ammonia compressor efficiency to calculate mass flow 1

[themroc]

I need to evaluate the performance of a heat exchanger (evaporator)

The exchanger evaporates ammonia
In order to do the calculation I need to know the ammonia mass flow. Which is not meassured directly.

The only information I have got is about the compressor which runs after the evaporator and has the following conditions meassured:

the ammonia vapour is compressed with a compressor from 82mbar (abs)(Saturation pressure) to 13.1 bar (abs)Discharge temperature (84.9°C)(superheated)

The Motor Ampere is meassured with 257 Ampere
The Voltage is unknown, I pressume it is 380V

My question:
Can I calculate the mass flow by
m * (enthalpy outlet - enthalpy inlet) = P compressor = 257 * 380. ???

I pressume I need to take into account the efficency of the compressor. I do not have any information about the type of compressor, Can anyone advice?
What kond of efficiency can one use??

 

[25362]

Before attempting to answer are you sure the suction pressure isn't gage pressure, namely 1.082 bar abs. ?

 

[themroc]

sorry you are half right, it is actually 0.82 bar and the temperature is about -33° the vapour is slightly over heated (about 4K)

thanks for the hint

 

[Montemayor]

themroc:

Is this a mechanical refrigeration application with an Ammonia evaporator acting as the cold source for your process or utility fluid? If so, why not just tell us that; it would be much simpler because then we could analyze and inquire why you don't just supply the vaporizer heat load. That's the way a conventional mechanical refrigeration system is usually designed - not by trying to find out how much refrigerant vapor your compressor is going to handle.

I have some misgivings about your basic data besides what I've stated:

1) what exactly are you trying to figure out - in total? You say you want to "evaluate" the evaporator and then you ask about how to calculate the compressor horsepower (or whatever you mean by "P compressor".
2) You state you do not have any information about the type of compressor. This is not only strange, it's not what a responsible engineer would accept.

Is this a real-life application or is this a conceptual or academic question? Please tell us ALL of the facts and the basic data as well as your scope of work - and then we can help you out in an organized and intelligent manner. Otherwise, we're only half-guessing at just exactly what you have.

 

[25362]

FWIW, you may compare results with the following ballpark data I took from old notes for an oil-flooded twin-rotor screw compressor on anhydrous ammonia working on a properly-maintained system.

With the evaporator and condenser conditions indicated by you and no economizer, the motor would be consuming about 3.5 HP/TR, and the ammonia flow rate to the evaporator would be about 12 kg/h per TR.

1 TR = 3,000 kcal/h.

 

[chicopee]

With a reciprocating compressor you can also estimate CFM
by knowing displacement and RPM of cylinders. Knowing an average specific volume between intake and outlet you'll have an estimated mass rate.

 

[dcasto]

380 volts, not familar with that. If I ssume 480 Volts and 257 amps on 3 phase, that appears to be 150 HP or 112 KW. At the available conditions, the compressor could compress 680 Kg/hr (based on a reciprocating compressor of 82% effiency. The resulting evaporator could remove 175,000 Kcal/hr. If the compressor is a lower effiency screw type, the values would drop to 425 Kg/hr and 110,000 Kcal/hr.

 

[Montemayor]

dcasto:

With all due respect, your estimates are just whispers in the wind. There is no way you can estimate/predict any duties, horsepower requirement, or condenser cooling requirements where the Original Poster (OP) hasn't revealed anything about his refrigeration cycle yet other than he's using Ammonia and his compressor suction & discharge pressures. This is not enough information to generate what you state. One needs to know the evaporator conditions and the condenser conditions.

Of course, one can establish a base - like ASHRAE does - and fix the conditions of both the evaporator and condenser. But you haven't stated that. Consequently I have to say that your estimates are meaningless unless they have a basis. This doesn't mean your estimates are wrong; it just means they lack a basis - and that's essential in engineering.

Again, without any concrete basic data and a scope of work explaining what the OP is doing, we're all guessing (or as we would state it here in Texas, "p_ _ _ing in the wind").

 

[dcasto]

i stated my basis, 150 HP at 82% effiency, with the pressures set by themroc. Simple refrigeration cycle. I gave an alternative at a lower effiency (62%). Now if themroc measures the heat rejection near one of the values I've stated he can enterpolate the effiency and then ask himself it thats a reasonable value or can they make an economic justification to replace the system.

