View Full Version : A question about the pressure drop in coil .

jackcn

22-04-2004, 11:14 AM

Usually in heat exchanger designing,the less the pressure drop, the better.

But a professor said, in an air cooled condenser designing, we must keep an appropriate Presure drop. (otherwise the two phase refrigerant will go to the expansion valve.)

and the pressure drop of the refrigerant (here R22) in the tube of a condenser coil shouldn't less than 4 psia.(The unit is about 100 tons capacity,and the pressure drop in our first selection is about 1.8psia)

It's really a new idea for me. I wondered if there are some experience pressure drop date for it?

I can't find informations about it. so I come here find help.

Thanks for all reply.

shogun7

01-05-2004, 02:46 AM

The pressure drop for an air cooled condenser is about the same as it is for an evaporator and that’s a maxium of 3 psi. Air cooled condensers normally have one or more fans and motors to move air usually at a rate of 500 to 1000 cfm per compressor hp. Air cooled condenser capacity are based on the total external area, temperature, time , and air velocity. Using these values, the capacity of an air cooled condenser varies between 1 and 4 btu/cuft/hr/ *f

Hi :)

I would tend to think in the terms of saturated temperature loss say 1k for a condenser. 3 psig is a good round number, but relates to a different temperature drop for each refrigerant, better to talk about a drop in saturation (R12 would have a much greater temperature drop for 3 psig than say R744)

Kind Regards. Andy :)

Prof Sporlan

01-05-2004, 01:42 PM

Maintaining a sufficient two-phase refrigerant velocity would be the primary concern for someone designing an evaporator or condenser coil. This is to allow proper wetting of the inside surfaces of the tubes which provides the desired heat transfer.

Pressure drop, however, is a function of refrigerant velocity. So one can think of a "desired" pressure drop for a coil if it is being related to velocity. But this Prof finds this to be an awkward way of thinking. In his humble opinion, one should be more concerned about a maximum desired pressure drop for the coil.

Specifically, pressure drop is roughy proportional to the length of the circuit and the square of the velocity.

[QUOTE=Prof Sporlan]

Pressure drop, however, is a function of refrigerant velocity. So one can think of a "desired" pressure drop for a coil if it is being related to velocity. But this Prof finds this to be an awkward way of thinking. In his humble opinion, one should be more concerned about a maximum desired pressure drop for the coil.

QUOTE]

So you are saying pressure drop is unimportant :confused:

I have never had the need to design a coil (yet) so please elaborate

I have been taught to work to an agreed temperature drop, but in doing this keep a velocity in mind. In hot gas lines this would be 15m/s I am assuming a little less would be appropiate in a condenser coil.

Please comment as I am learning here :D

Kind Regards. Andy :)

shogun7

02-05-2004, 12:32 AM

We in the colonies like to think in terms of pressure drops when we consider heat transfer in closed system heat xchangers. I have never considered t in terms of temperature, although thats probably just as good if one thinks in terms of a particuar application. :rolleyes:

Prof Sporlan

02-05-2004, 04:29 PM

Pressure drop is a fine metric to use in evaluating coil sizing, particularly with respect to making sure pressure drop is not excessive. But the Prof has a bit of a problem with the statement that a condenser coil must have x-amount of pressure drop to be at its most efficient when we don't know the circuit length, tube OD, refrigerant being used, tube material (steel, aluminum, copper).

It is safer to state that the two phase refrigerant velocity must be within a certain range to obtain optimal performance.

For example, all other things being equal, the condenser coil having longer circuits, or a smaller tube OD, will require greater pressure drop for optimal performance.

Designing coils is a matter of selecting the tube OD, circuit length, number of circuits, and doing the proper circuiting so that refrigerant velocity falls within an acceptable range yet results in minimal pressure drop.

Hi Professor :)

Pressure drop is a fine metric to use in evaluating coil sizing, particularly with respect to making sure pressure drop is not excessive. But the Prof has a bit of a problem with the statement that a condenser coil must have x-amount of pressure drop to be at its most efficient when we don't know the circuit length, tube OD, refrigerant being used, tube material (steel, aluminum, copper).

It is safer to state that the two phase refrigerant velocity must be within a certain range to obtain optimal performance.

For example, all other things being equal, the condenser coil having longer circuits, or a smaller tube OD, will require greater pressure drop for optimal performance.

Designing coils is a matter of selecting the tube OD, circuit length, number of circuits, and doing the proper circuiting so that refrigerant velocity falls within an acceptable range yet results in minimal pressure drop.

A very eloquent way of saying there is much more to coil design that pressure drop :)

Any chance of a worked example to enlighten thick heads like me :D

Kind Regards. Andy :)

Prof Sporlan

04-05-2004, 03:56 AM

When modeling pressure drop due to fluid flow thru a pipe, we invariably start with the Darcy-Weisbach equation:

hf = f * Le * v² / (D * 2 * g)

where:

hf = head loss, ft

f = friction factor, dimensionless

Le = equivalent length, ft

v = velocity, ft/sec

D = pipe diameter, ft

g = standard acceleration due to gravity, 32.174 ft/sec²

The effect of velocity and equivalent length is apparent from this equation. The problematic variable here is the friction factor. Friction factor is a function of the effective roughness of the pipe, its diameter, and the Reynold's number. For commercial copper tubing, an effective roughness of 5.0e-6 ft is used.

