Solutions for obtaining domestic hot water
A solar installation intended for domestic hot water is made up of one or more solar panels (also called solar collectors), to which are added components of connection to the sanitary installation, catching on the roof, one or two boilers with one or two coils, recirculation pump, electronic elements (controller), temperature sensors and mixing valves.
The simplest solar system for domestic hot water heating consists of a solar panel, a boiler, a pumping group, an expansion vessel and a solar automation. The system is recommended for medium and small homes. If the boiler has two coils, it can also be connected to a boiler on wood, gas or pellets.
If the existing boiler has 2 coils that can be dedicated only to the solar installation, they can be connected either in series or in parallel. The serial connection will homogenize the water temperature in the boiler. Using a diversion valve, it is possible to manage the parallel connection of the boiler and its stratified heating.
In the situation where the roof has no south orientation but only east-west, a solar panel can be installed on the eastern roof and another on the western facade. Each panel will have its own sensor and the start of the recirculation pumps will be determined by the temperature difference of the two solar collectors and the temperature in the basin. In practice, one of the pumps will operate from morning to noon and the other from noon to evening. This system uses 3 pipe circuits from panels to boiler, instead of 2.
This diagram is similar to system 3, in addition to the stratified boiler heating system which uses a deflection valve.
This system is dedicated to the roofs with east-west orientation in which 2 groups of solar panels will be used. The control system of the installation will consist of a diversion valve and a pumping group. The system analyzes the temperatures of the 2 groups of solar panels and deflects the valve towards the hottest solar panel.
For situations when it is desired to heat two tanks, the use of system 6 allows the solar fluid to be diverted between tank 1 and tank 2 using a diversion valve. The deviation can be controlled depending on the temperatures in the 2 tanks using successive heating or parallel heating techniques.
This scheme is similar to diagram 6, except that it uses a control system consisting of two pumping groups, without the use of a deflection valve.
System 8 is used in situations where two tanks will be heated and the panels will be located with an east-west orientation. Two pumping groups and one bypass valve will provide the logic of heating the solar system.
This system allows the transfer of the thermal energy stored in the solar tank to another tank, using a recirculation pump.
Solar installations for heating domestic water
Domestic water heating is by far the area where most solar installations are installed.
Density difference recirculation systems
As the hot water has a specific weight less than the cold water, when it is heated it goes up into the manifold and the pipe system, reaching the tank (which is mounted above), while on the other side the colder water flows from the tank to the collector. . Circulation occurs when the water in the collector is warmer than the water in the tank and is even more powerful as the amount of radiation is higher and thus the temperature difference between the collector and the tank increases. This principle is called recirculation based on density difference or thermosiphon.
Advantages: The recirculation takes place almost automatically, the solar circuit works without pump, without additional heat source and without regulating devices. For this reason, this principle is used everywhere where it is desired to install small, simple installations or where there is no electricity grid. Unlike the “standard type solar installation”, the pump and the adjustment system are no longer needed, which causes a significant decrease in the price of the installation.
Moreover, thermosyphon systems can operate relatively smoothly with an open solar circuit (if the pump is recirculated, this requires a certain lower pressure limit). Thus, the installation is further simplified, as it no longer has to be pressure resistant and no longer require devices such as the pressure gauge, pressure valve or vent valve.
Disadvantages: disadvantages are the marginal hydraulic conditions: the flow velocity, respectively the flow, are not adjustable, and the driving force is relatively low. Thus, relatively low flow rates result, which increase the temperatures in the collector and decrease its efficiency. This can be avoided, however, as with the low-flow type systems, by adopting a suitable technique, in particular by a solar-type heat exchanger. However, the hydraulic resistance opposite the current must be low because otherwise the thermosyphonic flow may stop. For this it is usually fitted, if necessary, with heat exchangers with housing.
