Pressure loss versus speed in suction line

[Peter_1]

Would like your opinion on this: when calculating a suction line, we must take in account a certain speed to ensure oil return (I take 8 m/s or 26ft/s) and a max allowable pressure drop of 2K.
What if you have a long line and speed is already at it's minimum (5 m/s or 16 ft/s) but max allowable pressure drop is 4K?

[Peter_1]

Sorry forgot, my opinion on this is that the oil return is the determining factor, so the speed must stay above min allowable speed.
Pressure drop will give bigger losses but better this then oil staying in the circuit.

One of my students asked me a smart question: suppose a hypothectical system wit a pressure drop of 6 K along the suction line. Waht pressure or temeprature does he need to take to calculate the suction line? The evaporating pressure in teh evaporator or the pressure measured at the compressor inlet?

[LRAC]

Would like your opinion on this: when calculating a suction line, we must take in account a certain speed to ensure oil return (I take 8 m/s or 26ft/s) and a max allowable pressure drop of 2K.
What if you have a long line and speed is already at it's minimum (5 m/s or 16 ft/s) but max allowable pressure drop is 4K?

Hi Peter 1

Get an oil sep on it quick.

Regards
Lrac

[chillin out]

Hi Peter 1

Get an oil sep on it quick.

Regards
Lrac

LOL... you should know Peter better by now.

Chillin

[US Iceman]

What pressure or temperature does he need to take to calculate the Suction Line? The evaporating pressure in the Evaporator or the pressure measured at the Compressor inlet?

Since one of your concerns is the compressor capacity I say you should use the evaporating pressure. The evaporator capacity is based on the rating per degree of temperature difference.

Therefore, the total evaporator capacity is Q (kW/° or BTUH/°) X TD (°K or °F). The TD determines the evaporating temperature and pressure.

The suction line pressure loss determines the compressor suction pressure (& saturated temperature). This is one of the required operating conditions to find the compressor capacity so that it matches the evaporator capacity.

With this method you always know what the compressor is supposed to be operating at.

my opinion on this is that the oil return is the determining factor, so the speed must stay above min allowable speed.

I agree. This is one of the penalties for operating a "*****" system. However, the problem is increased when the compressor has capacity control. In this case, you have additional concerns.

[lana]

Hi Peter,

As far as I have read, the minimum velocity for oil return in horizontal pipe is 2.5m/s (of course with a 1% slope to wards the compressor).
And for vertical risers the minimum velocity is 5m/s.
I think if ASHRAE tables are used then, they are calculated for those velocities.

For the pressure I think we must take the compressor suction pressure because this is the actual inlet pressure which compressor takes. It is obvious on a P-h diagram. This is why compressor capacity decreases with high suction pressure drop.

Cheers

[ernestlin]

I consider the mini-velocities of refrigrant are determined by different tubes' figuration, refrigrants and compressor types. 2.5 m/s in horizontal pipe is not safe enough for oil return, 4 m/s may be better in normal feron system.

"Therefore, the total Evaporator capacity is Q (kW/° or BTUH/°) X TD (°K or °F). The TD determines the evaporating temperature and pressure."

Iceman, your words make me confusedly. Theoretically, cooling capacity is determined by the latent heat quantity under the definite evap pressure, refrigerant flux; it also can be caculated by TD X K(heat exchange ration) X heat transfer Face, but I can't agree with your last expression, I think the pressure determines phase transition temperature/evap temp, the TD determines the Cooling Capacity.

[lana]

Hi There,

The paragraph below is taken from Mcquay Refrigerant Piping Design Guide.