 

[25362]

To dcasto, the temperatures of the gas entering the compressor at say, 0.8 bar, and the liquid upstream the reducing valve weren't given.

If, through heat exchange with the condensed ammonia, usually done to remove any droplets by vaporization, vapors entering the positive displacement compressor heat up by 40-50^oC, their specific volume may increase by more than 20% defeating the estimate of throughput.

Of course, this enthalpy exchange wouldn't be lost since the liquid entering the evaporator would be cooler. Agree ?

BTW, one interesting item I found many years ago when dealing with (anhydrous) ammonia refrigeration is that the liquid has a specific heat capacity equal or greater than water. Any comment ?

 

[25362]

My own comment: the change in BHP/kg ammonia vapor compressed as a result of superheat at suction would change, but not by much, so as to thwart a rough estimate.

 

[25362]

Let's clarify the subject during the weekend.

Since BHP_adiabatic/(kg/h) is proportional to the absolute suction temperature, to [(k/(k-1)] and to [(p₂/p₁)^(k-1/k)-1], roughly speaking a 20% increase in absolute suction temperature would result in a 14% increase in BHP/(kg/h) for a constant volumetric efficiency and the given pressure ratio.

On the other hand, 4 K superheat wouldn't mean an appreciable change in the said BHP to mass rate ratio.

 

[themroc]

Dear all thanks for your replies.
I was out of office for some days, therefor no clarification.

We designed and delivered an heat ammonia evaporator.
When designing it I based my design on the information delivered.
At this time it was not clear for what purpose the exchanger was designed.
The following conditions were given:
Evaporation on the tube side with ammonia (quality 0.067) entering the exchanger at a pressure of 1.3bar absolute (t sat = -28.9°C) Requirement was to evaporate 1320kg/hr with a superheat of about 4K.
This was the design sucction pressure for an compressor which compressed the gas.
The ammonia is heated via syltherm a heat transfer fluid. (t inlet -17.2 / outlet - 25°C) Heat transfered 457kW.

Now in service we got the feedback that the exchanger does not perform. It is based on the heat load of the shell side heating fluid. Which only meassured about 280kW.

The thing is according to them this is the value they base the exchanger duty on. Apparently they do not measure the ammonia mass flow.
But they state that the exchanger is not performing well. In addition they give the compressore information of the current perfomance so I wondered whether it is possible to calculate from this the mass flow in order to calculate the tube side duty of the exchanger?
Following conditions meassured of the compressor:
___________
The ammonia vapour is compressed with a compressor from 82mbar (abs)(Saturation pressure) to 13.1 bar (abs)Discharge temperature (84.9°C)(superheated)

The Motor Ampere is meassured with 257 Ampere
The Voltage is unknown, I pressume it is 380V
__________

In addition they state that there are dropplets present in the succion stream. But they meassure a temperature of -33°
The saturation temperature at this point would be about -37°C. Indicating a superheat of the ammonia. Is it possible that you get droplets in a superheated flow?

 

[25362]

Themroc,

Can you indicate how is the ammonia preflash attained ? Is there a level-controlled flooder feeding the exchanger by gravity while simultaneously venting the vapor produced in the preflash ? Can you explain the 1.3 bar at the inlet of the vaporizer and 0.82 bar at the outlet ?

The data just now supplied indicates the original design considered a degree of subcooling for the liquid ammonia prior to the expansion valve. Can you comment on that ?

Yes, droplets can coexist with superheated vapor for a certain time until equilibrium is reached. If the residence time in the suction pipe, from the vaporizer to the compressor, is short enough droplets may reach the compressor. Is there a KOD at the compressor suction ?

 

[themroc]

25361,
The 1.3 bar was the design condition.

Now they run the exchanger under different conditions, for the 1.3 the outlet condition due to the pressure drop was about 1.18 bar.

I refer to the 0.82 because I have got for this value data from logs for the compressor. For the 0.82 bar the inlet pressure of the exchanger was about 0.89 bar.

What do you understand under the ammonia pre flash? Is that commonly used in this kind of applications?

According to the operator the ammonia is not subcooled when entering the evaporator ir as a quality of about 5%.

What is a KOD.

Thanks themroc?

