Heating Media - Water and Other Fluids in Heating Systems
6 października 2022 | Heating
What flows in our pipes, radiators - generally in heating systems? Most often it's just plain water. However, there are other fluids that can be found in heating systems. These are primarily glycols, and in the past, brines.
Water
The most commonly encountered fluid in installations. This is because it's the cheapest, most common liquid. Additionally, water has a high specific heat capacity of 4.2 kJ/kg*K - this is a value higher than that of glycols. This physical property means that to transport a specific thermal power, less water flow is needed than glycol. Additionally, water has lower viscosity than glycols, which directly affects the value of linear and local resistances in pipes and fittings. Both these properties translate into the operating costs of circulation pumps. Additionally, water is neutral to the environment and human health. Any leaks are therefore not a problem in this regard. When it comes to the pipes and fittings themselves, problems may arise after some time in the case of hard water. Fortunately, water softeners for boiler water and corrosion inhibitors are common to protect heating installations and equipment.
The only downside to using water as a heating medium appears in installations located outside buildings or in unheated rooms, where the temperature can drop below zero degrees Celsius and there are also system shutdowns. The water in the pipes will then simply freeze. This has a destructive effect on the fittings. To prevent this, there are two solutions. Use a non-freezing fluid to the required negative temperature, e.g., to -25°C, or wrap the pipes with electric heating cables and insulate them. Unfortunately, using the second method is not 100% certain, as power supply interruptions or failures may occur.
Propylene Glycol
Glycol filling stations for
installations
An organic chemical compound from the group of dihydroxy alcohols, or diols. At room temperature, it is a colorless, odorless, oily liquid with a sweet taste and high viscosity. It is considered a compound harmless to health, or of very low harmfulness. It has not been found to cause allergies, does not show carcinogenicity, and is not mutagenic. Contact with undiluted propylene glycol may cause eye and skin irritation, but these are not serious and usually subside with the cessation of contact. In the body, it is quickly converted to lactic acid (in a manner similar to the conversion of sugar to energy in muscles). It easily biodegrades in the environment. It is a safe substance even at a 50% concentration in the finished preparation. In installations, glycol solutions of appropriate concentration are used, depending on the temperature to which we need our solution not to freeze.
Glycol concentration [%] | 80 | 50 | 42 | 37 | 33 |
|---|---|---|---|---|---|
Crystallization temp. [°C] | * | -35 | -25 | -20 | -15 |
Kin. viscosity [mm2/s] at 50°C | 6.02 | 2.14 | 1.61 | 1.31 | 1.18 |
Specific heat [kJ/kgK] | 2.93 | 3.58 | 3.70 | 3.77 | 3.84 |
Table 1. Properties of propylene glycol
Ethylene Glycol
An organic chemical compound that is the simplest polyhydroxy alcohol (diol), and also the simplest sugar alcohol. It is widely used as a component of automotive coolants, as well as a precursor for polymers. In pure form, it is a colorless, odorless, syrupy liquid with a sweet taste. Ethylene glycol is toxic, and its ingestion can cause death. It is a harmful substance that has a depressive effect on the central nervous system. It irritates the mucous membranes of the nose and conjunctiva. The body absorbs it through the respiratory tract, skin, and digestive tract. Through the digestive route, glycol initially causes symptoms similar to alcohol intoxication, then (after several or a dozen hours) leads to metabolic acidosis. When heating glycol, the resulting vapor can lead to loss of consciousness, while in small concentrations it causes irritation of the nose and throat and headaches. Therefore, special caution should be exercised when working with glycol.
Glycol concentration [%] | 93 | 48 | 40 | 35 | 28 |
Crystallization temp. [°C] | * | -35 | -25 | -20 | -15 |
Kin. viscosity [mm2/s] at 50°C | 6.53 | 1.59 | 1.43 | 1.08 | 0.94 |
Specific heat [kJ/kgK] | 2.50 | 3.38 | 3.54 | 3.63 | 3.77 |
Table 2. Properties of ethylene glycol
Disadvantages of Glycols
Glycols have higher viscosity than water, thus causing greater pumping resistance in the installation. Additionally, they have lower specific heat, which means that to deliver a given heating power, more glycol flow is needed than water. These disadvantages affect greater pumping resistance in the installation. These alcohols are also more expensive than water and require a different approach when making the installation filling system. Due to its toxicity, ethylene glycol is rather used as a coolant for car radiators than in sanitary installations.
Summary
We fill installations that have no contact with negative temperatures with water. This is the most optimal and simple solution. In the case of installations exposed to "frosts," it's best to use propylene glycol.
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