In addition, the size and feasibility of such systems are limited by the marginal conditions. In any case, the tank must be mounted above the collectors in order for the supply to be transferred to the tank; it is avoided to install long horizontal tubes because in this way the recirculation would not take place properly. The hydraulic resistance opposite to the current in the tubes must be very small, which means that the cross-section of the tubes is large and the length of the tubes between the collector and the reservoir is very small. In order to avoid the unwanted recirculation of the thermal agent between the collector and the tank at night, when the tank is significantly warmer than the collector, it is recommended to install an easily maneuverable closing flap.
In addition to the space and installation constraints (mounting the tank on the roof can be a problem from a space point of view), there are also the limitations imposed by static (roof loading), aesthetic effects (tanks that cannot be seen from outside). a positive aesthetic effect), as well as limitations imposed by the usual installation practice, respectively connection to the existing installation.
Areas of use
In Central Europe, density-based installations are rarely used: the solar tank is usually mounted in the cellar, as close to the heating system as the external tanks have too high thermal losses due to the climate in this area. The situation is quite different in Southern Europe, respectively in the Mediterranean area: the climate is milder (but not warm enough so that no more hot water is needed), and the external tanks have lower thermal losses; here, the central heating installations are just as widespread as the cellars, and the claims regarding comfort and aesthetic effect are not (yet) as great. For these reasons, thermo-siphon installations are more widespread in the Mediterranean area, being found on thousands of (flat) roofs and having an area of up to 10 m2 so that the roof is not overloaded. Beside the largest market in the world for solar energy, namely China, a country dominated by simple thermo-siphon installations, there are also countries such as Australia, South America and India. Thus, thermo-siphon systems are by far the most widespread solar installations.
Advantages and disadvantages of different configurations of installations
- open system termosyphone with a circuit
Advantages: compared to the described system there is no heat exchanger or expansion vessel, because the solar circuit is an open one. However, a float is installed, which stops the supply of cold water to a certain level. Very simple tanks and components can also be used, which must not be pressure resistant. Another important advantage of the systems with a circuit is a good thermal transfer, because there are no losses of the heat exchanger.
Disadvantages: in open systems the risk of corrosion due to oxygen penetration is quite high. In addition, impurities can reach the reservoir, which means that it should not be excluded that the domestic water is dirty. This is the reason why this system is not approved in Germany for the use of drinking water. As a single circuit system it cannot operate with antifreeze.
Areas of use: the simplest and cheapest thermosiphon system is used in regions with low pressure of the drinking water network, or where the central system of drinking water supply is often interrupted. The tank is at the same time a storage tank, and the domestic water pressure is only determined by the difference in height between the tank (usually mounted on the roof) and the feed pump (static pressure). The system should be emptied if there is a risk of frost. Such configurations are found, for example, in areas of northern Africa.
- closed system thermosyphon with a circuit
Advantages: in this case it is a simple and cost-effective concept. Due to the fact that it is a closed system, as opposed to the system described above, the risk of impurities entering here is excluded, while maintaining a good thermal transfer between the collector and the consumer circuit. Through the direct connection to the water network there is a risk of corrosion as a result of the oxygen entering through the fresh water.
Disadvantages: in the main circuit is the pressure of the water network which, having a value in 4-bar tubes usually in Central Europe, implies the installation of additional devices to ensure the safety of the installation, being especially the installation of a membrane expansion vessel. and a safety valve. However, absorbent elements are rarely resistant to such high pressure. In many countries of the world, the pressure in the drinking water network is very low. Often there is only one water tank (plastic battery) on the roof, where the pressure is formed due to the height difference from the consumer. Since the solar system is also on the roof, most of the time, problems can occur due to too low pressure of the installation.
Areas of use: this type of installation is usually installed in regions without frost risk, where the low water pressure in the tubes (not more than 2 bars) is dominant.
. 2-circuit thermosiphon open system
Advantages: due to the separation of the solar circuit from the consumer one can use for the solar circuit an antifreeze mixture for the protection against frost. Disadvantages: as in any open system there is the risk of corrosion due to the penetration of oxygen and impurities, here referring especially to the drinking water in the tank.
Fields of use: The fields of use are similar to those described in the previous cases. However, this configuration of the installation is relatively rare, because in most areas where it can be mounted there is no risk of frost, and the separation of the two circuits would mean only higher costs and fewer advantages.
