Microwave vacuum and conventional drying characteristics of round Dendrocalamus asper

Abstract

Drying round bamboo is important to ensure its dimensional stability, to protect it from biological agents and for making high quality products. Conventional kiln drying of bamboo requires extended drying periods. Microwave drying of bamboo has been studied recently and has been reported to be a very fast method of drying. Dendrocalamus asper (Schult. & Schult.f.) Backer is an important, large-sized bamboo, preferred for structural purposes and handicrafts manufacturing. However, it also shows a propensity to develop defects such as cracks during drying. In this study, eight culms of mature D. asper (about 3–3.5 years old) were felled and cut into two halves: upper half and lower half. Both parts were then cut into 900 mm long sections. Twenty sections each from the upper half were dried in a conventional kiln (CKD) at 50 °C and 70% RH; and another 20 sections from the upper half were subjected to microwave-vacuum drying (MWVD) at bamboo wall temperature 60–70 °C and 150 mbar pressure. Similarly, twenty sections each from the lower half bamboo sections were dried using CKD (50 °C, 70% RH), MWVD (bamboo wall at 60–80 °C, 150 mbar) and combination drying in which bamboo sections were first dried in CKD until the moisture content (MC%) dropped to below 30% (26%) and thereafter, further dried using MWVD. The time taken to dry the sections, MC% drop rate, defects, and energy consumption were analysed. MWVD took 12 h (upper half) to 20 h (bottom half) to dry the bamboo sections, whereas CKD took 12–15 days. Combination drying did not offer any major advantages. A lower number of defects were found in MWVD sections compared sections dried by CKD. On average, 3.78 units of electrical energy were consumed for the removal of 1 kg of water from bamboo during MWVD.