Question

A summer air conditioning system with recirculating air stream and BPF = 0 has a room sensible heat load of 20 kW and a room latent heat load of 2.22 kW. The required inside conditions are 22°C  and 50% RH, and the outdoor design conditions are 36°C  and 60% RH. The mass flow rate of the outside air is 15% of the mass flow rate of the supply air.

Find a) RSHF b) the point s on the chart, mass flow rate of supply air, moisture content of supply air and the mass flow rate of the re-circulated air, c) temperature of the mixed air streams; d) Sensible, latent and total heat loads on the coil.

 

Answer 1

In above problem , room and outdoor conditions are given, so these two points ca be easily located on Psychrometric chart. Mixing point also can be shown by percent of fresh air in total air. RSHF can be obtained from given loads and drawing supply line from room point is possible. BPF is zer so supply point will be on ADP point of coil on 100% RH curve (for this case). Now taking required values from chart , asked values are calculated.

Givens BPF- DO room sensible heat load (RSH)=20kW = 20 ks/see. Room latent heat load (RLH) = 2.22 kW=2.22 kJ/see. Inside conditions, td2= 22°C.=db, P2= 50'). - RH. Outdoor design conditions, tdı = 36% = db $.= 60.4. = RH. outside air %=15% of supply air Solution: SHF = 0.9 h3 ha hs' 3 ADP S ted2=22°C td1=36°C Psychrometric chart. Pt. 1 - shows outdoor condition. Pt-2 shows room Condition. 15-4. of mass flow rate of supply air (total air) given for outdoor air. so marking miscing point 3 as: dength (2-3) = 0.15 * x length (1-2). Sensible heat factor, SHF = Room Sensible heat load Room total heat

20 20 SHF RSH RSH FRLH -= 0.90 20 +2:22 22. 22 base line 0.90 SHF on SHF SO Drawing a of scale, & then drawing 2-5(ADP) line parallel to base line for obtaining supply points, which will be on 100% RH. By pass factor = 0, air learing cooling coil will be apparatus dew point (ADP) temp. The same point supply air point. (9) RSHF (Room sensible heat factor) same to shf, already calculated. RSHF 0.90 -- Ans. RSHIRLH will be It is RSH (b) Mass flow rate of supply air at pt. s. ms Room total heat Cheahs) 2 from, chart, hz=43-2 kJ/kg. hs = 24.8 kJ/kg. ms 1.658 20+ 2.22 (43.2-29.8) kg/see Ans.

Moisture content of supply air, Ws = 7,8 grams kg of dry air from Psychrometric Ans chart Mass flow rate of re-circulated air: Mr = 85% of supply air. = 0.85 X 1.658 : 1.409 kg/sec. - Ans Temp. of the mixed air streams": From Psychrometric chart, temp-at point 3, which shows miseed flow condition, td3= 24.3°C dry bulb temp. twa=18.3°C wet bulb temp. व d load on Sensible heat load on cooling coil = mg Cha-hs) On Psychrometic chart, process 3-A shows latent heat coil & process A-S shows sensible load. from chart, ha=43.8kJ 1kg ha = 43.8 kJ lkg, h3 = 51k3/kg. hs=29.8 kJ 1kg 1. sensible heat load = 1.658 (43.8-29.8) - 23.21 kJ/sec - 23.21 kw Latent heat load - Ms Cha-ha)=1-658(51-43.8) = 11.93 ks/sec: 11.93 kw Total heat loads ms Cha-hs) =ms (51-24-8)= 35:14kW