Environmental Engineering. Family of heating appliances

Transcription

1 Fakulta strojní Ústav techniky prostředí Family of heating appliances Ing. Ondrej Hojer, Ph.D. Table of contents: A) Design of Floor heating B) Design of a safety valve and an expansion vessel C) Introduction to large space heating 2

2 Design of Floor heating Characteristic floor number m = 2 Λ + Λ a b π 2 k d d [/m] Heat distribution layer (the one with tubes) where: Λ a [W/m 2.K] Λ b [W/m 2.K] k d [W/m.K] d [m] heat transfer coefficient upwards from the tubes (on the fig. red arrows) heat transfer coefficient downwards from the tubes (on the fig. yellow arrows) thermal conductivity of heat distribution layer material (the layer with the tubes) external tube diameter partial heat transmissivity upwards from the tubes Λ = a d i + k h i where : d [m] particular thicknesses of layers above the tubes, k i [W/m.K] thermal conductivities of layers above the tubes, h = h + h = 4, , 25 = 0, 05 W / m 2.K R C partial heat transmissivity downwards from the tubes Λ [W/m 2.K] = = b d i + R + ceil k h h i summary heat transfer coefficient on the floor heat transfer coeff. by convection and by radiation where : R ceil [m 2.K/W] thermal resistance of the ceiling, h [W/m 2.K] summary heat transfer coefficient on the bottom side of the heated floor (standard - in case there is another room this coefficient is chosen as h = 8 W/m 2.K) 2

3 Floor surface temperature is calculated from equation l Λ tgh m 2 t t = a ( t t ) s i h m i l m 2 [ C] where : t m [ C] mean water temperature (t w +t w2 )/2 t i [ C] indoor design temperature m [m - ] characteristic floor number Λ a [W/m 2.K] heat transfer coefficient upwards from the tubes h [W/m 2.K] summary heat transfer coefficient by convection and radiation upwards l [m] tubes distance Principle of the calculation is to keep surface temperature bellow the hygienic limits (floor heating = 29 C). Hence maximal thermal output is limited. Mean surface temperature t s shall not from physiological point of view exceed the value: t s = 27 to 29 C at room for permanent stay (rooms for living, offices,...), t s = 30 to 32 C at service rooms, where people are just temporary (halls, corridors, stairs,...), t s = 32 to 34 C at rooms, where people are mainly walking barefooted (swimming pools, spa, bath rooms...). At given water temperatures t m... (t w +t w2 )/2 and internal design air temperature t i, the mean surface temperature t s depends mainly on distance between tubes l. Other values are either almost constant or they have just small influence on results. The specific thermal heat flux to the room is than defined: q = h t t s i [W/m 2 ] and specific heat flux of the floor surface downwards when both air design temperatures in rooms upwards and downwards are the same h q = Λ t t b Λ s i [W/m 2 ] a 3

4 We the design temperatures up and down are different t i t i downward is calculated q = Λ h b Λ t t + Λ t t s i b i i a [W/m 2 ], specific heat flux This heat flux represents heat loss that has to be minimized as possible. In case there is an unheated room bellow the floor it is necessary to choose for the layers located bellow the tubes materials with lower thermal conductivity than in cases when there is another heated room. Mostly it is required that heat flux downwards shouldn t be higher than approx. 0 up to 5 % of the total heat output of the floor For rooms located below other heated rooms the total heated surface is calculated A P ΦHL,i = q + q [m 2 ] whereϕ HL,i is total heating load of the room (EN 283). In rooms at ground floor or on the highest floor the heated surface is calculated from an equation: A P = φ HL,i q [m 2 ] and total thermal power needϕ PN is for both cases given by equation φ = (q + q ).A P PN [W] 4

5 Design of an Expansion vessel 9 B EV NP M PR PR PR h PR B EV NP M h V et Panel radiator Boiler Expansion vessel Neutral point Manometer Height of water column between highest point of system and neutral point NP =,3 V n o η 0 5

6 t max = t max t min feed water temperature (mostly 0 C) maximal operation temperature η = p p h,max,a p h,max,a d,min,a 2 6

7 3 p =, ρ g h 0 + d,dov,a p B 3 Pressure of the dry air or nitrogen is set to the value: p et,set. = (, až,3).ρ.g.h.0-3 [ kpa ]. 4 7

8 Central air distr. system Floor heating Local air distr. system Radiant Strips Plaque radiant heaters Tube radiant heaters Floor (panel) heating 8

9 Radiant strips (panels) Tube radiant heaters 9

10 Plaque (luminous) radiant heaters Local air-distribution systems 0

11 Central air-distribution systems Vertical space characterization A Occupancy zone B Neutral zone C Roof zone