- thermosiphon system with 2 circuits, with open circuit and closed circuit to the consumer
Advantages: As an open circuit, the solar circuit can be made up, cost-effective from simple components, which are not necessarily pressure resistant and can be filled with antifreeze water. Due to the consumer’s closed circuit there is no risk of impurities entering the drinking water.
Disadvantages: In the open solar circuit there is a high risk of corrosion due to the penetration of oxygen. The tank must be pressure resistant.
Areas of use: The main area of use of these systems is the countries of the western Mediterranean. The system can operate throughout the year and in countries at risk of frost.
It is also possible to install a thermosiphon system with 2 circuits, of which the solar circuit is closed and the consumer circuit is open. However, such a system configuration does not make much sense and, therefore, is not implemented too much.
- closed thermosyphon system with 2 circuits
The advantages over the other systems represent a low risk of co
corrosion and penetration of impurities in drinking water, as well as the possibility of introducing antifreeze into the solar circuit.
Disadvantages: Of all the types of systems listed so far, this one is definitely the most expensive, since it is necessary to install pressure-resistant components and, because of the closed solar circuit, it is not possible to abandon additional devices such as the vessel expansion membrane or safety valve.
Areas of use: systems of this type are usually installed in the Mediterranean area, especially in cities, where sufficient financial resources are usually available to invest in a solar system and where the water pressure in the pipes is so high. stable, so that such a system can be mounted.
Systems with forced recirculation
In contrast to the thermosyphon systems, at the solar installations with forced recirculation, a pump is installed which has the role of recirculating the thermal agent from the solar circuit. To operate the pump, it is necessary to install a control system. An important feature of the forced recirculation installations is the presence of the pump and the control or control units.
Advantages: The installation of a recirculation pump with control system no longer has the limitations of the heating system. The places of installation of the collector and the tank, as well as the distance between the two elements are no longer as important and no longer have to be adapted to the local conditions. Moreover, by properly sizing all the elements, the system can operate at maximum efficiency.
Disadvantages: The disadvantage is that additional effort is required to operate the pump and control system, as well as to install and adjust them.
Areas of use: since the advantages play an important role in practice, solar systems with forced recirculation system have become small standard systems for Central and Northern Europe. Large installations cannot be done in any other way.
Advantages and disadvantages of different configurations of installations
In the solar installations for domestic water in the regions with central European climate, despite the great effort of operation, the closed system with heat exchanger (in the solar circuit) is used, because it is frost resistant and complies with the local rules of the installation technology.
The systems without heat exchanger do not work all year long except in the southern countries, ie where there is no frost and where the installations should not be emptied. Due to their qualities (good yield, low costs) these areas are spread especially as a thermo-siphon system. In Germany, this system is only discussed for swimming pools.
Open systems are not installed in this region at all, either because they are difficult to integrate into the pressurized water supply system (in the case of systems without a heat exchanger), or because part of the thermal agent turns into vapors in the vessel. open expansion (for heat exchanger systems), which means huge maintenance costs. In addition, open systems are prone to corrosion as a result of oxygen entering through the open expansion vessel. In the Mediterranean countries, respectively in the countries with subtropical climate, the open systems, especially the small thermosyphon are of greater importance, because the non-pressurized mode of construction corresponds to the water supply system of the households (the water tank is mounted on the roof).
System with two circuits with forced recirculation and internal heat exchanger (for the solar circuit)
This is the standard system found in Germany for small installations (<10 m2), by far the most commonly used system for domestic water heating.
Advantages: for the simple and very flexible concept of this installation there are countless experiences and a variety of standard prefabricated components, which means that such installations are relatively inexpensive and are delivered according to size (prefabricated packages).
Disadvantages: Compared to thermo-siphon systems, this type of installation requires more effort and is more expensive.
Areas of use: domestic heating of drinking water for up to 8 people (collector (<10 m2).
