ANS:C - high labour cost & low heat economy.

In the context of batch tray dryers, high labor cost and low heat economy refer to two distinct but related issues:

1. High labor cost: Batch tray dryers typically require manual loading and unloading of the product onto the trays. This manual handling of the product adds to the labor costs associated with the drying process. Additionally, batch operations may require frequent monitoring and adjustment of parameters such as temperature and airflow, further increasing labor requirements. Overall, the manual labor involved in operating batch tray dryers contributes to higher labor costs compared to automated or continuous drying methods.
2. Low heat economy: Batch tray dryers often operate on intermittent heating cycles, where the heating elements are turned on and off to maintain the desired temperature within the dryer. This cyclic heating can result in fluctuations in the internal temperature of the dryer, leading to inefficient use of heat energy. Moreover, since batch tray dryers are designed to process a fixed quantity of product per batch, there may be periods of time during the drying cycle when the dryer is not fully utilized, resulting in wasted heat energy. These inefficiencies in heat utilization contribute to a low heat economy, where a significant portion of the energy input is not effectively utilized for drying the product.

Addressing these issues often involves implementing process improvements or adopting alternative drying technologies that offer higher levels of automation, energy efficiency, and process control. This may include upgrading to continuous drying systems, implementing advanced control systems, or optimizing the design and operation of batch tray dryers to minimize energy consumption and labor requirements.