The proper functioning of a thermostat depends primarily on the correct choice of the component, but also the conditions of its installation. Conditions used to calibrate regulating and control equipment in the factory are always ideal laboratory conditions, ensuring measurement accuracy and repeatability. These conditions are rarely those found in practice when installing thermostats. However, with a minimum of constraints, it is possible to optimize assemblies.
One’s will always bear in mind these two essential rules:
- A thermostat measures the temperature where the sensing element is located, and it is therefore necessary that this place is representative of the temperature that must be controlled
- The thermal inertia is the most common causes of poor regulation. A thermostat does not have an instant response to a temperature change.
1. GENERAL RULES
1.1 THERMAL CONDUCTIVITY
The temperature of a medium (liquid, air, metal) decreases progressively as the distance from the heat source. This decrease, called thermal gradient is inversely proportional to the thermal conductivity of the medium. For good temperature control, first step is to make this decrease as low as possible: by stirring the liquid, stirring the air, using metals that are good conductors of heat.
In unstirred liquid baths, thermal variations that rise several tens of degrees between different measurement points are quite common. It is the same in the air.
1.2 RESPONSE TIME
Practically speaking, the time taken by a device to change temperature is proportional to its mass and inversely proportional to its thermal conductivity.
Subject to the same variation of temperature, a large block of copper takes longer to heat up than a little. A block of pure silver of the same weight will react much more quickly.
In one room, sun exposure will raise rapidly the temperature of the ambient air because its mass is low, but the walls will react much more slowly because they are much more massive, even if their thermal conductivity is higher. Therefore, to control the air conditioner, make sure that the thermostat measure the temperature of the air and not of the walls.
Thermal conductivity of some materials
|Materials||Thermal conductivity at 20°C (W•m-1•K-1)||Materials||Thermal conductivity at 20°C (W•m-1•K-1)|
|Ait (atmospheric pressure)||0,026||304 Staineless steel||26|
|Fiber glass wool||0,043||Platinium||72|
|Wood (Average)||0,16||Cast iron||100|
|Epoxy||0,25||Aluminium alloy (with SiC)||150-200|
|Nylon||0,25||Pure aluminium (99,9 %)||237|
|PPS (Ryton)||0,3||Massive silicium carbide||250|
|Vulcanized rubber (EPDM)||0,4||Gold||317|
It is easily possible to see that if a thermal gradient takes 1 second to be transmitted in a silver part, it will take 1.1 seconds in copper, 2.5 seconds in aluminum alloy, 4.3 seconds in iron, 6.3 seconds in mild steel, 16.5 seconds in stainless steel, 680 seconds (more than 11 minutes) in non-stirred water and 16500 seconds (more than 4 hours) in still air.
1.3 TIME NEEDED TO HEAT
An issue frequently raised, and that many consider as associated with thermostats is the time it takes to heat a product. In fact, at constant power, the amount of heat (energy) required to heat a product depends on its mass and its heat capacity, and not on the thermostat.
Specific Heat capacity (or specific thermal capacity) is the energy it takes to bring a body to raise its temperature by one degree kelvin for a mass of one kilogram. It is expressed in joules per kelvin per kilogram (J / K). It originates from the “calorie” that was defined as the amount of heat required to raise 15 ° C to 16 ° C the temperature of one gram of water.
The table below gives some common values
|Materials||Specific heat capacity (J*kg-1*K-1)||Materials||Specific heat capacity (J*kg-1*K-1)|
|304 Stainless steel||510||Alcohol||2450|
One can easily notice that the same power, whether 600 seconds are needed to heat one kilogram of water, it will take only 290S for oil, 145s for air, 73s for stainless steel, 55s for copper, and 18s for gold. The heat capacity is an extremely important parameter in the definition of a thermal system.
1.4 OVERHEAT AND HEAT ACCUMULATION
Many heating systems accumulate heat before transmitting it to the environment.
This is especially the case with sheathed heating elements, where heating wires are coated with magnesia, and then covered with stainless steel tube. Before the stainless steel sheath begins to warm, the entire interior of the heating elements has heated up.
When the power is then turned off, the heat accumulated inside will continue to dissipate, and the temperature of the outer shell will continue to rise. A temperature control which regulate by measuring the temperature of the outer shell will be false.
2. WALL AND PIPE MOUTING THERMOSTATS
The following requirements must be respected:
- In the case of thermostats with a flat sensitive part, the mounting wall must be In particular, if it is needed to measure the temperature of a small diameter tube, it is mandatory to weld or solder a heat conductive part made of copper or brass on the tube surface, with a flat surface on the side facing the thermostat.
- In the case of thermostats whose sensitive part is curved to match the shape of the wall (tanks, pipes): use thermal contact grease between the thermostat sensing face and the wall, insulate the thermostat body to limit the influence of the ambient temperature, have in mind that the whole thermostat must withstand the maximum or minimum temperature of the wall. Check if these temperatures are compatible.
3. AIR DUCTS THERMOSTATS
Attention to the use of bimetallic rod thermostats in air ducts: These devices generally have very fast response time to tempera- ture changes, and some models are not suitable for use as safety device because they trigger too fast.
4. ROD THERMOSTATS
The whole sensitive part of the rod must be immersed in the air or liquid that it must control.
Do not mount the thermostat on a stack of fittings and rod must be in an area representative of the temperature of the tank. Avoid areas without natural convection or no stirring.
Whatever the installation, the thermostat head must not exceed the maximum allowable temperature. In particular, when mounting thermostats on high temperature equipment, the head must be kept away from hot walls.
Use pockets adapted to the rod diameter, and do not hinder expansion movements. If you want to get accurate settings and low differential, put thermal grease between the pocket and the rod.
5. BULB AND CAPILLARY THERMOSTATS
6. WINDING THERMOSTATS
7. THERMAL CUT OUT
Their terminals are heat conductors: welding or soldering them can cause the fuse to open by thermal conductivity.
Do not make soldering at less than 15mm of the housing. The soldering duration shall not exceed 3 seconds. Terminals wires are also sensitive to strength and torsion. Be careful not to apply significant forces (1.3 N max).
Bending terminal wires should be preferably made with a wire bending machine. Do not bend or crimp at less than 5mm from the body. Do not crush the body.
Sensitivity to temperature: thermal fuses must not remain continuously exposed to temperatures that are too close to their cutoff temperature. Respect the maximum allowed permanent temperature given in technical data sheets. They are also sensitive to current and can trigger by Joule effect if the rating is too high.
8. VAPOR PRESSURE BULB AND CAPILLARY THERMOSTATS
9. EXPLOSION PROOF THERMOSTATS
The explosion-proof devices require special care during assembly. A specific mounting and assembly instruction manual is supplied with each unit.
- Explosion-proof enclosures: These housings are designed to withstand an explosion occurring within the It is therefore important to take particular care that the screws of the cover (these screws cannot be replaced by others models with different mechanical resistance), to ensure the cleanliness of the sealing surfaces, not drill holes in the boxes, not to replace original cable glands by others, properly tighten the cable glands, ensuring that their gasket is adapted to the diameter of the cable used.
- Explosion proof Switches: In thermostats using this system, only the electrical part of the switch mechanism is enclosed in a flameproof enclosure. By this way, the outer casing of the thermostat does not provide explosion protection, but only requires at least IP65 ingress Electrical connections must be made on the cable coming out of the unit, outside the hazardous area or in a suitable junction box.