Solar system with two circuits with forced recirculation external heat exchanger (for the solar circuit) as installation with two tanks
Advantages: heat is better transferred through an external heat exchanger. The first tank can be a simple pressurized tank with no attachments. This system can be provided with an external heat exchanger instead of an internal one.
Disadvantages: The external heat exchanger requires additional effort for the second pump and for the pipe connection. Both tanks require space relatively large, and the thermal losses are higher than those of a single tank with the same volume of storage. Both tanks must be pressure water tanks for domestic water.
Areas of use: this system is used in larger installations (> 10 m2), but also in connection with an existing system where there is already a second one regarding the last case, the system is the standard solution, and the second one tank is usually much smaller, as in conventional heating installations. In this case, it is cost-effective to install an additional heat exchanger and another pump at the top of the second tank; the latter has the role of pumping water from the additional tank to the other tank if there is no blockage.
- system with three circuits with forced recirculation and internal heat exchanger (for the solar circuit)
Advantages: by separating the consumer circuit from the tank by an additional heat exchanger, another buffer tank can be fitted, without pressure. For example, heating water can be found in the tank.
Disadvantages: through the additional component elements (external heat exchanger, pump plus adjustment system, pipe connection) the system requires more energy and is more expensive,
Areas of use: The system is used when the water in the tank is needed for another circuit (for example for the heating circuit). Therefore, it is installed where the tank volume exceeds 400 l and requires special protection measures against the legionella bacterium.
- 3-circuit system with forced recirculation and external heat exchanger
Advantages: The external heat exchanger separates all the circuits from one another. As a solar tank, simple buffer tanks can also be fitted, so that small drinking water and domestic water tanks can also be installed in large installations.
Disadvantages: This system requires a lot of effort due to the numerous additional elements and the necessary expenses for the pipe connection. Thermal losses increase in proportion to the number of tanks.
Areas of use: the described configuration represents a connection way for large installations (collectors with an area> 50 m2). The connection of several tanks in series (installations with 2, 3 or more tanks) is often necessary (due to the available space, etc.). This example shows that large installations are not simply enlarged copies of smaller installations, but largely require a different conception and a correspondingly greater effort.
Direct power supply
In large drinking water networks, such as those for large homes, hospitals, etc., solar energy supply can also be achieved directly, without a tank. This is possible because when the solar system produces energy, energy releases almost always occur, which means there is a need for (consumption) of energy. In addition, at such installations, however, a tank is installed which can be used if necessary. For this purpose, prefabricated stations have been developed, through which the existing networks can subsequently be completed relatively easily.
Sizing installations for domestic hot water
Determination of energy requirement for hot water
In order to adapt the energy production to the consumption it is important to determine the hot water consumption and the energy needed to heat it. In ideal cases, this is done by measuring consumption over a longer period, which means, however, that too much effort is needed for small uses (households with a family).
Apart from the individual settlements, the different standards of the hot water distribution systems in the households must also be taken into account. In particular, the pipe systems in old buildings need to be upgraded: oversized pipe diameters, insufficiently thick insulation and circulation during continuous operation can accelerate the energy required for hot water preparation.
Often, therefore, it is worth modernizing or replacing the hot water distribution system when installing a solar installation.
In the industrial field or for other uses, the determination of the need for hot water can be done with the help of hot water meters. Moreover, in this case, particular attention must be paid to the temporal structure of consumption (eg higher consumption peaks in the hotel industry in the morning). For the planning of objects with a structure of common use there are special manuals for specialized engineers.
When determining consumption, it must be checked whether there is the possibility of storing domestic water that can be used consistently. Thus, the installation of quality mixing batteries in the sink and tub and some valves that limit the flow to the supply pump can help to avoid more water than is needed. Also included in the category are some shower heads that need less water to ensure the same degree of comfort.
Measures of this kind have several effects on a lower consumption of domestic hot water means not only a lower energy requirement, but also a more solar installation.
low and therefore lower costs.
Determination of the solar radiation supply to the collector
Now you have to calculate the supply of solar radiation from the spot. An important role plays it:
· The place where the collector is fitted (climatic considerations)
· Tilt angle and collector orientation (architectural considerations)
· The potential degree of shading due to surrounding houses or trees, etc.
