Arrangements of heating systems in compliance with official regulations

Transcription

1 Arrangements of heating systems in compliance with official regulations Michael Hartmann, chartered engineer Hydraulic problems in heating systems and related difficulties are very common in everyday life. This paper is aimed at explaining why the popular remedial measures ultimately lead to the increased consumption of energy and what the possibilities of creating a system that operates reliably with consideration for the dynamics of a heating system are. Official regulations recommend hydraulic compensation In the statutory regulation of the German Industrial Standard no in Item it is stated: Sections of a system should be set in such a way as to achieve the required functions and efficiency and so that the basic requirements are fulfilled. The hydraulic compensation should be conducted in such a way that when the operation is in accordance with the regulations, i.e. even when there is a drop of temperature in rooms or during breaks of operation, all users are provided with heat adequately. The standards reflect the general condition of technology. Hence the hydraulic compensation is a proven automatic efficiency. But what does this term refer to in practice and how to achieve this aim? Operation in accordance with regulations means, following the regulations on heating systems, the control of the initial temperature depending on the ambient temperature and the control of temperature in rooms by thermostat valves. When the initial temperature is reduced automatically, thermostat valves may open entirely depending on the temperature in the room and the settings. During the repeated heating and if the system is not compensated, the radiators with better hydraulic positions are supplied with the excess of water. Those radiators that are located in worse positions in terms of the hydraulic system get respectively less water, which leads to the prolonged time of heating (Photo 1). The users who are subject to such longer heating may complain about chill. Such results of wrong compensation in practice cause the increased efficiency of a pump or the rise of the initial temperature. Consequently, the higher efficiency of the pump leads,

2 apart from the higher consumption of fuel, to the situation when the radiators located in good hydraulic positions receive even more water. The efficiency of the pump becomes so high that ultimately even the radiators with bad hydraulic positions are supplied with a sufficient amount of water. The increase of the initial temperature results in the higher consumption of energy at the beginning due to the losses in distribution. Hence, a user will not complain about chill but will virtually burn huge amounts of money. As there is no possibility of comparison, sometimes it will remain unnoticed how expensive the use of such an incorrectly set heating system is. In such an unbalanced heating system the consumption of energy may rise even by 15%. Another popular method of compensating the system, which also gives only limited results, is the application of so-called control valves. Such valves limit the volume of water in parts of the system depending on the arrangement. The kv value of the valve is calculated based on the required stream of the medium needed to meet the demand for the heat of the valve and the available pressure difference for the valve. Of course, there are radiators with good and bad hydraulic locations, e.g. situated close or far from the pump and radiators of variegated sizes. Other problems may concern the distribution of heat among the specific radiators in the system. Therefore the regulation stipulates the exact guidelines concerning the compensation. In Item the regulation states: For water heating systems there must be a possibility of reducing the volume of flow in each heating surface. Adjustable thermostat valves give the particularly ample opportunity to limit the volume of flow directly at the radiator (Photo 2). The setting of such a radiator is possible for a fitter without much work and the setting is easily checked by removing the body of the valve. Proper pressure is the key Apart from the distribution of water in the system the proper allocation of pressures should also be kept in mind. The Regulation of the German Industrial Standard explains in Item : Circulating pumps, the accessories and the pipe efficiency should be regulated in such a way that within the foreseen variations of operating

3 conditions the sufficient amount of water supply is ensured and that the permissible level of noise in the water-level gauge is not exceeded. If, for example, the excessive value of pressure difference is foreseen when the operation is weak, proper steps are to be taken, e.g. devices regulating the pressure difference should be fitted. Thermostatic valves endure higher differences of pressures, however it cannot, due to acoustic reasons, exceed 0.2 bar. An adjustable pump may be used as a device controlling the pressure difference. In order to control the pressure difference in smaller systems, where the tolerance of pressure difference does not exceed 0.2 bar, the pump controlling pressure difference is sufficient. For larger systems with a higher tolerance of pressure difference, it is not sufficient. On the valves located close to the pump the pressure is always higher than the designed values. In the remote valves, however, the pressure difference for the reverse flow may rise above the acceptable level. How do the reverse flow of the stream and the increased pressure difference originate. Thermostatic valves are independent controllers. They control the temperature of a room by changing the volume of water flowing to radiators. If the temperature rises, a thermostatic valve reduces the supply of water to the radiator. Nonetheless, thermostat valves also receive heat from other sources, such as sunshine, heat emitted by bulbs, television sets, electric stoves etc. The presence of a larger number of persons in a room also leads to the increase of temperature and results in the reverse movement of water. As a result only in some rare cases a designed amount of water circulates in the system. In modern systems the amount of water is always variegated. Another cause of the reverse stream are excessive initial temperatures, which compensate the faulty hydraulic compensation. If after a night reduction the temperature in the room reaches the level set at the thermostat, the stand-by mode turns on. This means that the thermostat valve provides the amount of water that is necessary to cover the demand for the losses of heat in rooms. When the initial temperature rises by just 5K, the efficiency of a radiator may be read from the diagram when the specific gravity of the reverse movement of water is 70% of the designed stream. Obviously, the habits of the users also lead to the reduction of the amount of water. Some rooms, for example bedrooms, are rarely heated or the temperature there is temporarily lowered. The combination of all these effects results in the fact that in most heating systems only 50-60% of the designed amount of water flows. This dynamics of the system should

