Design Supply Air Temp for Peak Load

When we design the air con, we need to specifies how much the supply air volume is. If we need to calculate the supply air volume, we need to specifies supply air temp. So, this post I'm going to discuss about design supply air temp.
Most common design supply air temp is 13 C to 15C dependent on cooling source and cooling coil configuration.

My personal default Design Supply Air Temp is 14C because most common coil default leaving air temp is 12.8C (55 F). So, If I choose design supply temp is 14C, leaving air temp from coil will be around 12.7C to 13C.  


By right,before we specifies the supply air temp,we need to specifies operation temp and RH %. So, typical common set point is 23+-1 & 50% RH.

For Singapore Design Parameter,Based on SS-554, maximum RH % is less than 65 on both Peak & Part Load Condition. So, our Peak Load condition should be 50 % ~ 55% coz if peak load is about 60%, part load may be more than 65 % and may not comply.

Just check with sample.
Space Area is 100m2 and Ceiling High is 3m.
Sensible Heat Load is 10 kW and Latent Load is 1.76 kW.
Meaning that the space have 85% sensible heat load & latent heat load is 15%.
So,SHR is 0.85 (SH/TH).
Space Operation Temp is 24C & RH-50% (Less than 55%) at Peak Load condition.
What's the design supply air temp and air volume?

First step, draw the line on Psychrometric chart by Standard Point and SHR.

So,  As per Psychrometric Chart, If the space have 0.85 SHR and we need to get exactly 50% RH, cooling coil ADP should be 11.3C. And then Space dew point is about 13C, so that cooling coil temp should be less than or equal 13C in order to get not more than 55% RH.
As I said above, normal cooling coil leaving air temp is about 12.8C (55F) and cooling coil bypass factor is 0.1 (10%). The room operation temp is 24C and then mixing temp with fresh air is about 25C. So back work with formula, Coil ADP = {Ts-(TmxBF) }/ 1-BF.
Supply air temp will be 14C. So, normally I did choose Design Supply Air temp is 14C.
Let check again with Design Supply Air Temp-14C, how much the RH in the space.

Just draw the line from point Coil ADP-12.8 to parallel with the line.
(The line from SHR to standard point).

Intersection point with 14C DB (Supply Air Temp) is leaving air temp 
and the room operation temp is 24C & 54% RH at Peak Load.
So, the leaving air enthalpy is about 37 kJ/kg.
Most common leaving air enthalpy is 35 ~38 kJ/kg coz supply air temp is 13C~15C.

So, The room Operation temp 24C & RH-54% is acceptable,
Make the supply air temp is 14C.

If supply air temp is 14C and the space sensible load is 10kw, we can calculate the design supply air volume. 
Q =  m x c x ∆T 
Q = 10 kw (kJ/s)
 c  = 1.004 kJ/kg
∆T = the room operation temp - design supply air temp (24C - 14C)

So, m = Q/( c x ∆T)............. (kg/s)
m = 10 / 1.004 x 10 = 0.996 kg/s.
V = m x v 
v = Humid Volume (m3/kg)
From Psychrometric Chart, Supply Air v is 0.825 m3/kg.
V = m x v = 0.996 x 0.825 (m3/s)
V = 0.822 m/s = 2960 m3/hr (CMH).

So, The space sensible load is 10 kw ,SHR is 0.85 & Supply air temp is 14C,
 Supply air volume should be 2960 cmh. After calculate the supply air volume,
we need to check it that it's sufficient air circulation to meet minimum air circulation.
It's checked by minimun MV requirement. For office building type, minimum air circulation should be 6 ACH. That's mean Area x Ceiling High x mini ACH= 100 x 3 x 6 =1800 cmh.
So,our supply air volume 2960 cmh is more than enough compare with minimum circulation.

Let check the cooling coil load for the room if mixing air temp is 25C and RH is 60% by fresh air make up.

Draw the line 14C db for supply air temp and then Mixing Air Temp-25C & RH-60%.
The cooling coil rate is 
Q = m x ∆h
m = 0.996 kg/s
∆h = (h2-h1) (kJ/kg)
h2 = mixing air temp enthalpy -55 kJ/kg
h1= Leaving Air temp enthalpy- 37 kJ/kg.
Q = m x ∆h = 0.996 x (55-37) = 17.93 kW.

Summary will be-
Total Cooling Coil is 18 kW.
Design Supply Air Temp is 14C.
Design Supply Air Volume is 2960 cmh.

Thanks.
Zaw Moe Khine.