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Purpose This study aims to simulate the flow and heat transfer through an air handling unit to reduce its energy consumption by a novel creative idea of using an air-to-air heat exchanger. Design/methodology/approach To do this, both first and second laws of thermodynamics energy and exergy balance equations were solved numerically by an appropriate developed computer code. Findings Using the air-to-air heat exchanger in dry conditions decreases the cooling coil load by 0.9 per cent, whereas the reduction for humid conditions is 27 per cent. Similarly, using air-to-air heat exchanger leads to an increase in the first law of efficiency in dry and humid conditions by 0.9 per cent and 36.8 per cent, respectively. Originality/value The second law of efficiency increases by 1.55 per cent and 2.77 per cent in dry and humid conditions, respectively. In other words, the effect of using an air-to-air heat exchanger in humid conditions is more than that in dry conditions.

Wireless sensor networks have attracted much attention because of many applications in the fields of industry, military, medicine, agriculture, and education. In addition, the vast majority of research has been done to expand their applications and improve their efficiency. However, there are still many challenges for increasing the efficiency in different parts of this network. One of the most important parts is to improve the network lifetime in the wireless sensor network. Since the sensor nodes are generally powered by batteries, the most important issue to consider in these types of networks is to reduce the power consumption of the nodes in such a way as to increase the network lifetime to an acceptable level. The contribution of this paper is using topology control, the threshold for the remaining energy in nodes, and two metaheuristic algorithms, namely SA (Simulated Annealing) and VNS (Variable Neighbourhood Search), to increase the energy remaining in the sensors. Moreover, using a low-cost spanning tree, an appropriate connectivity control among nodes is created in the network in order to increase the network lifetime. The results of simulations show that the proposed method improves the sensor lifetime and reduces the energy consumed.