Essential Components of Closed-Circuit Water Heating Systems

Posted on Dec 7, 2024 in Technology

Gauge

A gauge is a measuring device used for determining the pressure of fluids contained in closed containers. There are two primary types: those for fluids and those for gases. Many pieces of equipment measure atmospheric pressure using it as a baseline and measure the difference between real or absolute pressure and atmospheric pressure. This value is called gauge pressure. Such devices are called pressure gauges and operate on the same principles that underlie mercury barometers and aneroid barometers. The gauge pressure is expressed either above or below atmospheric pressure. The pressure gauges used to measure pressures below atmospheric pressure are called pressure gauges, vacuum gauges, or vacuómetros.

The simplest gauge consists of a bent glass tube containing a suitable liquid (mercury, water, oil, etc.). One branch of the tube is open to the atmosphere; the other is connected to the tank containing the fluid whose pressure is to be measured. The fluid enters the container part of the tube, making contact with the liquid column. Fluids reach an equilibrium configuration from which it is easy to deduce the gauge pressure in the tank.

In the industry, metal or aneroid manometers are almost exclusively used. These are modified aneroid barometers where the unknown pressure to be measured acts within the box, and the outside air pressure acts outside. The most common is the Bourdon gauge, consisting of a rolled, sealed metal tube, closed at one end and wound in a spiral. The open end communicates with the reservoir containing the fluid whose pressure is to be measured. Then, as the pressure inside the tube increases, it tends to unwind and sets in motion a pointer against a scale calibrated in units of pressure.

Circulation Pump

Electrocirculadores or pumps facilitate the transport of heat transfer fluid from the collectors to storage and later consumption. Driven by an electric motor, they provide the necessary energy to the fluid to transport it through the circuit at a given pressure. There are three types of centrifugal electrocirculadores:

• Submerged Rotor: These are silent, require low maintenance, and are mounted in line with the pipe and the horizontal axis.
• Monobloc: With the shaft in any position.
• Motor-coupling electrocirculador of different axes: These are noisy.

The behavior of an electrocirculador is represented by P = C * p, where P is the power required, C is the flow (l/sec.) between two points on a pipe with a pressure difference of ΔP. This means that the pump power is a function of pressure drop and flow. The manufacturer represents these two axes in its characteristic curve, each pump having its own characteristic curve.

Over time, pipes accumulate corrosion, so the pressure drop increases. Additionally, calculations are often done as if the system only had water, while antifreeze is often added. For this reason, the chosen pump should be slightly oversized.

Pumps often have multiple speeds, and the manufacturer states this in its graphics. It is advisable to work at a medium speed so you can raise or lower the speed if the pump is undersized or oversized, respectively.

By associating two pumps in series, the gauge height increases significantly, and the flow increases a little, whereas if they are associated in parallel, the flow increases significantly, and the pressure increases a little.

The pump has to counteract the pressure drop only in the worst circuit. However, if the circuit is balanced, one will be selected at random.

Elements Associated with the Electrocirculador

The circuit is preceded by a filter to keep out impurities in the weld and the rest of the installation at the pump. It also carries a check valve to prevent fluid from decreasing heat transfer from the collector to the pump. Keys numbered 3 and 4 are used in case of pump failure to allow for replacement.

Closing key 1 and leaving key 2 open, we obtain the discharge pressure gauge. Closing key 2 and opening key 1, we get the suction pressure gauge. If we subtract the results, we obtain the pressure loss of the facility, which must match the installation.

At the rear, the electrocirculador must have a small pressure to be able to start. Legislation states that a minimum should be 0.2 bar or 0.5 bar for high temperatures.

If the pump had a smaller drop than necessary, it will create a depression in the pipes, and the water flowing will easily become a gas. As the pump is designed to move only liquid fluid, this will cause an acceleration of the pump and an implosion in the fluid, ultimately damaging the electrocirculador. The sum of both reactions is called gravitation.

Check Valves

Check valves, also called non-return valves, one-way valves, or “check” valves, are intended to completely close the passage of circulating fluid—either gas or liquid—in one direction and release it in the opposite direction. They have the advantage of a minimum travel of the disc or shutter to the fully open position.