Piping Design Basics

Good piping design results in a balance between the initial cost, pressure drop, and system
reliability. The initial cost is impacted by the diameter and layout of the piping. The pressure drop
in the piping must be minimized to avoid adversely affecting performance and capacity. Because
almost all field-piped systems have compressor oil passing through the refrigeration circuit and back
to the compressor, a minimum velocity must be maintained in the piping so that sufficient oil is
returned to the compressor sump at full and part load conditions. A good rule of thumb is a
minimum of:

• 500 feet per minute (fpm) or 2.54 meters per second (mps) for horizontal suction and hot
gas lines

• 1000 fpm (5.08 mps) for suction and hot gas risers

• Less than 300 fpm (1.54 mps) to avoid liquid hammering from occurring when the solenoid
closes on liquid lines
Hard drawn copper tubing is used for halocarbon refrigeration systems. Types L and K are approved

Cheers

[Peter_1]

Lana, if I can go as low as 500fpm, then my problem is solved of course.

But you are speaking about compressor suction pressure and US Iceman is speaking about evaporator suction pressure. This is of course not a big difference but it's worth making once the exercise.
I think that pure theoretically that the tube must gradually expand the more we approach the compressor.
So starting with a small tube at the evaporator outlet and expanding towards the compressor.

PS: Ispell is saying that 'towards' is wrong. What's the correct expression then?

[lana]

Hi Peter,

PS: Ispell is saying that 'towards' is wrong. What's the correct expression then?

I had the same warning. I don't know. We understand each other, whatever Ispell says .
I think this is one of the words which does not need "s" at the end.

Cheers

[taz24]

So starting with a small tube at the evaporator outlet and expanding towards the compressor.

PS: Ispell is saying that 'towards' is wrong. What's the correct expression then?

I think that the term would be expand away from the evap?
Not sure!!
But like you stated we all knew what you meant .

Cheers taz.

[lana]

Hi Peter,

The following text is from ASHRAE Refrigeration handbook (chapter 2) :

A pressure drop in the suction line reduces a system’s capacity because it forces the compressor to operate at a lower suction pressure to maintain a desired evaporating temperature in the coil. The suction line is normally sized to have a pressure drop from friction no greater than the equivalent of about a 1 K change in saturation temperature.

Hope this helps.

BTW : about the word "towards" , I found out that "toward" is American English and "towards" is British. SO Ispell is American .....
I like British one, I am little bit of Anglo-Saxon .

Cheers

[US Iceman]

ernestlin,

Here in the US an air-cooling evaporator used in refrigeration applications is rated based on BTUH/°F of temperature difference (TD).

The entering air temperature to the evaporator is considered the room temperature, therefore room air temperature - TD = evaporating temperature.

The TD fixes the the evaporating temperature, which in turn determines the evaporating pressure for a specific refrigerant.

Therefore, if you take the evaporating temperature - the equivalent temperature loss of the suction line you have the saturated suction temperature of the compressor.

The important issue to remember is not whether you use the suction pressure or the evaporating pressure, but to make sure you include the suction line pressure loss so that it does not affect the system capacity.

If you select the compressor based only on evaporating temperature, the evaporator TD will re-balance at a lower TD and loose evaporator capacity...

I think that pure theoretically that the tube must gradually expand the more we approach the compressor.

Theoretically, I agree. Although I do not do this. What you are describing Peter is similar to designing air duct work for a constant static pressure loss.

In horizontal lines I would rather have them sloped back to the compressor, but large enough for minimum pressure loss. But, I also look at the minimum velocities as a comparison.

The way I look at this problem is to carefully evaluate the risers and use of traps to get the oil back into a horizontal line back to the compressor.

Once the oil is in the horizontal line, most of the problems are solved anyway.

[ernestlin]

Hi, Iceman, I got your point, we just look at this issue from a different angle, for design or selection, TD always is a specific empiric value, right?
Lana, your datum is right, but mine isn't incorrect, it comes from tecumesh's guide manual. For various consideration, different Co. may afford different values, they're all based on expriments.

[lana]

Hi ernestlin,

Lana, your datum is right, but mine isn't incorrect, it comes from tecumesh's guide manual. For various consideration, different Co. may afford different values, they're all based on expriments

I didn't say your values are incorrect. I was talking about the "minimum" velocities. Any velocities above those values would be more than safe.

Cheers