Circulation pumps for heating systems

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In this Article: The History of Circulation Pumps device and principle of operation; types of pumps for heating; how to choose a circulation pump; where and how to install the pump for heating.

Circulation pumps for heating systems

If the total area of ​​heated rooms is hundreds of square meters and if these very meters occupy several floors, then classical heating based on the natural circulation of the coolant will not be enough. And this is not surprising – the pressure in systems with natural circulation does not exceed 0.6 MPa. There are only two ways to increase the pressure and improve the circulation of water in such heating systems – to build a closed system with large-diameter pipes or to introduce a circulation pump into it. Large-diameter pipes will not be cheap, so the best solution for heating areas from 100-150 m2 – circulation pump.

Heating pumps – history

A century ago, engineers tried to solve the problem of the circulation of the coolant in water heating systems, trying to somehow entrust this task to a pump with an electric motor. But the electric motors existing at the beginning of the 20th century had open contacts, the ingress of water on them led to immediate accidents.

In the 1920s, German engineer Gottlob Bauknecht, who founded the Bauknecht company, created the first hermetically sealed electric motor. A few years later, Wilhelm Oplander, owner and founder of Wilo, created a circulating pump that used a Bauknecht electric motor. In the “dry” Oplender pump, the drive from the engine to the axial wheel installed in the pipe elbow was performed by a shaft sealed with stuffing box seals. Wilhelm Oplender called his circulation pump “circulation accelerator”; from 1929 to 1955, pumps of this design were produced and used in heating systems in Europe and the United States everywhere.

The main drawback of the Opleder circulation pump was the stuffing box seal, which wears out quickly at the slightest irregularities on the shaft surface, and the stuffing box material was not particularly durable. Required frequent replacement of the stuffing box packing, shaft surface needed periodic grinding and polishing.

70 years ago, the first “wet” circulation pump was created – it was invented by Karl Rutchi, a Swiss engineer and founder of Rutschi pumpen AG. The electric motor in the Ryutchi pump was mounted on a knee, through which water was pumped, and was reliably sealed. In this case, water was assigned the role of a lubricant.

Heating pumps

Later, the knee, along which the coolant passed, was replaced by a “snail”, from that moment the “snail” is used in the design of every modern pump for heating systems.

Device and principle of operation

Circulation pumps have a narrow specialization – they are designed for forced circulation of the coolant (water) in closed heating systems. In their structure, they are similar to drainage pumps: a body made of stainless metals or alloys (steel, cast iron, aluminum, brass or bronze); steel or ceramic rotor; the rotor shaft is equipped with an impeller-impeller; rotor motor.

Being installed in the heating system, the pump sucks in water from one side and pumps it into the pipeline on the other due to the centrifugal force arising from the rotation of the impeller – a vacuum occurs in the inlet pipe, and compression on the outlet pipe. With uniform pump operation, the coolant level in the expansion tank does not change, i.e. with its help, it will not be possible to raise the pressure in the heating system – for this task you will need a booster pump. The task of the circulation pump is to assist the coolant in overcoming the resistance arising in certain sections of heating systems.

Types of circulation pumps

Basically, heating pumps are divided into two types – “dry” and “wet”.

In constructions of the first type, the rotor does not come into contact with the pumped water, its working part is separated from the electric motor by O-rings made most often of carbon agglomerate, less often of stainless steel or ceramics, aluminum oxide or tungsten carbide (the material of the end seal depends on the type of coolant). When the pump engine is started, the O-rings rotate in relation to each other – between the polished and carefully fitted rings there is a thin layer of water film, which seals the connection due to the pressure difference in the external atmosphere and in the heating system (the pressure is higher in the heating system). The spring pushes one sealing ring to another, during operation the rings wear out and self-adjust to each other, their service life will be at least 3 years – they are more effective than the stuffing box packing, which needs constant lubrication and cooling. The efficiency of circulation pumps with a dry rotor is up to 80%. Compared to “wet” pumps, dry rotor pumps emit a loud noise during operation, so they are installed in a separate room with good sound insulation.

Types of circulation pumps

When using pumps with a dry rotor with sliding mechanical seals, carefully monitor the presence of suspended matter in the pumped water and the state of dustiness in the air in the room where the pump is installed. The operation of a “dry” pump causes air turbulence that attracts dust particles – dust particles and suspended matter in the coolant can damage the surfaces of the seal rings, impairing their tightness.

Regardless of the type of seal, whether it is a stuffing box or a sliding mechanical seal, in the operation of a “dry” pump, they are destroyed, so they need the presence of liquid to act as a lubricant – in the absence of it, destruction of the mechanical seal is inevitable.

