Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 152 (2016) 327 – 331
International Conference on Oil and Gas Engineering, OGE-2016
The calculation features of consecutively switched on condensers at their composite cooling Maksimenko V.A.a, Fot A.N.a* a
Omsk State Technical University, 11, Mira Pr., Omsk, 644050, Russian Federation
Abstract Realization of the condensers composite cooling system allows stabilizing condensation pressure, to cut power and water consumption on cold production. The calculation technique providing determination of rational condensation pressure in such systems is presented. © 2016 2016The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University. Peer-review under responsibility of the Omsk State Technical University Keywords: composite cooling, condenser unit, water and air cooling, ecology, economy of energy resources, mathematical model,
cold producing machine
1. Introduction The price of electric energy and fresh water is increasing currently, and their consumption affects the cost of the products increasingly [1-5,10,13]. Cold producing machines represent a class of equipment consuming the most of these resources at the enterprises of various branches. The analysis of the cold producing machine operation shows that pressure (temperature) of condensation and a method of condensers cooling affect energy resources consumption the most. The method of cooling condensers with water and air have various thermodynamic and operational characteristics. Some of them change during the operation period. Therefore, applying only one cooling methods it is impossible to achieve the best operational indicators of the cold producing machine. Applying schemes of condensers cooling with water and air combined is connected with their calculation and regulation issues. In this article, the features of the composite scheme for condensers cooling and its calculation are considered.
* Corresponding author. Tel.: +7-913-966-3464. E-mail address:
[email protected]
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the Omsk State Technical University
doi:10.1016/j.proeng.2016.07.711
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2. Study subject In works [6,11,12,15] various schemes of switching-on heat-exchange apparatus are proposed. The main of them are a consecutive and parallel switching on along the cooled product. Parallel switching-on of the cold producing machine condensers is considered the most preferable since it allows providing low hydraulic losses in a condenser unit, and also solves the problem of a heat-exchange surface flooding by condensate removal from every heat exchanger. The drawback of switching on the condensers in such a way is a high requirement to the hydraulic resistance uniformity of every condenser in parallel, and the need for complete symmetry of the connecting pipelines. Non-compliance with these requirements leads to flooding of the condenser with the smallest hydraulic resistance and decrease in a useful heat-exchange surface. Considering these features, as a rule, identical condensers are used at parallel connection. At composite cooling of the condensation unit, it is clear that to provide equal hydraulic resistance values for condensers of water and air cooling is quite complicated. Therefore, the scheme of consecutive condensers switching-on (Fig. 1) is to be considered, since it allows neglecting the hydraulic features of heat-exchange apparatus.
Fig. 1. The diagram of the cold producing machine with a condensation unit of the composite cooling. Patterns of the coolant flow in condensation unit: "from air to water condenser", "from water to air condenser". Tа is the temperature of the cooling air, Tw is the temperature of the cooling water, ts1 and ts2 are the product temperatures at the entrance and the exit of the evaporator, 1 is the compressor, 2 is the condenser of air cooling, 3 is the condenser of water cooling, 4 is the liquid separator.
When condensers are in series, it is necessary to solve a problem of condensate removal after the first condenser along the coolant flow. The need for condensate removal after the first condenser is explained by danger of the subsequent condenser flooding and, also by the essential hydraulic losses along two-phase coolant flow in the condenser [7,11,14,15]. An opportunity to change the sequence of coolant flow in condensers is provided in the diagram of the cold producing machine with condensers in series (Fig. 1), that is demonstrated by a solid line (cooling in water, then in the air condenser) and a dotted one (the inverse sequence).
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At composite condensers cooling, the degree of the refrigerating agent dryness at the exit from the first condenser can accept values from 0 to 1, therefore, the amount of the condensed coolant can change significantly. For effective removal of the condensed coolant the liquid separator is used (Fig. 1) [8]. The main advantage of the combined condensation unit scheme is an essential opportunity to set any condensation pressure (temperature) corresponding to the current temperatures range for the cooling water and air. This is achieved by redistribution of condensation heat in a necessary proportion between condensers of water and air cooling. 3. Methods To calculate the condenser's thermal balance formula (1) is used in various publications [14,15].
