Heating Systems with Pumped Circulation

Heating Systems with Pumped Circulation

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As we have said many times before, the main disadvantage of gravity systems is low circulational head (especially in an apartment system), and therefore large diameter pipes are necessary. It is possible to make just a little mistake in choosing the pipe diameter and the system will be «clamped» and unable to overcome hydraulic resistance. Unclamping the system can be done without too much effort: add a circulating pump to the system (see Figure 12) and move the expansion tank from the supply line to the return line. It should be noted that moving the expansion tank to the return pipeline is not always necessary. With simple changes of an uncomplicated heating system (e.g. in an apartment), it is possible to leave the expansion tank where it was. But to reconstruct correctly or in building a new system, the expansion tank should be moved and changed from an open one to a closed one.

Circulational Pump
Figure 12. Circulational Pump

What must the power be of a circulational pump? How and where should it be installed?

Pumps for domestic heating systems have low energy consumption, 60–100 watts, like the usual light bulb. They do not raise the water, they only help it overcome local resistances in the pipes. These pumps can be compared with a ship’s propeller: a propeller pushes water and provides the motion of the ship, but the amount of water in the ocean is not increased or decreased and the total balance of the water stays the same. The circulational pump which is connected to the pipeline pushes the water but it doesn’t matter how much water is pushed because the same amount of water comes from the other side. So there is no fear that the pump will push the water through the open expansion tank: the heating system is a closed contour and the amount of water in it never changes. In these systems, along with circulational pumps, lifting pumps can be included, and these are able to increase the pressure and raise up the water. These pumps should be called simply «pumps», while the circulational pumps should be called «fans». It doesn’t matter how much an air fan works in a room: the only thing is its ability to create wind (circulation of air); but it is unable to change the atmospheric pressure, even in a closed insulated room.

As a result of using a circulational pump, the radius of action of a heating system is much greater, diameters of the pipes are reduced, and it is possible to attach the system to the boiler with higher values of the parameters (temperature, circulational head, speed of motion, etc) of the warmth carrier. To provide a noiseless hydronic pumped system, the speed of the water in the pipelines which service the main rooms of inhabited buildings with nominal bore of pipes 10; 15; 20 mm and more; should not be more than 1.5 m/sec, 1.2 m/s, 1.0 m/s , respectively. In pipelines which go in less important rooms of inhabited buildings, the speed of the water should be not more than 1.5 m/s. In pipelines which go in less important buildings, the speed should be not more than 2 m/s.

To provide a noiseless system which provides the required volume of warmth carrier, it is necessary to make some calculations. We already know how to calculate approximately the power of the boiler, in Kw, based on the area of the heated rooms. The optimal expenditure of water which goes through the boiler recommended by many boiler manufacturers is calculated using the simple empirical formula: Q = P, where Q is the expenditure of warmth carrier through the boiler, in litres per minute; and P is the power of the boiler in kilowatts. For example, for a boiler with a power of 30 Kw, the expenditure of water is approximately 30 litres/min. To calculate the expenditure of warmth carrier on any section of a circulational loop, we use the same formula, knowing the power of the radiators in that section — for example, to make the calculation of the expenditure of water for radiators installed in one room. Suppose that the power of the radiators is 6 Kw, then the expenditure will be approximately 6 litres/min. From the expenditure of water, we arrive at the diameter of the pipes (see Chart 1).

The numbers in this chart give the correspondences between volume of water (in litres) per minute and pipe size (in inches) and have been obtained from practical experience and are for a speed of the warmth carrier of not more than 1.5 m/sec.

Chart 1. Relationship between pipe diameter and expenditure of water carrier
Expenditure of water, l/min 5,7 15 30 53 83 170 320
Diameter of the pipes, inches 1/2 3/4 1 2

 

Next we must determine the power of the circulating pump. For each 10 metres of length of a circulational loop, 0.6 metres are required for the head of pump. For example, if the total length of the loop is 90 metres, then the head of the pump should be 5.4 metres. We go to a store or choose in a catalogue and buy a pump with the necessary head. If the pipes are of smaller diameter than were recommended in the previous paragraph, then the power of the pump should be increased because the thinner the pipes, the more hydraulic resistance they have. When using pipes with large diameters, the power of the pump can be decreased.

To provide constant circulation of water in these heating systems, it is desirable to install at least two circulating pumps. One of them works and the other is on bypass and reserved. Or, you can install one pump and the other can be put away as a replacement if the first one breaks.

It needs to be said that the calculation for the heating system given here is very primitive and doesn’t consider a lot of factors and peculiarities of individual cases.  If you build a house with a complicated architecture of heating system, then it is necessary to make exact calculations. This can be done only by heating engineers. To build an expensive building without proper analysis of the project which considers all the details of the building would be silly.

The circulating pump in the heating system is filled with water and gets equal (if the water is not being heated) hydrostatic pressure from two sides — from the inlet side (sucking) and from the outlet side (blowing) — which are connected with the pipes of the system. Modern circulating pumps which are made with water lubrication of the ball bearings can be placed either on the supply or return pipeline, but more often they are put on the return pipeline.