They are used when pressure is to be maintained in a pipeline in service and to prevent backflow. The fluid flow is directed from the inlet to the outlet, where usage is free, while in the opposite direction, it is blocked. They are also often called one-way valves.

Check valves are widely used in pipes connected to pumping systems to prevent water hammer, mainly in the discharge line of the pump.

Expansion Tank

It absorbs the expansion of water in hot water installations. When the pressure increases in the installation due to the expansion of the heat transfer fluid (temperature increase), the remaining fluid enters the tank and pushes the membrane. The gas is compressed, avoiding pressure variations.

The gas it contains should be nitrogen because oxygen oxidizes the membrane and damages it. It should always contain a minimum of fluid to avoid membrane slippage. The gas should never be placed above the fluid because air pockets would form, leading to malfunction and corroding the membrane. There are several types of expansion tanks; one has no membrane but contains a gas that does not mix with water.

If the tank is open, legislation indicates that it must have 0.2% of the facility’s water and be located at a height of 3.5 meters above the collectors. It is likely that doing this is currently prohibited.

Calculation of the Expansion Tank

Responds to the formula:

• VVC = Volume of closed expansion tank.
• VI = Volume of water in the system.
• CEXP = Coefficient of expansion due to the maximum operating temperature.
• COP = Coefficient of pressure. CP = Pmax – Pmin

Volume VI is the volume of the collector pipes + the volume of the exchanger. The volume of the collector and heat exchanger is provided by the manufacturer, while the volume of the pipes is calculated using a table from UNE depending on diameter, thickness, flow, etc.

Flow Sensor

This sensor is designed to detect the liquid moving through a pipe. It is suitable for all kinds of non-aggressive liquids. It is threaded into the side of the tube, with a tab inside in contact with the liquid. This tab is kept straight when the fluid moves to stand and move east. This movement is transmitted to the outside through a bellows spring that acts on a one-way switch. You can adjust the trigger point with an accessible screw and adapt it to various diameters by changing the size of the tongue. It is essential for facilities geared to the direction of flow in a straight and full pipe.

Glycol or Brine (Refrigerating Liquid)

Glycol (HO-CH2CH2-OH) is systematically called ethane-1,2-diol. It is the simplest diol, a name also used for any polyol. Its name derives from the Greek word “glyco” (sweet) and refers to the sweet taste of this substance. This property has been used in fraudulent activities trying to increase the sweetness of wine without the additive being recognized by seeking added sugar analysis. However, it is toxic and causes kidney dysfunction.

Properties

Glycol is a slightly viscous, colorless, and odorless substance with a high boiling point and a melting point of about -12°C (261 K). It mixes with water in any proportion.

In the flexible polyurethane industry, these products are generally called “polyol.”

Applications

Glycol is used as an antifreeze additive for water in the radiators of internal combustion engines. It is a lead compound in vehicle brake fluids and is also used in chemical processes such as the synthesis of polyurethanes, some polyesters, as a starting material in the synthesis of dioxane, synthesis of glycol dimethyl ether or glycol monomethyl ether as a solvent, etc.

Summary

Glycol is generated industrially from ethylene by oxidation with oxygen in the presence of silver oxide as a catalyst and hydrolysis of ethylene oxide generated in the first stage. Another way to synthesize it is by treatment with a cold, diluted, and basic solution of potassium permanganate, accomplished with a stereochemical “sin” (hydroxylation with permanganate).

The Heat Transfer Fluid

The heat transfer fluid passes through the absorber and transfers the heat to the utilization system (battery or interaccumulator) energy. The types most commonly used are water and antifreeze mixtures, but silicone oils or synthetic fluids may also be used.

