flow rate of 0.6 lb./min. and a 
nominal airflow of 19 lb./min. having an enthalpy of 92 BTU/lb. can be 
routinely derived to completely vaporize the injected water and to have 
the water flow initiated only when the temperature in the outlet duct 
reaches approximately 155.degree. F., a temperature which ensures that 
there will be no condensation within the ducting and the passenger 
compartment and that only a nominal 1.degree.-2.degree. F. change in 
passenger compartment temperature will be experienced. 
In FIG. 4, a graph showing the typical humidity level maintained in the 
passenger compartment of a typical 250 passenger wide-body aircraft at 
33,000 ft. is illustrated. Since the passengers themselves add humidity, 
the humidity level varies as the load factor--the ratio of the number of 
passengers actually carried to the maximum number of seats available. 
Because of clogging of the demisting filter and scale buildup within the 
tank, in particular on the evaporator plates, the humidity level drops 
with time. As is illustrated in the graph, over 200 hours of satisfactory 
operation can be obtained before the humidity level within the passenger 
compartment exceeds tolerances and the humidification system requires 
cleaning. 
While the humidification system has been described with reference to 
particular embodiments, it should be understood that such embodiments are 
merely illustrative as there are numerous variations and modifications 
which may be made by those skilled in the art. Thus, the invention is to 
be construed as being limited only by the spirit and scope of the appended 
claims. 
INDUSTRIAL APPLICATION 
The humidification system is useful on jet-powered commercial aircraft to 
provide increased passenger comfort.