“Dry” pumps are divided into three types: horizontal (cantilever), vertical and block. For pumps of the first type, the suction branch pipe is located on the end side of the “volute”, and the discharge branch pipe is located radially on the body. The electric motor of the console pumps is mounted horizontally.

Vertical pumps (in-line) are equipped with the same bore nozzles located along the same axis. The location of the electric motor in the design of such pumps is vertical.

The coolant enters the block pump in the direction of the axis, it is released in the radial direction.

“Wet” heating pumps differ from dry ones in that in their design the impeller is immersed in the coolant together with the rotor, while the coolant performs the functions of lubrication and cooling of the running engine. A metal bowl separating the rotor and stator, the material for which is stainless steel, ensures the tightness of that part of the electric motor that is energized. The rotor of a “wet” pump for heating systems is made of ceramics, bearings are ceramic or graphite, the casing is usually cast iron – for heating systems, “wet” circulation pumps in a brass or bronze casing are better suited. Compared to “dry” pumps, “wet” pumps are less noisy, do not require maintenance for years, and are easier to repair and adjust. But their main and significant disadvantage is their low efficiency, not exceeding 50%. The reason for the low performance of “wet” pumps is associated with the fact that it will be practically impossible to seal the sleeve separating the stator and the coolant with a larger rotor diameter. It is precisely because of the low efficiency that “wet” type pumps are used for the most part to improve circulation in heating systems of short length, i.e. in domestic heating.

Types of circulation pumps

Modern “wet” circulating pumps are modular in design. There are five such models: pump casing; electric motor with stator; box with terminal blocks; Working wheel; a cartridge containing a rotor and a shaft with bearings. A single cartridge unit makes it easy to eliminate air accumulated in the pump casing during start-up, and the modular design scheme itself facilitates repair work – you just need to replace the faulty module with a new one.

Accordingly, the capacity, “wet” pumps for heating are equipped with single- and three-phase electric motors. The pumps are fastened to the heating system pipeline with a threaded or flange connection – its type depends on the capacity of the pump.

Since water in pumps with a wet rotor plays the role of a lubricant, water must constantly flow to the bearings through the sleeve separating the coolant and the stator. The only way to ensure the bearings with sufficient lubrication is strictly horizontal position of the shaft – any other position of the shaft will cause the pump to malfunction and soon it will become unusable.

Heating pumps – how to choose them

First, let’s calculate how much of the coolant passes through the boiler per minute. Most manufacturers of heating boilers recommend using a simple calculation method – equating the boiler power to the water flow rate, i.e. at a power of 30 kW, 30 liters of water will pass through the boiler per minute. When calculating the flow rate of the coolant in relation to a specific section of the circulation ring, we will use the same method: we know the power of the heating radiators, and accordingly the water flow is calculated.

The next step is to calculate the flow rate of the coolant in the pipeline, according to the diameter of the pipes from which it is built:

  • in pipes with a diameter? in. the water flow rate will be 5.7 l / min;
  • in pipes with a diameter? inch the water flow rate will be 15 l / min;
  • in pipes with a diameter of 1 inch, the water consumption will be 30 l / min;
  • in pipes with a diameter of 1? inch the water flow rate will be 53 l / min;
  • with a pipe diameter of 1? in. the water consumption will be 83 l / min;
  • with a pipe diameter of 2 inches, the water flow will be 170 l / min;
  • with a pipe diameter of 2? inch, the water consumption will be 320 l / min.

The speed of movement of the coolant is taken as 1.5 m per second – as a rule, this is a sufficient speed for water in heating systems.

Let’s calculate the power of the pump for heating on the basis that a head of 0.6 m is required for a ten-meter section of the pipeline – accordingly, for a hundred-meter heating system, a pump will be needed that creates a head of 6 meters. According to the results obtained, the pump should be selected.

If your heating system uses pipes with a smaller diameter than those listed above, then you need to increase the set pump power, since the hydraulic resistance in them will be higher. And vice versa – with a larger diameter of pipes, a circulation pump of less power is required.

The above calculation of pump characteristics for heating systems is rather arbitrary and simple – if a calculation is required for a heating system of a large length and complex construction, then it would be most correct to contact a specialist in the field of heat engineering. You will not be able to independently calculate for a complex and multi-level heating system! But, if you nevertheless decide to try, the calculation formula is given in SNiP 2.04.05-91 *.

Circulation pump with minimum characteristics – power 30 W, maximum head 2 m, water flow 2 m3/ h, with an inch connection – costs an average of 4 300 rubles. The largest suppliers of domestic and industrial pumps for heating systems on the Russian market are Italian DAB, Lowara, Ebara and Pedrollo, Grundfos (Denmark), Wilo (Germany). Russian manufacturers, as a rule, produce industrial pumps, there are no domestic circulation pumps in their product line.

