Normal vacuum tube

COMMON VACUUM TUBE

Normal vacuum tube

Evergreen normal vacuum tube

Description

Normal vacuum tube

Vacuated tubes are the most improtant part of the solar water heater which works as an absorber. It absorbs solar energy and converting it into heat to transfor the heat into water.There are several types of evacuated tubes in use in the solar industry. It designs and the twin-glass tube".which is similar to a conventional Dewar flask.

Each vacuum tube consists of two borosilicate glass . the inner tube is coated with a sputtered solar selective coating (Al-N/Al) , this coated inner tube is closed at one end and sealed at the other end to the outer tube.the annular space between the outer tube and inner tube is evacuated to virtualy eliminate heat loss by conduction and convection. And the outer tube is transparent allowing light rays to pass through with minimal reflection. This "evacuation" of the gasses forms a vacuum, which is an important factor in the performance of the evacuated tubes.

Basic information of the vacuum tube:
1.Why a vacuum? As you would know if you have used a glass lined thermos flask, a vacuum is an excellent insulator. This is important because once the evacuated tube absorbs the radiation from the sun and converts it to heat, we don’t want to lose it!! The vacuum helps to achieve this. The insulation properties are so good that while the inside of the tube may be 150oC / 304oF , the outer tube is cold to touch. This means that evacuated tube
solar collector can perform well even in cold weather when flat plate collectors perform poorly due to heat loss (during high Delta-T conditions).

2.In order to maintain the vacuum between the two glass layers, a barium getter is used. During manufacture of the three target vacuum tube of this getter is exposed to high temperatures which causes the bottom of the evacuated tube to be coated with a pure layer of barium. This barium layer actively absorbs any CO, CO2, N2, O2, H2O and H2 out-gassed from the evacuated tube during storage and operation, thus helping to maintain the vacuum. The barium layer also provides a clear visual indicator of the vacuum status. The silver coloured barium layer will turn white if the vacuum is ever lost. This makes it easy to determine whether or not a tube is in good condition. See picture below.

Strucure		All-glass concentric dual tube geomer
Glass material		Borosilicate glass
Outer tube diameter and thickness		Φ=47±0.7mm & =1.5mm Φ=58±0.7mm & =2mm Φ=70±0.7mm & =2mm
Inter tube diameter and thickness		Φ=37±0.7mm & =1.5mm Φ=47±0.7mm & =1.6mm Φ=58±0.7mm & =1.6mm
Tube Length		1200(±5mm)1500(±5mm) 1700-1800(±5mm) 2000(±5mm)
Absorbing coating properties	Configulation	1200(±5mm) 　1500(±5mm) 1700-1800(±5mm) 2000( ±5mm)
	Mothod to deposition	AL-N/A1 selective absorptive coating
	absorptance	as 0.88-0.93
	Emittance	Σn:-.08-0.04
Vacuum		p ≤ 2 × 10-3 PA
Stagnation parameter		＞250M2C/KW
Sloar rediance exposure for obtaining a preset water temperature		＜2.8MJ/M2
Average heat loss coefficient		＜0.8W(M2C)
Lifetime		15years