Economizer with exhaust damper, fresh air damper, returan air damper and exhaust air damper. The dampers can be automaticlly controlled by PLC controller or by manual based on project needs.
This type HVAC system is another energy saving in the field of Air Conditioning.
Interlocked to run when return air dampers are being closed and supply air blower is in operation. The extraction fans run when outdoor air dampers are at least 50% open (adjustable). It is also overload protected. A gravity exhaust damper is supplied with this option to prevent air from entering the unit during shutdown.The power exhaust fans have been size to exhaust 50% of the nominal air flow of the unit.
The schematic diagram as below like free cooling system.
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Recently, it has been hot and rainy for a long time. It has caused some negative impacts on fishery production for many years. In particular, it is one of the leading products of Dantu District’s technology-based households. The four measures actually deal with the drought and ensure the dry years. Do not cut production, win the harvest. 
Four measures to cope with high temperatures and less rain to ensure no reduction in dry years
[China Agricultural Machinery Industry News] Fisheries refers to the social production sector that fish and other aquatic animals such as aquatic animals and seaweeds to obtain aquatic products. Generally divided into marine fisheries and freshwater fisheries. Fisheries provide food and industrial raw materials for people's lives and nation building.
Four measures to cope with high temperatures and less rain to ensure no reduction in dry years
First, do a good job in water quality regulation
Due to the limited water level in the pond and the large amount of evaporation, in order to prevent the water quality from deteriorating, in the current situation, the harmful substances in the water should be degraded regularly with the substrate and water quality improver, and the disinfectant should be used to eliminate the harmful pathogens on the fish body. .
“Aquaculture: for high temperatures and low rainfall in summer
Second, carry out classification guidance
The township fishery science and technology introductory instructors should go deep into Tangkou and classify the breeding species and Tangkou conditions to ensure that the aquatic products in Dantu District can safely cross the summer under the service platform of fishery science and technology.
Third, increase pond water storage
Demonstration households can take all possible means to obtain exogenous water sources, such as rivers and rivers. It is also possible to properly supplement the pond water with low density of cultivation to the ponds with high density of cultivation, or to treat the ponds according to their own farming conditions. The amount of water is properly concentrated. When entering the water, it is necessary to understand the water quality status of the external water, and ensure that the clean water enters the pond to ensure that the fish and the water in the pond are not harmed.
Fourth, strengthen the management of river crab ponds
The long-term high temperature has a great impact on the main main river crab culture in Dantu District. At present, we mainly do four aspects of work: one should add water as much as possible, increase the pond water level, and reduce the pond water temperature; second, we must maintain the reasonable distribution and quantity of water plants. Excessive removal, too little to be transplanted, and third use of microporous aeration and other facilities to increase dissolved oxygen and reduce harmful factors; Fourth, appropriate increase of plant-based bait, reduce feeding of animal bait.
(Original title: Aquaculture: Four measures for high temperatures and low rainfall in summer)