Please note that you will not be able to choose a pump that is 100% suitable – each heating system has its own characteristics, and the pumps are a serially produced unit with average parameters. Choosing a pump model with excessive power than is really necessary will cause noise in the pipes during operation. Therefore, it is worth choosing the pump model that has several adjustable operating modes and setting empirically the mode in which the pump works most efficiently. It will be correct to choose a pump whose power exceeds the required for this heating system by 5-10%.

Site selection and installation of the circulation pump

The “wet” pump can be installed in both the return and the supply pipelines. The popularity of the installation on the return pipeline is associated with the old models of pumps – they were installed only on the return line, because the passage of colder water through them prolonged the life of the stuffing box, rotor and bearings.

During the operation of the pump, different pressures are created in the pipeline before the expansion tank and in the pipeline after it: in the first case, compression, in the second, vacuum. The static pressure that the expansion vessel creates will affect the operation of the heating system with a circulation pump. It must be taken into account that the hydrostatic pressure in the pump delivery zone will be higher than the normal (at rest) water pressure. On the other hand, in that part of the heating system from where the pump sucks in the coolant, the pressure will be reduced, its level can not only drop to atmospheric, but also lead to vacuum. Differential pressures in the heating system can cause water to boil and air can be released or sucked in..

Heating pumps

The circulation of the coolant in the heating system will not be disturbed if, when constructing it, one condition is taken into account – at any point in the suction zone, the hydrostatic pressure should only be excessive. Compliance can be achieved in the following ways:

  1. Raise the expansion vessel 0.8 m above the highest point of the heating pipe. This method is most simple if the heating system with natural circulation is changed to forced circulation, however, its implementation is possible only with a sufficient height of the attic room and it will be necessary to insulate the expansion tank well;
  2. Place the expansion vessel at the top of the pipeline in order to lead the top section of the heating system into the pump discharge area. Modern heating systems (this technique is applicable for them), designed in advance for forced circulation, are built with a pipeline slope “to the boiler”, and not “from it”, as in heating systems with natural circulation. The goals are as follows: with such a slope construction, air bubbles will move along the water flow, carried away by the pressure from the circulation pump, i.e. the counterflow movement for air bubbles, which is common in natural circulation systems, will not be possible. As a result, the highest point in the heating system will not be on the main riser, but on the farthest. It is up to you to use this method or not, however, altering the existing heating system for it will be difficult, and building a new system based on it is not entirely convenient, because there are simpler ways;
  3. Transfer of the pipe with an expansion tank from the supply riser and its insertion into the return line not far from the circulation pump, in front of its suction pipe. With such a reconstruction of the existing heating system, we will obtain optimal conditions for the operation of forced pump circulation;
  4. This method is not suitable for all pump models – connecting the circulation pump to the supply section of the pipeline, directly behind the expansion tank entry point. Outwardly, such a modification of the existing heating system looks simple, but the temperature of the coolant in this section of the heating circuit will be especially high – make sure first that this pump model can really withstand such unfavorable operating conditions.

Having decided on the place of installation of the pump, we proceed to the installation itself. You will need a coarse filter, a check valve (for closed systems under pressure), a bypass and wrenches (from 19 to 36 mm) – all elements for the threaded diameter of the pump. On the main pipe, between the inlet and outlet of the cut-in bypass, it is necessary to install a shut-off valve along its diameter. It is especially convenient if the selected pump model has detachable threads, otherwise you will have to buy them separately.

A bypass used in heating systems is a small section of pipeline installed parallel to shut-off and control valves, its task is to switch the heating system to natural circulation in the event of a power failure and pump breakdown. For normal operation of heating devices, the diameter of the bypass pipe must be equal to the diameter of the riser into which it cuts.

The procedure for installing devices on the bypass, in the direction of the coolant: filter, check valve (if necessary) and circulation pump. The bypass inlets into the riser should be done through the stopcocks – when the system is switched to natural circulation and in the event of breakdowns of the devices on the bypass, these valves are closed, the stopcock under the bypass opens.

For efficient operation of the “wet” pump and to prevent accumulation of air, the bypass is installed strictly horizontally. Just in case, among the devices installed on the bypass, an automatic air vent can be installed – in any place, not important, but in an upright position. The advantages of an auto air vent over the classic Mayevsky tap, which are equipped with some heating radiators – the release and subsequent shutdown of this device is performed automatically, and the Mayevsky design valve must be unscrewed and screwed down manually.

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