Glycol and antifreeze are the most commonly used, specifically ethylene glycol and propylene glycol. The key features of antifreeze are:

• They are toxic: This is because they contain corrosion inhibitors, which are beneficial for installation devices. They must be prevented from mixing with drinking water (with secondary pressure greater than the primary, for prevention against a possible rupture of the exchanger).
• They are very sticky: Being denser, the liquid has more difficulty advancing, increasing the pressure loss factor to consider when choosing the pump, which is usually more powerful.
• Expands more than water when heated: To avoid overpressure, an expansion tank is used. If the tank were designed to hold a pressure as if it were only water, the membrane of the vessel would reach a point where it could not expand further, and there would be excessive pressure in the circuit.
• Unstable above 120°C: If this temperature is exceeded, it degrades, becoming a very corrosive acid that affects the life of the fixtures. It also loses its antifreeze properties. Some antifreezes endure higher temperatures but are more expensive.
• The boiling temperature decreases compared to water: This could be seen as an advantage because it means it absorbs more energy.
• The specific heat decreases compared to water: While it absorbs more energy, it also takes longer to lose it or give it away, so the advantage mentioned above vanishes as it does not transfer all the heat it has gained.

To calculate the amount of antifreeze to be added to an installation, you must first consult the historical temperature chart to determine the minimum temperature recorded in that city. Once you know this, you can use the glycol graph supplied by the manufacturer to determine the required percentage.

Safety Valve

This is an accessory required by law that protects against overpressure. It is a tank equipped with a spring, which protects the valve from a certain internal pressure. It allows water to flow in one direction in the distribution of drinking water.

Automatic Fill Valve

This valve allows the primary circuit to be filled with water.

Application

The automatic fill valve allows filling and refilling of a closed-circuit heating system. It combines a pressure-reducing valve with the possibility of shut-off in a single valve. The gauge connection allows for attaching a pressure gauge (available as an accessory) for precise control of pressure in the installation after filling.

Features

• Body with male and female threads
• The pressure is adjusted by turning the steering wheel
• The set pressure is read directly on the wheel
• The internal valve is made of high-quality synthetic material and can be replaced
• Approved check valve DIN / DVGW
• Balanced input pressure – variations in inlet pressure do not affect the output pressure
• Cutting function
• Optional fittings

Application Range

Closed-circuit heating (DIN 4751)

Construction

The automatic fill valve comprises:

• Body
• Cover with adjustment spring and graduated scale
• Spring
• Pressure Reducing Valve
• Internal check valve
• Shut-off valve with internal control groove

Materials

• Brass
• High-quality plastic cap resistant to heat
• High-quality plastic check valve resistant to heat
• Reinforced NBR diaphragm
• NBR seals
• Steel spring

Operating Mode

Pressure-reducing valves operate by spring balance of forces. The diaphragm exerts a force against the adjustable spring. The inlet pressure has no influence on the opening or closing of the valve. For this reason, variations in inlet pressure do not influence the outlet pressure, as it is balanced.

The internal valve protects the distribution of drinking water by allowing water to flow in one direction. The flow pushes the metal plate against the spring force, and the valve is opened by the pressure difference.

Trap

This is mainly used to remove any accumulation of air in the circuit.

Steam traps have the function of evacuating air from the solar loop during filling and maintenance operations. Traps must be installed at the highest point of the installation and always upright.

In solar installations, steam traps should always have a stopcock in front to isolate them once the installation is filled. This will prevent the fluid in the circuit from being drawn by the automatic trap when the temperature of the fluid reaches its evaporation point.

Outline of a Closed-Circuit Water System

In the primary circuit, water enters the network through the first shut-off valve. This valve is left open until the circuit is full and serves to isolate the circuit. The water reaches the parallel electrocirculador, where a pressure gauge with two keys is used to measure the pressure and the electrocirculador circuit. Then, there is a check valve to prevent unwanted thermosiphonic effects when the collector does not understand energy. It passes through the solar collectors, and the temperature begins to be measured to govern the operation of the circuit. When the temperature is too high or too low, it sends a signal to close the three-way valve, and the electrocirculador is operated. At the highest point is the trap used for expelling gas from the installation. There would also be a safety valve that expels fluid from the circuit if the pressure rises above the target level.

In the secondary circuit, there would be the battery together with an auxiliary power source (heater), which could be in series with it.

The advantages of this type of circuit are that it has a better control system and a number of security protections, increasing performance. It also increases the number of applications that can be sought and the geographical locations where it can be installed.

The disadvantages are that it requires an additional electrical circuit, a control system, and the price is higher. It also requires more maintenance.