AIR HANDLING THERMAL CONDITIONS Parts of air handling unit (=AHU): Outdoor air grille Air chamber Outdoor air damper Filter Heat recovery unit Heating coil Fan Sound absorber Mixing chamber Per-heating coil Humidifier Cooling coil Air conditioning systems consist of air handling units, ducts with components and air distribution devices Source: Rakennusten sisäilmasto ja LVI-tekniikka. O&M Seppänen. XAMK/Heikki Salomaa 1
HEAT RECOVERY Considerable share of exhaust air s heat content can be recovered using heat recovery unit (=HRU) commonly supply air is heated utilizing recovered heat - temperature level is low (< 25 o C) - heat pumps must be used, if higher temperature levels are necessary e.g. for heating or for hot water Heat transfer will be more efficient, when temperature difference is high Humidity of exhaust air will limit the effect of HRU during cold periods Source: XAMK/Heikki Salomaa 2
TEMPERATURE EFFICIENCY Realized temperature rise of supply air is compared with the theoretical maximum Theoretical maximum would be achieved, if supply air would be heated up to the temperature of exhaust air Supply air Exhaust air Outdoor air Exit air Source: In the figure +23 o C +10 o C -5 o C An example calculation XAMK/Heikki Salomaa 3
PLATE HEAT EXCHANGER Heat transfers from extract air to outdoor air through metal plates which are separating the air flows Direct recuperative heat exchanger Supply and extract air ducts must be combined to the same HRU-unit in the machine room => there must be enough space (i.e. height) Heat transfers according to cross-flow principles (counter flow would be better) Effect of heat exchanger depends mainly on the surface area Aim: wide surface area in a small volume Counterflow plate heat exchanger Cross-flow plate heat exchanger HRU-unit installed in the AHU Source: Rakennusten sisäilmasto ja LVI-tekniikka. O&M Seppänen. Source: Swegon XAMK/Heikki Salomaa 4
PLATE HEAT EXCHANGERS (2) Temperature efficiency 50-60% Supply and exhaust air flows are apart from each others Risk to move bacteria and odours to fresh air is very small Simple apparatus, no moving parts, no need for control devices or pumps Space requirement quite big Risk of frosting - condensate may freeze to ice when outdoor temperature drops below ca. -7 o C Effect adjustment and smelting quite difficult Recuterm Source: XAMK/Heikki Salomaa 5
LIQUID-COUPLED HEAT RECOVERY SYSTEM No need to install big exhaust and supply air ducts in the same place Usable also in renovations Indirect recuperative heat exchanger Efficiency is lower compared with plate heat exchanger Water with an anti-freeze additive (30 40%) is used as heat conveying medium Heat exchangers are composed of copper pipes with profiled aluminium or copper fins Principle of liquid-coupled heat recovery system. Heat is transferred from the exhaust air unit (above) to the supply air unit (below) utilizing liquid circulations Source: Rakennusten sisäilmasto ja LVI-tekniikka. O&M Seppänen. XAMK/Heikki Salomaa 6
LIQUID-COUPLED HRU (2) Temperature efficiency 45-55% Heat is transferred utilizing liquid circulation Exhaust and supply air duct can locate far away from each others No risk to mix exhaust air to supply air Supply and exhaust units can be separated Effect adjustment and anti-frost protection is easy to put into practise Ecoterm Lähde: XAMK/Heikki Salomaa 7
ROTARY HEAT EXCHANGER Rotating wheel accumulates heat The mass of the rotor gets warmer while exhaust air flows trough it Heat from the mass transfers to supply air and the wheel cools down again Regenerative (heat accumulating) heat exchanger Effective heat transfer is based on - rotor s small triangular ducts, made of thin aluminium material Rotating wheel can also transfer moisture Small amount of impurities is also moved to the supply air side Rotating and honeycombed wheel heat exchanger accumulates heat from exhaust air and transfers it to outdoor air Source: Rakennusten sisäilmasto ja LVI-tekniikka. O&M Seppänen. XAMK/Heikki Salomaa 8
ROTATING AND ACCUMULATING HEAT AND COOLING ENERGY RECOVERY Recovery device of heat and cooling energy is rotating (max ca. 15 r/min) Temperature efficiency: 75-85 % Moisture transfer and recovery of cooling energy is possible besides heat recovery Space requirement is quite small The drive apparatus has an electro motor for variable (or constant) speed, transmission and drive belt Control equipment regulates the rotor speed from top speed of ca. 20 rpm to minimum of ca. 0,5 rpm Sourcee: Regoterm, Turboterm XAMK/Heikki Salomaa 9
Air Heaters and Air Coolers Coils are commonly used for heating and cooling in AHU s A coil is built-up of tubes arranged in rows, in the direction of air flow and tubes are fitted into holes of the fins (aluminium) the design temperature of water used in heating coils is commonly 60 o C/40 o C (in cooling e.g. 7 o C/12 o C) Air is heated after heat recovery to supply air temperature e.g. +18 o C Source: Source:Motiva XAMK/Heikki Salomaa 10
Humidifiers Humidification principles of AHU`s Contact evaporative humidifier (e.g. http://www.flaktwoods.fi/4915673b-a161-460b-9610-fa20441d5ac7 ) - the temperature of the air drops down the energy for evaporation is taken from the air - hygienic risk needs good maintenance Stem humidifiers (e.g. http://www.flaktwoods.fi/ba87dc84-64aa-4f02-a4fa-7ddce8e8d312 ) - better hygiene - electric power is required for producing steam - negligible air resistance Nozzle humidifiers commonly air is also cleaned at the same time (or heat is recovered from dirty exhaust air) Moistening can be done also directly to the indoor air XAMK/Heikki Salomaa 11