The Moody diagram can be used to look up the friction factor. The Colebrook equation can also be used to calculate friction factor:

1 / f^0.5 = -2 * log10[ (e / D) / 3.7 + 2.51 / (R * f^0.5) ]

where:

f = friction factor, dimensionless

e = effective roughness, ft, 5.0e-6 for copper tubing

D = pipe diameter, ft

R = Reynold's number, dimensionless

As one will notice in short order, an iterative procedure is necessary to calculate friction factor with the Colebrook equation.

To calculate Reynold's number:

R = v * D * rho / (u * gc)

where:

v = velocity, ft/sec

D = pipe diameter, ft

rho = density, lbm/ft³

u = absolute viscosity, ft²/sec

gc = gravitational conversion factor, 32.174 lbm-ft/(lbf-sec²)

And that's simply if we're looking at a single phase fluid!

The problem we have with refrigerant flow in a condenser or evaporator is we're dealing with two phase flow. With two phase flow, there is no obvious way to determine viscosity, or for that matter, if the Moody diagram or Colebrook equation is even valid.

But as one might guess, past researchers have modeled two-phase flow with the above equations in mind, and have developed useful correlations. The Lockhart-Martinelli correlation was perhaps the first correlation done. Others have followed. Bottom line here if one has a handle on the above equations, one has a good understanding on how pressure drop, line length, and refrigerant velocity are related.

Hi Professor :)

I had to ask, hadn't I :D :o

That is one of thoses posts that I will have to down load and study, some of the stuff Renolds ect I remember from an earlier life. :D

Would these formulas be valid for say water/glycol mix. I am thinking about the head created on a pump in a ground loop system.

Many thanks for the equations, I will follow them up.

Kind Regards. Andy.

Prof Sporlan

05-05-2004, 01:24 AM

Would these formulas be valid for say water/glycol mix. I am thinking about the head created on a pump in a ground loop system.

Absolutely! The problem you would have with a water/glycol mix would again be determining an appropriate value for viscosity. But this would be simpler than with two-phase flow. One should not have much difficulty locating approx viscosity values for water/glycol mixes.

allanbaker

11-05-2004, 08:06 PM

Prof and andy

I would like to find out from you while we are on the condensor subject, about a condensor that was supplied to us with the Hot gas header ( 1 5/8)on a 83Kwatt R22 system -12*C Suction & 45* C discharge. The problem being that the incoming discharge pipe comes into the bottom of the header, we found liquid migrating back to the oil sep when the ambient droppped could this be the problem? liquid running back down the header when the gas velocity drops with cycling of the compressors?

Hi Allen :)

once worked on a packaged water chiller, blue box I think, that was wrecking compressors (low ambients on a wind swept roof) one of the two condenser circuits (the one with the wrecked compressors) was piped that way, hot gas in the bottom, with liquid out the bottom in the other header. Asked why and I was told it was probably ok in high ambients and hot countries. Not much hope of that in N.Ireland :(

So we stuck a discharge check valve on it. I know it helped, but we lost that contract to a lower priced contractor after that some I don't know if that was the final cure. :confused:

Kind Regards. Andy.

Hi Allen :)

you could always rise the discharge line above the condenser and cause a trap, space permitting (we couldn't), that would help a lot.

Kind Regards. Andy :)

allanbaker

11-05-2004, 08:40 PM

hi Andy

We have installed a check valve and it seems to have worked but we still have to wait for low ambients, which are coming now in the next month. ( WE are in South Africa) gets down to -5*C sometimes -10. Cold for us nothing for you i pressume.

The other advise that we recieved from the manufacturer was to install a P trap near the oil sep and that didnt make sense to me even installing it just below the header it would still migrate when she cycles????

[QUOTE=allanbaker]hi Andy

( WE are in South Africa) gets down to -5*C sometimes -10. Cold for us nothing for you i pressume.

QUOTE]

Much the same as us really, loest I remember was -13 deg C lots of burst water pipes and wrecked condenser pumps :eek:

Only thing top temp would be low 30 deg C but mostly not much more than 25 deg C and plenty of rain and then some :D

Kind Regards. Andy :)

P.S

N.Ireland is just a little bit smaller than South Africa :D :D

allanbaker

11-05-2004, 09:01 PM

Marc I'm sure Gravitate, even still they should'nt supply you with a coil with the header piped up the wrong way.

I am actually going into a meeting with the suppliers regarding this matter ..... only problem i have no documentation regarding this matter any suggestions

Prof Sporlan

12-05-2004, 01:54 AM

Seems to me they stuck the header on the wrong way round.

The Prof seconds this assertion... :)

superheat

23-08-2004, 06:09 PM

I would like to interject something.

You probably do some coil design to a lesser extent, Andy. What about changing the evap set-up from the high end of meduim temp to the low end. 45F evap changed to a 15F evap on the same system. The condenser might come into play on a marginal system complicated by high ambient temps variation. You could easily cut the velocity through the coil in half giving 1/4 the pressure drop.

Just rambling here, but I am going to apply that change of condition to the pressure drop equation Prof gave us. (I though it was a lot of work when I had to solve it in college many years ago. )

ISMED BURMAWI

07-11-2008, 08:18 AM

Thank your info, i can check this data

ISMED BURMAWI

13-12-2008, 05:10 PM

According to me, decrease the pressure on the condensing unit of media-related trade refrigerator as kalor (the air). While air temperature is always based on the change of time (day or night). To maintain different temperature between air and the incoming coil after coil out from the result of the movement of heat from the refrigerant to the air to the process of condensation in the perfect kondensor.

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