 

[25362]

Besides, can you explain the 1.3 bar at the inlet of the vaporizer and 820 mbar at the outlet ?

Has the condenser operation been checked ?

Is there a KOD at the compressor suction ?
Is there a strainer at the compressor suction ? What type of compressor is being used ?

On another direction altogether, has the user checked the flash point or the viscosity of the syltherm fluid to verify whether it suffered a change with time ?

 

[25362]

Sorry for the duplication of questions due to my pressing the submit post button too soon.

I understand the clients moved to "vacuum" conditions to get lower ammonia temperatures to compensate for the worsened performance of the exchanger. Right ?

I call preflash the vaporization that occurs after the expansion valve which is the 5% quality you mentioned. I assume the vaporizing takes place on the shell-side. Am I right ? What type of vaporizer is this ?

A KOD is knock out drum usually provided with a demister to remove liquid droplets from the vapor. It usually has a coil to subcool the ammonia from the condenser, while superheating the vapors to vaporize droplets coming from the vaporizer.

 

[themroc]

In fact the client did a whole series of check at different operation conditions in order to check the performance of the exchanger.

In this case the vapourisation takes place on the tube side.
It is a horizontal BEM type evaporator.

As far as I know there is no KOD installed, at leased I do not see anything on the flow sheet.

 

[Montemayor]

Themroc:

Thank you for finally giving us ALL of the facts. However, some things don’t make for common sense:

You say your client has a suction pressure on his compressor that registers 82 mbarA. This can’t be correct. Either you or they have made an error or are reading the pressure wrong. 82 mbarA yields an ammonia saturated temperature of -74 ^oC and I don’t believe that you have an operation that is producing that level of refrigeration. You also may have just made a typo error and really mean that you have 840 mbarA suction pressure. If so, please clarify. If you are going to insist on using the SI system, then I suggest you stick to one set of units (like barA) instead of measuring and reporting in different units which only creates conversion problems – as noted.

You state your ammonia vaporizer has the liquid ammonia vaporizing in the tube side. Is this a vertical unit? You also state that the vaporization is taking place at 1.3 barA and -28.9 ^oC. But then you say that the ammonia is compressed from 82 m barA (840 mbarA?) to 13.1 barA. That means that there must be a pressure drop of 460 mbar (6.7 psi) in the suction piping going to the ammonia compressor(s). This is an excessive amount of pressure loss for an ammonia mechanical refrigeration system. I would have expected 0.5 psi or thereabouts. My point here is that this is very inefficient.

You are correct in noting that the compressor suction cannot possibly have ammonia “droplets” if the actual temperature is -33 ^oC. This temperature, as you state, indicates superheating and droplets (liquid) cannot physically exist in a superheated state. Superheat applies only to the gaseous state – not the liquid state.

If the evaporator that you designed is not transferring the specified 457 kW (1,559,350 Btu/h) then something is obviously wrong. However, this would entail believing the information coming from people who also allege that liquid droplets exist in a superheated suction to the compressor. Consequently, I would suggest that you or your representative investigate the application and get to the real facts in the field.

From the application data you supply it is obvious to me that the ammonia compression is being done in a 2-stage cycle. This type of cycle usually has an “economizer” type of intercooler, of which there are at least 2 varieties. Depending on the mechanical configuration and type of intercooler, you will have varying refrigeration efficiencies and the Bhp requirements (as well as the compressor capacity) will vary depending on the efficiencies. That’s why I maintain that the best way to determine the amount of ammonia evaporated is to make an accurate heat balance around the evaporator. This should be easy to do since you can measure the Syltherm flow rate and the inlet/outlet temperatures. This is far more accurate than assuming a variety of conditions that, as seen with recent data, don’t exist.

 

[themroc]

Montemayor,
thanks for your comments, as I staed earlier (reply 3) there was atypo and it is in fact 820mBar absolute.

The 1.3 bar was the design pressure at the inlet of the evaporator

The 0.82bar was the actually meassured suction pressure at the compressor. (see sommend 12 Dec 06 8:32)

The evaporator type is horizontal.
It is probably better to do a heat balance around the evaporator. I intended to do this and therefore was quiet keen to know the mass flow of the ammonia. But because this was not meassured I thought there is a way to find it out using the compressure inlet and outlet conditions.

Thanks for the help.