Hello all,
Sorry for keeping quite for long time..and thank you for your valuable suggestions.
We have installed ammonia analyzer on top of Urea prill tower, near axial flow ID fans for analysis of ammonia emission. The analyzers require dry air (at 1 kg/cm2g) for continuous purging. We checked with supplier, who demands dry air.
Meanwhile I have assumed pipe in pipe (DPHE) exchanger with 3.5 barg saturated LP steam at annular side and compressed air in pipe and calculated the size as follows. Sizes assumed are inner pipe 1" and outer pipe 2".
Air requirements have changed as follows:
Air requirement = 320 m3/h at 1 kg/cm2g (~640 Nm3/h = 828 kg/h), heating from 40°C to 85°C. Heat duty = 9030 kcal/h (10.5 kW)
So LP steam requirement is estimated to be 18 kg/h.
I estimated steam side condensing HTC using equation :
For condensation on vertical tubes or plane surfaces hv = 0.94[(k3r2g/m) x (l/LDT)]0.25 (got from a website)
(all properties of condensate at wall temperature about 105°C, L= assumed as 5m for initial calculation), dT = average comp air temperature & wall temp (105-65°C = 40°C).
I got HTC, ho= 3188 W/m2K
For air side, used eqn 6.2 in Kern and got hi = 1227 W/m2K, or hio = 1227*24.2/33 = 900 W/m2K
(Calculation: Mass velocity G = 500 kg/s/m2
Overall HTC = hio x ho / (hio + ho) = 900 x 3188 / (900 +3188) = 702 W/m2K
SO area required = Q / UxLMTD = 10500 / (702 x 85) = 0.176 m2
Area per unit length of 1" pipe = pi x d x L = 3.14 x 0.033 x 1 = 0.103 m2
So total length of DPHE = 0.176 / 0.103 = 1.71 say 2m.
Is it OK? Please suggest.