Influence of the degree of inclination and orientation on energy gain is rarely underestimated. In a relatively large area the losses due to deviation from the ideal position are relatively low.
Example: for a roof with a south-facing inclination of 45 °, the correction factor is 1.06.
Sizing the surface of the collector
In practice there are several variants for determining the surface of the collector. However, an exact determination of this is possible, because the climatic influences and especially those of the user’s behavior (the heat requirement) can be radically changed. This is why the calculation methods presented in the following paragraphs often refer to average values or results. Besides, the exact measurement of the surface of the collector does not make sense either,
for the size of the collector field corresponds to commercial uses.
Determination using a basic formula
Based on the experiences of several decades of solar installations for domestic water heating, the following basic formula was imposed for small installations (from households with a family):
· 1 – 1.5 m2 from the surface of a flat collector / person, respectively
· 0.8 – 1.2 m2 from the surface of a vacuum tube / person collector
This formula is used for an average consumption of hot water and determines a solar coverage of the installation of 40 – 60%. Lower values mean lower solar coverage rates or are valid for higher yield collectors, while higher values mean higher solar coverage rates.
Of course, the formula of this type indicates approximate values, where the quality of the collector, the climatic differences, as well as the deviations from the ideal assembly of the collector are not taken into account. However, this formula is very important for estimating the size of the collector
Determination using a nomogram
Several manufacturers of solar installations also make measurement charts, the so-called nomograms, specially designed for their products. Thus, the surface of the collector can be determined easily and quickly, the same being valid most often for the volume of the tank. The results for a family with 4 persons are (45 ° roof inclination degree, south orientation, Warzburg region, 4 persons x 50 k / d.pers = 200l / d):
AK = 5 m2 for a collector manufactured by Wagner & Co at a coverage rate 58% solar output.
Determination based on the degree of use of the collector
Based on an equation for the degree of use of the collector (see paragraph with the definitions of terms) it is possible to calculate even the required surface of the AK collector, as long as the radiation is known
nk = QsK / Gs = QsK / (Ga * AK)
Now only the gross energy gain of the QsK collector needs to be calculated. It depends on the following factors:
· Solar coverage rate: the information in chapter 5.2 indicated that it is not cost-effective to cover more or less complete necessities with the help of a solar installation. An optimal cost-consumption ratio is obtained when the coverage rate is between 40-50%.
In the case of a family of 4 members with an annual consumption of 3,400 kWh, the solar installation must have a contribution of 1,700 kWh (at a coverage rate of 50%).
· Thermal losses in the system: the collector field itself must provide more than 1,700 kWh, since thermal losses occur between the collector and the consumer.
· The losses in the collector circuit depend to a large extent on the length of the tubes and on the quality of their insulation. Depending on the length of the tube and the daily running time. These represent between 10 and 20% of the gross thermal gain of the collector. Losses greater than 20% indicate inadequate connection of the pipes and, last but not least, their insulation.
In our example, the losses in the collector circuit represent 250 kWh per year, ie 11% of the collector’s annual gain of 2,250 kWh.
· Thermal losses to the tank can be calculated according to the value (U-A) of the tank indicated in the manufacturers manual. In this case, they represent a total of 500 kWh per year. Since the solar system transfers only 50% of the energy gain to the tank, it is assumed that the total losses of the solar installation are 50%, thus in our case it is 300 kWh and corresponding to a percentage of 13% of the collector’s gain.
· Other losses occur when using domestic water circulation. Since, however, in such households with a family, such a system is not usually installed, it is recommended, for economic reasons, to abandon it; it starts from the premise that there is no domestic water circulation and therefore no additional thermal losses.
Determination based on the degree of use of the system and the monthly average values
The principle of this method is represented by the monthly hot water requirement (daily requirement the number of days of a calendar month) as well as the monthly amount of radiation.
The surface of the collector is determined using the degree of use of the nSys system:
nsys = QNlunar / Glunar * AK).
