- Importance of Air Flow
In test chambers all we are concerned about is airflow speed. How fast should it be? Most test specifications don't say much on this subject, which makes it even harder to decide. In some cases we require low speed in some cases higher. Test chambers need to circulate the air so that it can pass over heaters and coolers, ensuring a uniform air temperature.
- Low speed: If test application doesn't specify temperature change rate, or if it is relatively slow (less than 5 degrees per minute), then lower air speed is required
- High speed: If primary concern is changing the temperature of samples quickly, then you should be more interested in airflow. The faster the airflow, the quicker heat can be transferred from the air to the samples.
Temperature change rate is reduced due to increased heat transfer. In addition, as it moves the air, it puts extra energy (i.e. heat) into the air, making cooling more difficult. WEIBER's line of test chambershave high airflow and reasonable change rates.
Methods of humidity generation in WEIBER test chambers
WEIBER employs one of the three humidification techniques:
- Water tank for steam generation
Water tank is equipped with an immersion heater. As the water in the tank is heated, steam rises up a tube to the chamber’s mixing plenum (where the main heaters and cooling coil are)and humidity is increased. Its benefits are:
- High volume of moisture can be generated (good for large chambers)
- Quality of water vapor is consistent
Atomizers use a fine-spray nozzle to “atomize” the water. This is done in the mixing plenum(where the main heaters and cooling coil are). As the atomized water passes the chamber heater, it becomes heated to a gas. Benefits:
- Can provide high volume of moisture
- No problems with heater burn-out
- Cools the air (beneficial when needed)
- Water Bath System
A small “bath” is located in the mixing plenum(where the main heaters and cooling coil are). As the chamber air is drawn into the plenum, it passes the bath, making water vapor. Benefits:
How Humidity Control is done in WEIBER Test Chambers
- Quick response due to small water amount
- Doesn’t have to be at boiling to provide moisture
- Less boiling means less mineral deposits to clean
- Can be inspected and cleaned quickly
Relative humidity is actually relative to the temperature. The higher the temperature, the more moisture the air can hold at a given temperature. Relative humidity is the ratio of the amount of moisture in the air vs. how much it can hold. And therefore lies the best reason to have temperature control: If the temperature changes, the relative humidity changes. So a stable temperature helps ensure a stable humidity level.WEIBER test chambers employ one of three dehumidification techniques to achieve lower humidity conditions.
This is an ideal standard method of controlling humidity. A cold cooling-coil in the chamber attracts moisture in the air, causing it to condense into water, which can then be drained away. So the refrigeration system helps control the temperature AND the humidity.De-humidification via refrigeration can get relative humidity down to 10% at higher temperatures.
Nitrogen gas is does not hold moisture, so it is naturally dry. By slowly feeding nitrogen gas into the chamber, moisture-laden air is purged, out of the chamber. We do not use nitrogen purge in chambers where people may go inside.
A dry-air purge system is more common than nitrogen. External compressed air is fed into a desiccant dryer system. A desiccant is a substance that absorbs water. It is most commonly used to remove humidity that would normally degrade or even destroy products sensitive to moisture, like the silica gel crystal packets you find packaged with some products.
The dry-air purge system doesn't need much maintenance. The system uses two desiccant “towers”. One dries the compressed air while the other tower is vented to the room to allow it to regenerate. You will often hear these systems “pop” a little compressed air while switching between towers. The desiccant can be used for years before replacement desiccant is needed.
Larger chambers are impractical to dry with nitrogen or compressed air. So, the chamber air is circulated to a desiccant for drying. The desiccant is in the form of a honeycomb wheel, where part of the wheel is being vented to the room to release moisture. These systems typically heat the air to make it easier to capture moisture, and will also have a cooler so the air will return at a controlled temperature that doesn't impact the chamber operation.
Measurement of Humidity in Test Chambers
The 2 different systems are provided for measurement of humidity in WEIBER test chambers- analog and digital. One of the systems for measurement is employed as per the requirement.
- Analog Method: Wet Bulb/Dry Bulb
A dry bulb is simply a dry thermocouple in the air. A wet bulb is a thermocouple with a fabric “sock” or “wick” that is kept wet by hanging the end in a small tank of water. When the wet and dry bulbs are placed in the air-stream of the chamber, the wet bulb is colder because the water on it is constantly evaporating. The chamber controller compares the temperatures of the two thermocouples and calculates the relative humidity. It is a fundamental measurement method because it is a direct application of thermodynamic principles. Its benefits are:
- Reliable, simple technology
- Easy to maintain
- No calibration required
- Easy to determine if malfunctioning (starts reading 100%)
- Only two typical failure modes: wick needs changing or water supply is gone
- Wicks are cheap to replace.
- Digital Method: Solid State Sensor
As the name says we use solid state sensor for digital measurement of relative humidity (RH). The sensor is made up of dielectric material. The mechanism says that dielectric permittivity of the insulating polymer, is linearly related to the ambient relative humidity. Therefore, the capacitance of a parallel-plate device using as the dielectric is a linear function of ambient RH.
The benefits of using solid state sensor are:
How Temperature is Raised in Test Chambers
- No wick to change
- Can read humidity below freezing and above boiling (although normal chambers can't control at these conditions)
Indirect heating system is provided in our units, comprising of air heaters made of high grade nichrome wire of suitable wattage. The warm air is evenly distributed throughout the chamber through efficient motor fansso the samples in the chamber are not exposed to radiant heat, ensuring very good temperature sensitivity.It is very easy to add heaters and increase the speed and/or high limit of a chamber.
Lowering the Temperature of the Test Chamber
An energy efficient cooling unit is installed in WEIBER test chambers to enable studies at lower room temperatures. We use ISI marked high end CFC free compressors of Kirloskar /Tecumseh / Bitzer/ Danfoss make, conforming to latest international standards and guidelines.
This temperature is achieved with a standard refrigeration using R-134A refrigerant. This is the same refrigerant that is currently used in new air conditioning systems and refrigerators. The type of refrigeration system is called "single stage", which basically means that there is one compressor
Chambers that can get this low temperature use "cascade" refrigeration i.e. using two compressors.. By using a "high stage" refrigeration circuit to cool the "low stage" circuit, these cold temperatures can be achieved. The low stage refrigerants (R-23 or R-508) would create too much pressure (which would blow-out the piping) if they were used alone, which is why the high stage is added.
Chambers that can get this low use liquid nitrogen for cooling. Nitrogen is a cheap method for cooling a chamber, however long term use of liquid nitrogen can get expensive.