4 be taken into consideration while designing a system. However, a commonly used model of a heating system is static, see the commented case! Manual control valves lose their efficiency under a partial load of the system Control valves are sometimes used to compensate the excessive difference or pressures in a system. Such valves are adjusted to the conditions of the system in terms of specific gravity. As it has already been mentioned that kv value for the setting is calculated using the specific stream and in the presented case the pressure difference limited by the valve. It is also recommended to reduce the control valve with a measuring computer. On the other hand valves lose their efficiency under a partial load. Here is an example of this phenomenon: in a medium-size system the centrally controlled pressure amounts to 0.4 bar. In the presented case the loss of pressure in the piping is 0.1 bar. At the thermostatic valves this pressure should also drop by 0.1 bar. Theoretically, the control valve shall be loaded with the pressure limited by 0.2 bar. According to the formula the kv value shall be obtained that should be set at the control valve: k v Drossel = V Nenn pnenn (translation: German: Drossel gland, Nenn specific) If the stream gets smaller in actual operation by 50%, the following condition prevails in the system: the loss of pressure in the piping is reduced by bar. The actual lost pressure on the control valve amounts additionally only to 0.05 bar! according to the calculation: P P Drossel 50% Drossel 50% V50% = kv Drossel = 0,25 p 2 Nenn

5 On the thermostat valve the differential pressure, in spite of the control valve, amounts to: pthv pthv = 0,4 - prohr50% = 0,325 bar - pdrossel50% (translation; Rohr pipes) Hence under a partial load the differential pressures are transmitted from the piping onto the control valve on the thermostat valve. The permissible border values are exceeded. This hypothetical calculation is binding only when the central pressure is maintained at a constant level by a controlled pump. An uncontrolled pump shall produce high differential pressure in the reverse stream, so that the conditions at the thermostat valve shall be even worse (Photo 4). Manual control valves are not compliant with the statutory requirements concerning the systems controlling the pressure difference, as they are not capable of compensating the effects of the reverse stream. Controllers of the differential pressure in a system fulfil the requirements of regulations The controllers of differential pressures maintain the differential pressure in a system at a constant, set level, irrespective of the conditions in which the system operates (Photo 5). Hence it is not possible to exceed the permissible value of pressure difference and thermostat valves will not have to function as limiters. The latest generation of controllers has been designed specifically for such applications. Thanks to the use of an individual control membrane for each exit width and the arrangement of the operation elements in a 90-degree angle bar it is characterized by the optimal qualities of control and an exceptional compact construction (Photo 6). The application of pressure difference regulators in a network offers additional advantages.

6 In Item the Decree of the German Industrial Standard explains: For thermostat valves in radiators the hydraulic compensation is required, so that the valves offer sufficient resistance on the device limiting the difference or pressures connected with the system in relation to the maximal possible differential pressure. Hence the characteristics of a valve are at question. It is obviously better controlled here using the controller of the differential pressure, as the maximal possible differential pressure of a network is fixed. Hence the characteristics of the valve do not need to be modified. The application of the connection of a controller of the differential pressure in a system with an adjustable thermostat valve Owing to the use of adjustable combinations of thermostat valves and the controller of differential pressures in a system both the requirements of the official regulations are met and the steps are taken to control the differential pressure. At the same time the characteristics of the valve required by the official regulations are fulfilled. The system with the constant operation, which treats the initial energy economically, can be set in an easy way. High operating costs and complaints are avoided. Satisfied clients are the best advertisement for each designing office and a company installing the system. Photos: Danfoss Heating and Cooling Technology Ltd., Heusenstamm