To be able to calculate now the surface of the collector must know the degree of use of the system nsy ,. It can be influenced by several variables, including radiation, type of collector (flat or vacuum tube collector), wind, length of tubes, quality of insulation, size of tank and, first of all, temperatures (temperature Thus, it is possible to determine the degree of use of the system of a solar installation, being between 20 and 40%. It is assumed that the average degree of use is 34%.
Thus, one can calculate the surface of the collector for each month of the year by which a solar coverage rate of 100% can be obtained.
Sizing the volume of the tank
The upper value should be chosen when it is desired that the solar coverage rate be higher than 60%, and the lower value should be chosen when the solar coverage rate is less than 40%. Based on the following recommendations, the volume of the tank can be determined:
1 volume of the tank = the daily requirement
+ a supplement of 20 to 50% for the tips of
+ 20 1 / m2 from the surface of the collector
where the supplement for consumption peaks depends on the size of the installation:
5 to 10 kWh / d + 50%
10 to 15 kWh / d + 40%
15 to 20 kWh / d + 30%
20 to 25 kWh / d + 20%
What volume should be kept at a certain temperature level by heating (conventional, electric current) and the height at which the tank must be heated subsequently are two factors that depend on the availability of additional energy:
• In the additional heating with a heating boiler (with a power between 8 and 20 kW), the heat exchanger must be mounted in the upper part of the tank, so “to provide between 25 and 500 of the daily heat requirement.
• when additional electric heating is carried out by means of a heating plunger (with a power of 2 kW), the daily water requirement must be heated so that the heating stick is in the middle of the tank. For ecological and economic reasons it is advisable to avoid additional electric heating.
From an energy point of view c
it is more cost-effective to use additional heating with a water flow heater (gas heater), because in this case the entire volume of the tank is available for solar energy.
Sizing the heat exchanger
Its size depends primarily on:
§ the transferred power, which depends on the surface of the collector
· The average temperature difference between the heat exchanger’s supply and outlet connection, which depends on the system’s operation. For high-flow installations this difference should not be less than 10 K, and for a total radiation it should not fall below 20 K. At Low-Flow installations, the average temperature difference is between 20 and 40 K.
– the glycol concentration in the solar circuit. The indicative values mentioned below refer to the concentration used by approx. 40%.
· Flow behaviors
· The type of construction of the heat exchanger.
The pressure losses from the heat exchanger must be small, because at higher values the required efficiency of the pump increases significantly. The pressure loss is usually between 70 and 200 mbar, a flow velocity of 0.5 – 1/5 m / s (information provided by the manufacturers or the thermal loss diagram). At thermo-siphon installations: this guidance value is too high; In this case it must be assumed that the pressure loss values of 10 mbar correspond to a flow of 101 / h per m2 from the surface of the collector.
Internal heat exchangers (sunken) for high-flow installations are dimensioned according to the following rule:
· Heat exchanger with smooth tubes: the surface of the exchanger will be 0.2 m2 per m2 of collector surface
· Heat exchanger with ribbed tubes: the surface of the exchanger will be 0.35 m2 per m2 of collector surface
Sizing of tubes
Since the tubes in the collector circuit and its liquid content represent a “dead” storage capacity and the thermal losses also occur, the diameter of the tubes must not be too large. On the other hand, a specific one must be chosen. minimum diameter in relation to the least possible operation of the pumps.
The required diameter of the tubes depends on the surface of the installed manifold (the flow velocity of the tube is max. 0.5 up to 1 m / s) and the total length (forward and backward) of the tube (pressure drop in tubes <200 mbar) .
Sizing the recirculation pump
In the case of small installations with a surface of the collector up to 10 m2 and a length of tubes of 50 m can usually be given a detailed calculation of the recirculation pump, as long as no special components with a flow resistance are fitted. extremely large. The most commonly used adjustable recirculation pumps meet this demand.
For larger installations or special installations it is necessary to calculate the resistance to flow in the circuit, only thus being able to choose the appropriate pump, the resistance to the flow or the pressure losses depend very much on the flow, that is on the flow of the total volume of the VKK circuit of the collector: the higher the flow, the higher the pressure losses.
