Your company has just developed a new product or builded a test lab, now you are responsible for selecting some environment test chamber for the lab. The product must be tested at various conditions to ensure its quality and reliability. Where do you start? What will determine your selection criteria? Price, quality, capabilities….
Quality, reliability, and service after the sale all add up to the value of your purchase. Value often is mistaken for low price. Over the life of the unit, that can become a costly mistake.
Many different types of test chambers are available. Your application is primarily going to determine the type of chamber you need. The better you relay that information to your supplier, the better your chamber will match your needs. How big must the chamber be? What is being tested? Is it going to be air cooled or water cooled? What temperature range is needed? These are just a few of the necessary questions that must be answered before purchasing a test chamber.
Chamber Types and Sizes
What kind of chamber you need: Temperature, humidity, altitude, Thermal Shock, rain, solar and vibration are just a few.
Reference: Types of environmental testing chambers of GPower Vietnam
Which size you need: standard chamber, walk-in chamber or drive-in chamber.
Temperature Range
The next major distinction is the temperature range in which the chamber will operate. Most manufacturers have a standard high temperature range of 177°C to 190°C.
To cool the chamber, several options are available. The two major categories are expendable refrigerant and mechanically cooled. Expendable refrigerants are liquids/gases that can be injected directly into the space being cooled. As the liquid enters the chamber, directly or through a fin coil, it absorbs heat and flashes to a gas. The gas then is vented out of the chamber. The two most popular refrigerants are liquid nitrogen (LN2) and liquid carbon dioxide (CO2). Cryogenic temperatures down to -184°C can be achieved with LN2. On the other hand, CO2 can only achieve temperature down to -68°C. Both of these gases are environmentally safe and can be vented to the atmosphere. Mechanically cooled refrigeration systems are similar to the type used in home refrigerators. They use a compressor and circulate a refrigerant around a closed-loop system. The ultimate low temperature required by your testing determines the type of refrigeration system needed. Single-stage refrigeration systems typically can pull the temperature in the chamber down to - 34°C. Some manufacturers rate their singlestage systems down to -40°C. However, due to the refrigerant used, there is very little cooling capacity available at -40°C. For continuous operation at -40°C and below, most manufacturers recommend a cascade refrigeration system. Cascade refrigeration systems have two separate systems working to cool the chamber down to a low of -85°C. The firststage refrigeration system cools and condenses the refrigerant in the second stage. The secondstage refrigerant flows through an evaporator located in the chamber that cools the air. These systems can become very complex depending on the application.
Humidity range
The standard temperature/humidity range for most chambers is 7°C to 85°C with 10% to 98% RH, limited by a 5°C dew point. The limitation of a 5°C dew point can be very confusing.
Since the amount of moisture varies at every temperature, the chamber manufacturers use dew point to describe the RH limitation. Inside the chamber, there is a refrigerated coil controlled at 5°C or slightly below. Moisture in the chamber will be attracted to the cold surface and condense. The accumulated water is drained out of the chamber, lowering the relative humidity. The refrigerated coil is never below freezing so frost will not develop. The best way to understand this is to refer to Figure below If you follow the bottom line of the standard range section of the graph, those temperatures and humidities represent the 5°C dew point.
Cooling rate/heating rate
By incorporating faster change rates, total test time can be reduced. Products also can be thermally stressed at faster change rates to identify reliability problems. However, be careful assuming the part temperature is changing at the same rate as the air.
Every chamber manufacturer has different airflow volumes inside their chambers. The airflow must have enough volume to support the refrigeration system. The typical air velocity in most reach-in chambers is approximately 100 ft/min through the work space. This velocity works well for steady-state and temperaturecycling testing. However, the part temperature will lag behind the air temperature with this airflow. Air velocity across the part should be much higher to keep it closer to the chamber air temperature during transitions. Typically, 500 ft/min or more is required to move the part temperature at a similar rate to the air temperature. It is a necessity in thermal shock applications to have airflow this high. For most temperature and humidity applications, the airflow in reachin chambers is adequate for the test. As the temperature is raised or lowered in the chamber, the air expands and contracts. Since the chamber is a sealed compartment with the exception of a small drain, the expansion and contraction of the air cause positive and negative pressure to be generated. When the air temperature in the chamber is changed quickly (10°C, 20°C, 30°C/min), more than 0.25 psi differential can be created. This does not sound like a lot, but over a large surface like a chamber wall, the pounds of force can become large. For example, a 32 ft2 chamber has a side wall that is 38s × 50s (1,900 in.2) 1,900 in.2 × 0.25 lbf/in.2 = 475 lb of force exerted on the wall, this force actually will be exerted on all the walls. To offset this reaction, most manufacturers install some type of pressure-relief vent. The vents normally are closed to limit the infiltration of outside air during operation. When a fast transition is started, the vent either will draw in outside air for a pull-down or exhaust chamber air during a heat-up. By allowing the chamber to breathe, the force on the walls is greatly reduced. It still is not uncommon to see the walls deflect in or out during a fast transition. Since this movement stresses the construction of the chamber, it is imperative that the chamber be well built.
Construction
Chamber construction is a critical area that needs to be evaluated when making a purchase. Most chambers have painted exteriors and stainless steel liners. It is easy to believe they are all built the same way. However, when you evaluate the details, you will see differences that can greatly affect the long-term reliability of the chamber. Most of these differences relate to the way seams are connected for the stainless steel liner. Are the seams welded, pop-riveted, or screwed together? How are the ports fastened to the stainless steel liner and the outer cabinet? When leaks occur in the stainless steel liner, a path is opened for moisture to travel in and out of the chamber, especially when the chamber is used for temperature and humidity testing. The humid air in the chamber finds the leak and condenses in the insulated area. Most chamber manufacturers use fiberglass insulation similar to what is installed in the walls of your house. When moisture condenses on the insulation, it becomes saturated like a sponge and loses its insulating capability. The walls of the chamber then have less insulation, and that can affect the temperature and humidity performance. The water in the insulated space eventually will rust the outer sheet metal and allow water to leak onto the facility floor. A chamber with continuously welded seams is much less likely to develop leaks than a chamber assembled with other methods. Another point of consideration should be the access ports. As the pressure in the chamber goes up and down during temperature transitions, the walls will deflect. The ports connect the inside chamber to the external cabinet. As the inside walls move from the expansion and contraction of air, the port transfers that movement to the outer cabinet. For that reason, the connection between the port and the chamber must be extremely durable to withstand the frequent movement. Chamber manufacturers use several methods to install ports, ranging from pop-riveted and caulked to fully welded. Again, a welded port will hold up better than other fastening methods.
Summary,
It is critical that you provide all the information for your test requirements to your testchamber supplier. This will ensure the best chamber for your application. Other items to keep in mind are the construction methods used by your supplier, the location of the chamber, and supplier service after the sale.
