Christchurch

New Zealand - Closed-cycle compression heat pump

 

Summary This technology demonstration at a wood drying kiln in Christchurch was started in December 2000 with the purpose to evaluate the technology and cost of new modular designs for potential commercial production and marketing purpose. The existing compressors were replaced with a new the new modular heat pump system. The process is currently undergoing optimisation and hence performance data is not available. This case study can be viewed in parallel with a similar example presented in Performance.

Industry/process

 

Industry type:	Wood
Location:	Christchurch, New Zealand
Description of the process: The timber is loaded into the kiln, which is then preheated by a conventional method to a required temperature. The dehumidification process is started and continued until the timber is dried.

Heat pump system

 

Application:	Wood drying
Heat pump type(s):	Closed cycle compression
Heat pump installed capacity (kW)	2 x 15 (compressors)
Drive energy:	Electricity
Refrigerant:	R134a
If substituted, what was the previous refrigerant::	R22 used in old system
Heat pump system completion date:	December 2000
Description of the heat flows:	No data.

Performance
Data for this particular installation is unavailable as the process is undergoing optimisation, which is due to be completed in early 2002. Compared to the old systems the efficiency has improved by 50%.

Operational experience and other comments
The heat pump has run well and reliably since the commissioning. The overall drying cycle average energy efficiency, or specific moisture extraction rate (SMER), is between 2-3 kg H2O/kWh, depending on other conditions. The dehumidifier capacity needs to be matched with kiln fan airflow systems.

Contacts

 

Contact 1:	Dr Guang Chen
Company:	Energy Group Ltd
Role:	Research engineer
Address:	PO Box 738 Dunedin NEW ZEALAND
Telephone:	+ 64 3 474 2082
Fax:	+ 64 3 479 0759
Email:	Guang @ eglnet.com

Additional information
The following report describes a set of experiments carried out on wood drying kiln in Dunedin, New Zealand: Paul Bannister, Bipan Bansal, Heat pump dehumidifier drying, Final trial and general summary: Atlas Timber trials, 28 June 1996, Energy Research, 14A Wickliffe Street, PO Box 738, Dunedin, New Zealand.

 

Energy Research Otago Limited carried out some wood drying trials at Atlas Timber Components Ltd in Dunedin. Several modifications were made in this kiln configuration during the trials because the power required for the four kiln air circulation fans appeared to be compromising the overall efficiency and the apparent level of air leakage was excessive. The modifications included:

• Disconnection of two of the kiln circulation fans and thus halving the kiln fan power. A reasonably even flow of air was maintained in the kiln by retaining the two outer fans in operation while disconnecting and blocking the central two fans. The measured average air velocity through the timber was 1 m/s.

• Removal of the damaged polystyrene sheets, which insulated the concrete floor of the kiln in the first three trials.

• Installation of a humidistat control to enable modulating the dehumidifier so as to prevent the kiln humidity from dropping below 35-40% (as compared to the figures of 20% and below observed in the final stages of the earlier trials). The low relative humidity was considered to be a possible cause of stress in the timber.

• The use of timber of a different shape (more square) in the fourth trial.

The use of a reduced level of fan power appears to have resulted in an improvement in process efficiency, although other factors may also have contributed. Furthermore, the use of humidistat control on the dehumidifier to maintain a minimum level of humidity at the end of the trial appears to have resulted in a reduction in stress in the timber. In the fourth trial about 19% and 47% less energy was used for the dehumidifier and the kiln fans, respectively. The run time was several times longer.

 

The peak specific moisture extraction ratio (SMER) achieved by the dehumidifier in the trials was 5.06 kg/kWh, and the peak moisture extraction rate was 25.0 kg/hr. The highest average specific moisture extraction ratio achieved by the total kiln (including circulation fans) was 2.53 kg/kWh, and the highest average moisture extraction rate was 18.6 kg/hr. The average moisture extraction rate was slightly lower than in most of the previous trials, probably due to the lower temperature achieved and the operation of the humidity control in the final stages. The overall process efficiency achieved in the fourth trial was the highest of all the trials conducted at Atlas Timber.

 

The inadequate level of insulation provided by the kiln had a negative impact on the performances achieved in the trials. The high level of heat losses prevented the optimum operating conditions for the dehumidifier being reached, which in turn depressed both the efficiency and capacity of the unit.

 

The kiln air circulation system is also identified as an area where significant improvements in energy efficiency could be made. It is shown that reversal of the air flow through the timber stack has potential for the improvement of efficiency and drying rate. Further work is required to optimise operating schedules and kiln design to achieve a good compromise between efficiency, drying rate, and product quality.

 

 

Last updated: 1 March 2004

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