At standard solar installations it is recommended that the flow be
between 30 and 50 l / h / m2 from the surface of the collector, and for low-flow installations it should be between 8 and 15 l / m2 * h.
The multiplication of the active surface of the collector results in a total flow: this represents for example on a surface of the collector of 5 m2, 150-250 l / h, respectively 40-75 l / h in low-flow installations. When choosing a recirculation pump or determining its operating location, the thermal losses in the collector circuit must be calculated.
Depending on the collector linking between them (series, parallel or combined series and parallel), the flow of each collector can be calculated separately. The manufacturers supply for each collector a diagram from which the flow resistance can be deduced as a function of the flow. At the collectors with an area of approx. 2 m2 and a volume flow of approx. 50l / (m2h), the pressure losses represent about 5-35 mbar. When connected in parallel, the pressure drop of the collector field corresponds to a collector, and to the series connection the resistors are added. The total heat loss from large installations (with an area of up to 30 m2) must not exceed the maximum value of 0.1 – 0.2 bar. This can be achieved by combining serial and parallel bonding quality.
· heat exchanger
In order to obtain a good thermal transfer between the heat agent and the exchange wall (turbulent flow) it is necessary that the flow velocity in the exchanger should be approx. 1 m / s. The pressure loss of the manifold can be calculated on the basis of the diagram provided by the manufacturer of the heat exchanger. There is the possibility that at higher flow rates it may be necessary to install two or more skis in parallel heat bath.
The total losses in the tubes result from multiplying the specific value by the length of the tubes (supply and discharge). At elbows and at T-elbows, reductions, valves and other fittings, the thermal losses are calculated separately – as long as they are not too small to be neglected – with the help of the resistance values in relation to the flow velocity and / or the equivalent length. of the tube.
Choosing the expansion vessel and the safety valve
For choosing the membrane expansion vessel and for calculating the volume of the thermal agent – the VA volume of the liquid in the circuit must be calculated first
VA = VK + VWT + VL
Installation volume = volume of collector + volume of heat exchanger + volume of tubes
The specific volume of the manifold and the heat exchanger are indicated in the sheets provided by the manufacturer.
The volume of VD expansion depends on the concept of the installation and the safety one
1. In the simplest case, the liquid will dilate until it reaches a stagnation temperature. The increase in volume depends on the so-called expansion coefficient, which in water is approx. 5%, and when mixing water with 40/60 propylene glycol it is 10% (including a 30% safety addition). This results in a minimum volume of VD1 dilation:
VD1 = 0.05 • VA for water as a thermal agent and
VD1 = 0.10 * VA for glycol mixtures with water.
2. Intrinsically safe installations must be able to retrieve in addition to the expansion of the liquid and the entire VK volume of the collector:
VD2 = VK + VD1 = VK + 0.1 • VK
The VG volume of the vessel is not the same as the VD volume of dilation.
Since the pressure in the manifold varies between the minimum and maximum operating pressure, it means that the entire volume of the expansion vessel is not available, but only the part around the gas cushion that changes during this change in pressure. Similarly, the minimum VGmin size of the vessel can be calculated according to the following equation:
VGmin = VD * (pBmax + 1) / (pBma-pvor)
due to the over pressure at which the installation is filled,
pBmax maximum operating pressure.
The expression (pBma-pvor) / (pBmax + 1) indicates the actual volume of the membrane expansion vessel. It should represent a maximum of 0.5, corresponding to a percentage of 50% of the volume of the vessel – otherwise there are risks of over-dilating the membrane.
· The overpressure must be so high that it does not penetrate the air in the system (for example, by means of a fast automatic fan) nor in winter, when the outside temperatures are very low. If the system is cold, the overpressure should be 4.5 to 1 bar at the highest point of the system, to avoid air entering the circuit. If the operating pressure is measured at another point and not at the highest point of the installation (usually in the cellar or near the tank), then besides the minimum pressure the pressure of the water column where at 10 m column must be calculated of water corresponds to 1 bar.