К.т.н. Кошкин И.В., Виничук Н.В.

Костанайский Государственный Университет имени Ахмета Байтурсынова

The most significant hot-pump units’ problems analysis and possible ways to solve them under the weather conditions of Northern Kazakhstan

At the present time heat pump units are becoming more and more popular. The implementation of heat pumps in the energy consumption mix of European countries , China and the U.S. is growing every year. In consequence of that the question of promoting co-efficiency of performance in electric power consumption is hotly discussed.

The following significant problems were observed while analyzing existing heat pump units:

1) A throttling process - working substance undergoes throttling at high temperature, which results in lower specific capacity cycle.

2) The use of ozone-depleting substances - a large number of heat pump units are operating on R22 and R12, banned by Kyoto Protocol.

3) Large temperature difference between the evaporator and condenser is problem related to the climate of North Kazakhstan as a consequence of low freezing temperatures during the winter period.

There are different ways to solve these problems.

The above-mentioned problem of a big temperature difference between the evaporator and condenser is quite typical for the northern region of Kazakhstan. The problem is that the low-grade heat source temperature is quite low (air), while the heating temperature of consuming water has to be rather high (≥ 60 ° C), which requires more intensive compression ratio of the working substance. This operation is not compatible with all working substances, which significantly increases the cost of equipment and materials.

The optimal solution to this problem is two-stage compression cycle. Two-stage compression implies usage of two reciprocating compressors, standing in a sequential order, in a cycle. This solution allows achieving the intended temperature difference at the optimum pressure difference before and after using the compressor.

An important disadvantage of heat pump units exploitation is usage of ozone-depleting substances. A prospective solution is usage of natural working substances (carbon dioxide , ammonia, water vapor) . However, usage of the substances is possible only on the recently implemented or planned facilities. In units, that are in use at the moment, new interchangeable working substances with similar characteristics to freon can be implemented. One of such substances is the prospective working substance elaborated by DuPont company

- retrofit ISCEON ® MO29 (R 422D), which is almost identical to the characteristics of R 22.

Benefits of ISCEON ® MO29:

• non-ozone-depleting HFC working substance with the intermediate formation of the ozonide (ODP = 0) (meets the Montreal Protocol) .

• in accordance with European standards it is not toxic or destructive to health.

• non-flammable, in accordance with the terms of ASTM test E681 -01 ( this applies to both the original composition and accidental release ) .

• compatible with mineral and polyether oils and alkyl benzene oils (in most cases there is no need to change the type of oil while changing the working substance ) .

• lower temperature of the compressed gas in comparison with R- 22 (positively affect the capacitor lifetime) .

It must be observed that the substance is a mixture of refrigerant and it satisfies the concept of the so-called temperature «glide ". This phenomenon implies a evaporation temperature difference between the constituent elements of the working substance . In case of system integrity damage one of the components will evaporate more actively resulting in a loss of the mixture characteristics as a whole and it will make it impossible to refuel. In consequence there are the special requirements to the system integrity are and mode of operation cycle.

 Partial solution to the problem of hot throttling is the implementation of regenerative heat exchanger in the cycle. It is installed in front of the throttle , it cools the working substance and warms up it on entering the compressor at the same time . This process is called internal regeneration cycle.

Complete solution of the hot- throttling problem is possible in case of usage of an additional element in a cycle – condensate cooler. Normally usage of condensate cooler provides the needs of domestic hot water (DHW). However, usage of hot water is not constant, which does not allow to talk about the complete solution of the problem.

In case of usage of the two-stage compression cycle, it is advisable to use the heat of the condensate cooler for warming up of working substance after the first compression stage . This process is called external regeneration.

Negative aspect of external regeneration may be a higher temperature of working fluid after heat exchanger of consumer during sudden thaws , etc. In this case, higher- temperature working substance enters the intermediate stage of compression, wherein at the second compression stage the temperature may rise above the optimum operating parameters that would lead to increased wear of the compressor , and partial decomposition of the working substance.

One of the solutions of above-mentioned problem is to use an accumulator tank as an external source of regeneration. The point of the proposed solution is that the external regeneration is not constant in the cycle, it inputs or outputs depending on the working substance temperature after the heat exchanger and the first stage of the compressor .

The advantages of using this scheme are the following:

1. In comparison with single-stage , multi-stage compression of the working fluid with an intermediate heat sink using accumulator tank can significantly improve the thermodynamic efficiency for big temperature differences in the evaporator and condenser . Theoretical calculation for R 422D showed that usage of regenerative heat exchanger will reduce the cost of vapor compression unit in a real heat pump unit cycle installation by14% .

2. Application of ozone friendly working substance ISCEON ® MO29 is possible without changing the oil in the existing units.

3. Heating the working substance after the first stage of compression due to the heat of the working substance before throttling (external regeneration) allows to minimize unavoidable losses from throttling of the working fluid . This type of regeneration is universal; it can be applied in almost all cases of usage of heat pump units.

4 . Opportunity to ensure consistent performance of consumers heating water through the use of temperature sensors and control accumulator tank.

Литература:

1. President's letter of the Republic of Kazakhstan – Leader of the nation Nursultan Nazarbayev to the people of Kazakhstan «Strategy «Kazakhstan -2050». – 2012. – P.14-18.

2. Tleuov А.Х. Nontraditional sources of energy / А.Х. Tleuov. – Astana: Publishing house «Фолиант». – 2009 – P.1-4.

3. Amerkhanov Р.А. Thermal pumps. - М.: ЭАИ, 2005. –P. 50-84.

4. Бэс Т. Exergy in processes of heating, isushka air conditioning // Questions of the thermodynamic analysis (eksergetichesky method)/ М.: Мир, 1965. - P. 139-149.