steam capacitors

[salvatore87]

Hello everyone, I have to dimension an air condenser, with tubes for a steam system, predetermined geometry, which exchanges the power of 25,6mw. the condensation temperature is 39°c while the air input and output temperature is 20°c and 29°c. the main problem is that, with these data, the air flow to be sent on the tube beam is 3ton/s unthinkable for fans. So I thought to break the system in 3 or 4 modules, so as to reduce the power for each module, and have smaller flow rates per module. the arrangement adopted would be that in parallel. What do you think?
p.s I cannot use water capacitors! !

 

[paulpaul]

Hi.
Yes, what you hypothesize is a common solution in the field of drycooling.
if you are looking on the net aircooled condensers you will find several examples.

 

[salvatore87]

Hi.
Yes, what you hypothesize is a common solution in the field of drycooling.
if you are looking on the net aircooled condensers you will find several examples.

Thank you! what are the maximum air flow values that can be achieved? from my calculations I arrived at about 500kg/s with a speed of less than 20 m/s

 

[paulpaul]

I don't know how to tell you exactly what the maximum course is: the plants that I have seen have much lower powers than those that serve you, that are custom plants and not catalog.
In any case, it seems to me that the speed of 20 m/s can be a plausible upper limit.

 

[mir]

25 m/s I would say

 

[athlon]

I'm not from the field, but on these fans with so powerful fans I've always wondered if the fans themselves didn't warm the air, either by compression or by heat given by engines, in that case it would be better to put the fans downstream of the heat exchanger

 

[paulpaul]

I'm not from the field, but on these fans with so powerful fans I've always wondered if the fans themselves didn't warm the air, either by compression or by heat given by engines, in that case it would be better to put the fans downstream of the heat exchanger

I have no direct experience with such powerful fans: However in the past weeks I selected two aircoolers whose function was to cool both gas interstage and cooling water shirts and oil for a compressor: Below is the essential data of one of the two and some of my considerations on the two aspects you mentioned, which I have frankly never thought about so much.

thermal power: about 150 kw
air flow: v = 5,6 m3/s
prevalence: dp = 28 mm h2o (275 pa)
power m.e. Blower: p = 5.5 kw

1) motor absorption: I don't remember how much it was loaded, hypotize 80%: 4,4 kw of which (1-n) lost as heat, being n the yield of m.e. (we assume n = 0.7 per engine of this size). about q=1,3 kw of thermal power that increase the air flow temperature. hypothesized that the whole range is heated, setting an energy budget I get q = m x cp x dt.

being
m = rho (air) * v = 1.2 x 5,6 = 6,72 kg/s air flow
cp = 1 kj/kg°c specific air heat (p = cost)
dt increase in temperature

dt = 0.19 °C

2) compression (suppose isoentropica)

(k-1)/k)

p1 = 101325 pa suction pressure
p2 = 101600 pa mandata pressure
k (air) = cp/cv = 1.4

t2/t1 = 1,0008

Supposing air entering at 40 °c, I would have a temperature, due to 1) + 2) of 40 + 0.19 + 0.031 = 40,22 °c, definitely negligible.

I would say that by definition a fan is not designed to give the fluid pressure (if not a few hundred pa), which is why temperature increases due to 2) should always be negligible. generally with fans it is lawful to consider the flow as incomprehensible precisely for this reason.

1) I believe that in the end it gives a negligible contribution, much more considering that increasing the size of a me this increases its performance (around 160 kw we are already about 0,95, for a cage motor) and also increases the flow, with always "more air to heat".

in my activity (mainly as a user of these aircoolers) I nevertheless met both solutions, "previously" and "aspirant", but frankly I never posed a problem of efficiency: I believe that one or the other solution is, for practical purposes, dictated by layout needs.
But it would be interesting to know the opinion of someone who designs them by profession!

 

[athlon]

In fact, for 0.22° it's not worth it, the doubt came to me thinking more about the turbos of the machines, which must use an intercooler after the impeller to cool the air.

 

[paulpaul]

in fact that is a real compressor: we recently installed some gas engines cat g3412, always as compressor drivers, and their pressure full load was about 0.8 bar. the compression ratio would then be (1+0,8)/1 = 1,8 and applying the formula of the isoentropic written above would have a t2/t1 = 1.2. which means, for air entering at 25 °C, a fine compression temperature of about 75-80 °C, and in fact it was equipped with water intercoolers. the ratio t2/t1 is between absolute temperatures (in k): before I made the account with the temperatures in °c since the ratio was practically unitary. Considering that the internal performance of the compressor is less than one, the real compression fine temperature (not isoentropica) will be a little higher.
for diesel engines the boost pressure is averagely higher (also 1.5 bar full load), since there is no problem of self-ignition.