Ground source heat pump and its replacement for boilers

In the north (such as Beijing and Tianjin), some large and medium-sized cities and governments have successively introduced policies for coal-fired boiler reconstruction for the purpose of reducing pollution and protecting the environment. Ground source heat pump has the advantages of cooling, heating, cleaning, environmental protection, energy saving and stable operation. The operating cost is only equivalent to that of coal-fired boilers, which is far lower than that of general gas, oil-fired boilers and lithium bromide absorption units and becomes a coal-fired boiler A good alternative is favored by the old boiler heating system. Table 2 compares several heating methods.

In the old boiler heating system renovation plan, people very much hope that the new ground-source heat pump units can be the best direct replacement of the boiler, the radiator is still using hot water circulation heating, heating reform involved so far is only the boiler room Transformation, the old indoor radiator heating system will not be the slightest change, which will greatly reduce the amount of renovation and reconstruction costs.

However, this kind of direct replacement of boilers using ground-source heat pumps in practical engineering applications is subject to the following conditions:

Only to meet the above conditions, the use of high-temperature water source heat pump directly to replace the boiler operation is feasible, otherwise the indoor heating system must be modified, you can increase the heating area, double pipelines to increase the water supply pipe to achieve. Of course, it is also possible to transform the heating system into a central air conditioning terminal system while addressing the need for cooling in the summer.

5.2 heat pump water temperature problems

The hot water supply temperature of a heat pump unit can not be too high due to the limitations of the working range of the refrigerant, lubricating oil and compressor, including the exhaust temperature, the exhaust pressure, the return air temperature and the return air pressure.

High temperature water source heat pump unit is usually used R134a refrigerant, at the same saturation temperature, R134a lower than R22 saturation pressure, which means that the same design pressure in the compressor and heat exchanger, you can ensure that The unit with R134a has higher condensing temperature, which can release condensing heat to the hot water with higher temperature. Compared with the heat pump unit with R22 refrigerant, the outlet temperature can be increased to some extent. However, the corresponding condensing temperature and evaporation The temperature is still limited.

R134a also known as tetrafluoroethane, molecular formula C 2 H 2 F 4 , molecular weight of 102.0, the standard boiling point of -26.2 ℃, the solidification temperature -101.1 ℃, the critical temperature of 101.1 ℃, the critical pressure of 4.06MPa, ODP = 0, GWP = 1300. The same condensing pressure of 20bar, R134a unit condensing temperature of 67.5 ℃, while the R22 unit condensing temperature is only 51.5 ℃. However, the unit volumetric capacity of R134a refrigerant is about 30% lower than that of R22, which means the same unit uses R134a. The refrigerant capacity is 25% -30% lower than the unit using R22 refrigerant. In addition, R134a As a refrigerant, due to the poor solubility of R134a and mineral oil, the need for polyester oil for lubricating oil, have better mutual solubility, this lubricant is commonly referred to as POE oil, POE has a significant Features: Viscosity coefficient decreases significantly with increasing temperature, eg Mobil

EAL 22 has a viscosity coefficient of up to 23 cSt at 40 ° C. When the temperature reaches 100 ° C, the viscosity coefficient decreases to 4.7 cSt, and the decrease of the viscosity coefficient leads to a significant decrease of its lubricating performance. That is to say, when the compressor discharge temperature is too high , Lubricating oil performance deterioration, can not guarantee the full lubrication of the compressor.

Compressor manufacturers have a limited scope of work on the compressor, the provisions of the compressor exhaust temperature and exhaust temperature (approximate condensing temperature) such as:

Copeland provides its maximum exhaust temperature limit of 110 ℃, exhaust saturation temperature: scroll compressor 60 ℃, full seal piston compressor is 60 ℃, semi-sealed piston compressor is 60 ℃, the most advanced Dicus compressor 60 ° C.

Bitzer's exhaust temperature is set to: semi-sealed piston and screw compressor safety exhaust saturation temperature of 70 ℃, the use of spray cooling compressor safety exhaust saturation temperature of 75 ℃, even with the machine outside the oil cooling + spray The cooled exhaust compressor has a safety vent saturation temperature of 80 ° C, of ​​course, at this time, the economic efficiency is already poor.

The Carlyle 06D / E compressor, which provides compressors for Carrie and Trane, has a safe exhaust temperature of 105 ° C. The safe exhaust temperature is 30 ° C to the exhaust saturation temperature, which means that the safe exhaust saturation temperature is only 75 ° C. FrasCold compressors, which make special compressors, have also made no breakthrough in this regard.

Looking at the above compressor compressor manufacturers, safe and economical maximum exhaust temperature is also 75 ℃, even assuming the system design efficiency is very high, the exhaust pressure loss (usually 1 ℃ pressure loss), the largest Condenser condensing pressure can only reach 75 ℃, to maintain an economical condenser size, the general condenser cooling liquid (hot water) and the refrigerant condensing temperature of about 5 ℃ heat transfer temperature difference, so the hot water temperature is the highest Also at 70 ℃.

Therefore, hot water supply at a higher temperature is required, other auxiliary heating methods should be used, or only the heat pump solution can be discarded.

5.3 heat pump system energy utilization

Air-source heat pump in the cooling, the condensation of the heat most of the unnecessary waste. In order to take full advantage of this part of the energy, some manufacturers take "waste heat" recovery approach can be clever use of this part of the heat (used as domestic hot water), the practice is in the high temperature and pressure of gaseous refrigerant into the condenser before, Plus a set of heat recovery heat exchange device for heating domestic hot water, depending on the degree of superheat of the refrigerant exhaust, the temperature can be quite high hot water.

For ground-source heat pumps, because of their design features, they can also provide a considerable amount of hot water for domestic use in the cooling operation and recover wasted heat to produce sanitary hot water.

In short, with the increasing energy shortage and the development of heat pump technology, the ground-source heat pump will be recognized and applied by everyone for its outstanding work efficiency. At the same time, making full use of the "waste heat" of the heat pump will also be the development trend of the heat pump in the future.

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