Thermal behaviour of closed wet cooling towers for use with chilled ceilings

doi:10.1016/S1359-4311(99)00096-4

Applied Thermal Engineering

Volume 20, Issue 13, 1 September 2000, Pages 1225-1236

https://doi.org/10.1016/S1359-4311(99)00096-4 Get rights and content

Abstract

A new closed wet cooling tower, adapted for use with chilled ceilings in buildings, was tested. Experimental correlations were obtained for mass and heat transfer coefficients. Existing thermal models for this type of cooling tower were found to predict well thermal performance, if the above correlations are used.

Introduction

There is an increasing demand for cooling in buildings. Better insulated buildings and the increased use of working equipment (computers, etc.) led to higher cooling requirements, particularly in office and commercial buildings. The need for reducing energy consumption and CO₂ emissions, together with the need for using environmental-friendly refrigerants, justify a strong demand for CFC free, efficient and cheap cooling systems.

Chilled ceilings are a relatively new approach to cooling, with about 100,000 m² installed in central European countries in the last five years. Chilled ceilings offer several advantages over conventional alternatives. The use of water instead of air reduces energy requirements for energy transportation. It also allows a reduction of ventilation rate to a minimum level, for hygiene purposes. Heat transfer from indoor space to chilled ceilings is made by combined convection and radiation. Radiative heat transfer allows chilled ceilings to remove considerable heat loads at a relatively small temperature difference between room air and ceiling. This makes it possible to run the system with ceiling supply temperatures of about 18–20°C. Heat transfer rates between 25 and 75 W/m² are possible, with lower air velocity in the rooms and a resulting comfortable indoor environment.

Due to the above mentioned moderately high water temperatures used in chilled ceilings, it is possible to deliver cold water with a closed wet cooling tower during most of the cooling period. The cooling tower can be combined with a refrigeration machine or it can be used alone, if some overheating is allowed during short periods, or if energy storage or nightcooling techniques are used.

Closed wet cooling towers were conventionally used to remove excess heat from various industrial processes, with a usual range of 32–46°C hot water temperatures and typical cooling capacities above 40 kW. A recent research work performed in Switzerland, [1], showed that these towers are greatly overpowered in airflow and spray water rate, when applied to the range of 22–25°C hot water temperatures, as needed for chilled ceilings. Tower design for lower cooling loads — $≤10$ kW — leads to smaller tower dimensions.

Existing models to predict thermal performance of closed wet cooling towers were developed for conventional units and operating conditions: high water temperatures and cooling capacities. It is the objective of this work to verify the applicability of existing models to smaller towers, adapted for use with chilled ceilings. For that purpose, a new cooling tower built specifically for use with chilled ceilings was tested. Experimental results were used to introduce corrections to existing model correlations.

Section snippets

Thermal models for a closed wet cooling tower

Thermal models to predict cooling tower performance can be classified as detailed models or correlation based models. Detailed models are based on a CFD-type approach, involving the numerical solution of differential equations for air/spray water flow, energy and water vapour concentration. After velocity, temperature and humidity fields are calculated, transfer coefficients can be calculated as a result. Examples of such models, which need numerical codes and high performance computers, can be

New closed wet cooling tower and experimental results

A new closed wet cooling tower was designed in order to be used with chilled ceilings. Design conditions were a cooling capacity of 10 kW, for an inlet water temperature of 21°C, a water flow rate of 0.8 kg/s and an air wet bulb temperature of 16°C. The tower has a section of 0.6 × 1.2 m and a height of 1.55 m. The tube bundle has 228 staggered tubes of 10 mm outside diameter, with a total transfer area of 8.6 m². This corresponds to a much smaller size than usual towers. The load/volume ratio

Model results

The different thermal models described previously were applied to the new cooling tower geometry.

Model 2 was implemented with a fixed number of nodes, equal to 13 — number of tubes in one column + 1. Model 3, a finite differences model, can be implemented with any number of nodes. However, above 25 nodes no significant difference was noted in the calculated tower efficiency.

Fig. 7 shows experimental and calculated efficiencies using Mizushina’s correlations — , . Results for models 1, 3 and 4

Conclusions

Thermal models are very useful both for designing and predicting cooling tower performance. This is not always possible with experiment, since it is difficult to reproduce all possible operating conditions. Different thermal models to predict the performance of closed wet cooling towers were presented.

A new closed wet cooling tower, adapted for use with chilled ceilings, was tested and experimental correlations for mass and heat transfer coefficients were obtained.

Different models were used and

Acknowledgments

The authors wish to thank the Commission of the European Union (DGXII) for partially funding the work done, under Joule IV research programme. All partners of the ECOCOOL research project are greatly acknowledged: Sulzer Infra Lab (CH), Sulzer Escher Wyss Lindau (DE), University of Nottingham (UK) — Institute of Building Technology, Helsinki University of Technology (FIN) — HVAC Laboratory.

References (10)

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R.I.T Mizushina, H. Miyashita, Characteristics and methods of thermal design of evaporative coolers, Int. Chemical...

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