Types of fluids
PIROBLOC as specialist in the design and manufacture of thermal fluid heaters and other apparatuses that use that type of heat transmitting fluids, is today interesting to approach them to you and giving you more data on such.
- The synthetic fluids fulfill the majority specific functions. Some of them have problems due to the toxicity, to the high point of freezing or the high viscosity at low temperature.
- The mineral oils fulfill with the next described requirements, a rank of application of temperatures from -10º C to 300º C and an index of viscosity in the rank 0-100 ST.
The basic requirements of any fluid are the following ones:
- They must be thermally stable to the temperature of service.
- It has to have high thermal exchange coefficients.
- The pressure of vapour at the operating temperature must be low in order that the system can work at atmospheric temperature.
- It must have a low viscosity to favour a turbulent work conditions, and at the same time, the viscosity at low temperature must be sufficiently low to favouring the cold starting.
- It does not have to be corrosive nor toxic.
- It must be safe and easy to use.
Types of viscosity
In reference to the viscosity, it is considered from two points of view: separately and its variation in reference to the temperature.
The oils have advantages in comparison with other fluid, thanks to their better relation viscosity/temperature, that it is expressed in terms of the Index of Viscosity. It is calculated by an empirical formula from viscosities at 40º and at 100º C. A high value means a small change of viscosity in relation to the temperature.
The point of freezing of a fluid measures the temperature to which the oil stops flowing by simple gravity, important factor in relation at the moment of the starting and its properties to low temperatures. In order to reduce this point, most of fluids incorporate additives.
The density is the mass by volume unit. The density is marked in kg/l or g/ml in relation to 15º C. Most of oils have values around 0.85 – 0,9.
The specific heat measures the amount of heat absorbed by unit of weight when its temperature increases one degree Celsius. The oils have values between 0.45 and 0.70 in the rank of temperature of habitual work.
The flash point is the temperature to which, with certain test conditions, a flame applied to the surface of the oil causes the ignition of the vapour but not a continued combustion. It is very important not to confuse this temperature with the temperature of combustion or self-ignition.
Once approximated a flame or spark, the combustion point is the temperature to which, in the presence of comburent air (oxygen), the flame maintains at least 5 seconds ignited. The normal values are located around 210º C.
The self-ignition point is the minimum temperature to which the thermal fluid ignites by itself, without presence of flame or spark that initiates the combustion. Logically, the presence of comburent air is necessary.
Consequently, the temperature to be considered in reference to a possible escape of thermal fluid, it is the temperature of self-ignition.
It’s necessary to have a escape, because without it, there is no oxygen in the circuit and therefore, although we reach temperatures of point of self-ignition in the close circuit, it would not be possible a combustion.
By all it, the probability of a fire in an installation of thermal fluid PIROBLOC, is practically null. The kindness of this affirmation, it comes authenticated by the effective legislation in Spain, which considers the thermal fluids like NO INFLAMMABLE LIQUIDS (B.O.E. nº 120, 20/5/82).
In reference to our thermal fluid circuits, being closed and not existing oxygen presence in them, a combustion or fire is impossible, unless there is a escape.
Thermal fluids of the market
In the following picture we can observe the characteristic temperatures of the main thermal fluids of the market. We can observe that the maximum temperature of service (indicated by the manufacturer) and the temperatures of flash point and self-ignition do not keep a direct relation:
- Density at 15 ºC: 0,880 kg/l
- Viscosity at 40 ºC: 20 cSt
- Viscosity at 100 ºC: 4,5 cSt
- Viscosity index: 80/90
- Flash Point P.M.: 180 ºC
- Freezing Point: -45 ºC
- Specific Heat 50 ºC: 0,48
In our next blog, we will continue speaking about this subject with special attention to the white fluids (the unknowns by the users).