Qk
Ga 'i ,
(1)
where Qk is condensation heat, Ga is mass coolant consumption in the condenser, Δi is a difference of coolant enthalpies at the entrance and the exit of the condenser. In case of the composite cooling (Fig. 1), the refrigerating cycle takes the form shown in Fig. 2, therefore, formula (1) considers no condensed coolant drain and cannot be applied to describe operation of a composite cooling condensation unit in general.
Fig. 2. A refrigerating cycle with a composite cooling condensation unit.
Also it is necessary to pay attention to the fact that the refrigerating agent condensed in the first condenser is drained in saturated liquid state (Fig. 2), whereas after the second condenser the coolant can overcool (Fig. 2). Therefore, it is necessary to determine the state of the coolants mixture after the first and second condensers (Fig. 2). To calculate the thermal balance of the composite cooling condensation unit it is necessary to consider the amount of the drained coolant after the first condenser. Having used the diagram of a refrigerating cycle in Fig. 2 it is possible to make expression (2) for the mass of the coolant drained into a liquid separator.
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Gx
Ga 1 x
(2)
where Gx is the mass of the coolant drained into a liquid separator, and x is the coolant dryness degree. Thus, the equation of thermal balance for a composite cooling condensation unit will take the form of expression (3).
Qk
Q1 Q2
Ga i5 i8' ,
(3)
where Q1 and Q2 are condensation heat in the first and second condensers, and i1-9 is the coolant enthalpy according to Fig. 2. Based on expressions (2) and (3) and the known techniques of calculating cold producing machines' condensers [9, 15], operating conditions of a composite cooling condensation unit are possible to predict by means of calculation. 4. Results and discussion Calculation was carried out for the ammoniac refrigeration unit with boiling temperature of 269°K, the cooling environment temperatures are set according to statistical data for the Omsk Region. Obligatory condensation (at least, the production of saturated steam) in each condenser is accepted as a boundary condition. The condensers of water and air cooling are capable of removing the condensation heat completely. The calculations results are presented in Fig. 3.
Fig. 3. The condensers' specific loads when the loads distribution between the condensers and the condensers' connection scheme are changing. The condensers' connection scheme: 0 is natural coolant circulation, 1 is consecutive switching on of the condensers, firstly, the water cooling, then the air cooling, 2 is consecutive switching-on of the condensers, firstly, the air cooling, then the water cooling.
V.A. Maksimenko and A.N. Fot / Procedia Engineering 152 (2016) 327 – 331
When the temperature of the cooling environment changes, loads redistribution and the change of the condensers switching-on scheme take place (Fig. 3). When the temperature values for outside air are low, the most part of condensation heat is accounted for by the condenser of air cooling. The condenser of air cooling is connected as the second one along the coolant flow. When air temperature approaches water temperature, the thermal load is redistributed mainly to the condenser of water cooling. During this period the growth of the condensation unit's specific load of is especially pronounced. After the cooling environments temperatures reached the balance, the switching-on of the condensers is changed, i.e. the condenser of water cooling becomes the second one along the coolant flow. After that the condensation unit's specific load increases (Fig. 3). 5. Conclusion The offered equations (2) and (3) are recommended to calculate composite cooling condensation unit with intermediate drain of a liquid working body after the first condenser along the coolant flow. The calculations performed with the help of the proposed calculation method for a composite cooling condensation unit demonstrate the possibility of load distribution between the condensers, and, as a result, maintenance of necessary condensation pressure. Since the redistribution of condensation heat is provided by changing the cooling environments consumption, and the resulting condensation pressure determines the consumption of the energy spent on coolant compression, through combining these indicators it is possible to achieve the minimal resource consumption during cold production at all temperature ranges of the cooling environments.
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