In the beginning this was done for technical reasons: putting it in cooler water, the bearings, rotor, and packing (through which the shaft of the pump goes) worked for a longer time. And now, pumps are put on the return side out of habit, but from the point of view of creating artificial circulation of water in a closed contour, the placement of the circulational pump is unimportant. However, the placement of the pump on the supply pipeline where the hydrostatic pressure is usually less, is more reasonable. For example, if the expansion tank is placed in the system at a height of 10 metres above the boiler, then it creates a static pressure of 10 metres of water column; but this fact is correct only for the low pipeline. In the high pipeline, the pressure will be less because the column of water here is less than 10 metres. Wherever we put the pump, it will get equal pressure from both sides even if you put it on the vertical main supply line or return pipeline. The difference between pressures of the inlet and outlet of the pump will not be significant because the pumps are small.

But all this is not so easy. The pump acting in a closed-contour heating system increases circulation, pushing water into the pipeline from one side, and sucking from the other. The level of water in the expansion tank does not change when the circulating pump starts working because an evenly working pump only provides circulation with a fixed amount of water. In such conditions (even with the action of the pump and a constant volume of water in the system) the level of water in the expansion tank stays the same. It doesn’t matter if the pump is working or not, the hydrostatic pressure where the expansion tank is connected to the system pipes will be the same. This point is called «neutral» because the circulational pressure developed by the pump doesn’t affect the static pressure created by the expansion tank. In other words, the pressure of the circulating pump at this point is zero.

In a closed hydraulic system, the circulating pump uses an expansion tank as the starting point in which the pressure developed by the pump changes its sign: before this point the pump creating compression pushes the water; after this point the pump evokes a negative pressure and sucks the water. All the pipes of the system from the pump to the point of constant pressure (counting in the direction of the motion of water) will be in the region of «pushing» by the pump. All the pipes after this point will be in the «sucking» region. In other words, if the circulating pump is installed into the pipeline right after the point where the expansion tank is connected, then it will suck the water from the expansion tank and push it into the system. If the pump is installed before the point where the expansion tank is connected, then the pump will start pumping water from the system and push it to the expansion tank.

So what is the difference if the pump pumps the water from or to the expansion tank? In any case the water is circulated in the system. But there is a difference and it is very important. In the work of the system, static pressure is involved, created by the expansion tank. In the pipelines situated in the «pushing» region of the pump, it is necessary to count the increased hydrostatic pressure compared to the water pressure in a calm state. It is the opposite in the pipelines situated in the «sucking» region: it is necessary to consider the decrease in pressure and there can be a case when the hydrostatic pressure will not only be lowered to atmospheric pressure, but even below. Thus, as a result of differences in pressures in the system, there is a danger of sucking or freeing the air, or boiling of the water.

To avoid disturbing the water circulation because of boiling or sucking the air, while making the design of the hydronic heating system, a rule must be followed: in the «sucking» region, at any point of pipelines of the heating system, the hydrostatic pressure while the pump is operating should be surplus. There are four means by which this rule can be followed (see Figure 13).

The Principle Schemes of Heating Systems with Pump circulation and open Expansion Tank
Figure 13. The Principle Schemes of Heating Systems with Pump circulation and open Expansion Tank

1. The first way is to raise the expansion tank to a sufficient height (usually not less than 80 cm). This is quite an easy method when converting a gravity system into a pumped system, but it needs quite a high attic and very good heating insulation of the expansion tank.

2. Moving the expansion tank to the most dangerous upper point in order to include the high pipeline into the zone of blowing. Here I must explain something: in new heating systems, the supply pipelines with pumped circulation are inclined not downwards from the boiler but towards it. The reason is that air bubbles can move with the water because the initial force of the pump will not let them go against the flow as it was in the gravity systems.  Therefore the upper point of the system is not on the main vertical pipeline but on the furthest vertical pipeline.  While redesigning an old system with gravity circulation into a pumped system, this method is rather laborious because it is necessary to redo pipelines, and to create a new system.  It is not used because there are better alternatives.

3. Connecting the pipe of the expansion tank near the sucking pipe of the pump. In other words, if we redesign the previous gravity system, then we simply cut the expansion tank from the supply pipeline and put it on the return pipe, behind the pump and thus create the best conditions for the pump.

4. Instead of putting the pump in its usual position on the return pipeline, include it into the supply pipeline immediately after the connection point of the expansion tank. While converting a gravity system, this is the easiest method: we simply paste the pump into the supply pipeline, doing nothing else. However, one must choose the pump very carefully because we are putting it into a high temperature environment. The pump must serve reliably and for a long time and this can be guaranteed only by the best manufacturers.

The contemporary market for heating devices allows the possibility of replacing open-type expansion tanks with closed ones. In a closed expansion tank, there is no contact between the liquid in the system and the air: the warmth carrier does not get converted into steam and is not enriched with oxygen. It lessens the loss of warmth and of water and also the internal corrosion of the heating devices. The liquid will never leave the closed expansion tank.

The closed type expansion tank is a capsule of spherical or oval shape divided inside by a hermetic membrane into two parts: air and liquid. A mixture containing nitrogen is put into the air chamber under certain pressure. Before filling the heating system with water, the pressure of this gas mixture inside the tank firmly presses the diaphragm onto the water section of the tank. The heating of the water leads to the creation of pressure and increases the volume of the water carrier and the membrane bends towards the gas part of the tank. During the maximum working pressure and maximum increase of the volume of water, the filling of the water section of the tank occurs, and also maximum compression of the gas mixture. If the pressure continues to increase and the volume of the warmth carrier continues to grow, then the pressure-relief valve is activated (see Figure 14).

Membrane-type expansion tank
Figure 14. Membrane-type expansion tank

The volume of the tank is chosen in such a way that the tank’s useful volume is not less than the temperature expansion of the warmth carrier, and the preliminary pressure of the air in the gas part of the tank is made equal to the static pressure of the column of warmth carrier in the system. Such a choice of the pressure of the gas mixture allows the membrane to be held in a balanced state (not stretched) when the system is filled but not yet turned on.

A closed-type tank can be placed at any point of the system but as a rule it is installed before the boiler because the temperature of the liquid where the tank is installed should be as low as possible.

We already know that the circulating pump should be installed right after the expansion tank where for it (and also for the whole heating system) the best conditions are created (see Figure 15).

Principal schemes of pumped heating systems and closed expansion tank
Figure 15. Principal schemes of pumped heating systems and closed expansion tank

There are two problems in this kind of scheme for a heating system: getting rid of air and high pressure at the boiler.

Whereas in the systems with open expansion tanks, the air is removed through the tank with counter-flow (in gravity systems) or same-way flow (in pumped systems), with closed tanks, this doesn’t happen. The system is completely closed and there is no way for the air to leave. To get rid of air jams in the highest point of the pipeline, automatic air purgers are installed — devices which are equipped with floats and stop valves. When the pressure is increased, the stop valve is activated and the air escapes to the atmosphere. Alternatively, Mayevsky valves can be installed on each heating radiator. These valves get rid of air jams from the radiators directly. The Mayevsky valve is sometimes incorporated into the radiator, but more often they are bought separately.

Automatic air purger
Figure 16. Automatic air purger

The principle of how the air purger works (see Figure 16) is as follows: in the absence of air, the float inside the device holds the air-outflow valve closed. When air collects in the floating chamber, the level of water inside the air purger gets lower. The float then goes down and the air-outflow valve opens, and the air goes to the atmosphere. After the air has left, the water level in the air purger rises and the float does also and this causes the outflow valve to close. This process continues until air no longer collects in the floating chamber and the water level does not go down, lowering the float. Automatic air purgers are made with different designs, shapes and sizes and can be installed on the main pipeline or directly on radiators (these are L-shaped air purgers).

The Mayevsky valve is different from the automatic air purger. It is, generally speaking, the usual stopper with an air-out channel with a cone-shaped screw inserted into it: unscrewing this will release the channel and the air goes out. Screwing it again closes the channel. Also there may be air purgers in which, instead of a coned screw, a metal ball is used for blocking the air-out channel.

Instead of using an automatic purger or Mayevsky valve, it is possible to have an air separator in the heating system. This device is based on Henry’s Law. The air which is present in the heating system is partly in dissolved form and party in the form of microbubbles. While water is moving, carrying air with it, through the system, it comes to regions of different temperatures and pressures. According to Henry’s Law, in some regions, air will leave the water, and in others it becomes dissolved in it. In the boiler, the warmth carrier is heated to a high temperature, and that is why exactly in this place the greatest quantity of microbubbles will be released from the water. If you don’t get rid of them immediately, then they will become dissolved in other part of the system where the temperature is lower. If you get rid of microbubbles right after the boiler using the separator, then the output from the separator will have water without air, and this water will absorb air from later parts of the system. When this water returns to the boiler, the process repeats until eventually all the air has been removed from the water in the system.

Air Separator
Figure 17. Air Separator

The work of the air separator (see Figure 17) is based on the principle of merging of microbubbles. Practically, it means that small bubbles of air stick to the surface of the special rings and gather together to form big bubbles which can be separated and emerge into the air chamber of the separator. When the flow of liquid goes through the rings, it goes in many different directions and the construction of the rings is such that all the liquid going through them comes into contact with their surfaces, so that the microbubbles stick to the rings and combine to form larger bubbles.

Principal Schemes of pumped heating systems, with closed expansion tank and air separator
Figure 18. Principal Schemes of pumped heating systems, with closed expansion tank and air separator

Now let us leave the subject of air and return to that of the circulating pump. In a heating system with long pipelines and therefore with high hydraulic losses, it is not uncommon that rather powerful pumps are needed, but these create more pressure than the boiler is designed for. In other words, if the pump is placed on the return pipe line just before the boiler, the connections in the exchange-warmth pipe inside the boiler can start to leak. To prevent this, powerful circulating pumps should be installed not before the boiler but after it, on the supply pipeline. And immediately we have a question: where should the air separator be installed, before the pump or after it? The leading manufacturers of heating systems have solved this question and recommend that the separator be placed before the pump (see Figure 18) to prevent it from damages by air bubbles.

And now we will look into pumped systems in more detail.

 

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