Oil return/reduced evaporator capacity

[Peter_1]

I occasionally work on plants in cold climates where the loads are very low in the winter for an extended period of time.

This is a copy of a thread somewhere else (see post 6) because we drifted a little bit from the original topic
Just throw this here in this forum: is the load anyhow lower on the evaporator side (while running) in cold winter period?

Let me try to explain: let's take the summer condition.
We have to maintain a temperature in the coldroom of 1°C (33.8°F) and a DT of 2K before cooling again.
So when it becomes 3°C (37.4°C) the compressor starts.
So, what load is the evaporator seeing? An airflow of x m³/h at a RH of x% which must be cooled from 3°C to 1°C.

This load will be the same in the winter. The only thing which will change then very slightly is that the room will reach faster the set temperature because heat infiltration through the walls will be smaller (very minor influence if PU panels are used)
If the compressor is not maintained on a certain pre-set HP, compressor capacity will increase with a lower evaporating pressure (or a bigger DT over the coil) as a result.
Due to the colder outside temperature, liquid temperature will drop, so an increased enthalpy.
But the load seen by the evaporator doesn't change, so the mass flow of the refrigerant flowing will also stay +/- the same.
We service a poultry slaughtery (+/- 6000 chickens/hour) and the compressor were maintained the whole year at a pre-set Hp with pressostats.
They had +/- the same running hours during winter and summer. The only explanation for the longer running hours during summer were because we couldn't hold the HP low enough to the pre-set HP (fans were running continuously)

So the compressor

[NH3LVR]

Just throw this here in this forum: is the load anyhow lower on the evaporator side (while running) in cold winter period?

Peter_1;
Thank you for pointing out my error.I understand the point you are making. The load that a compressor sees is not dependent on the outside temperature and that the coil/room TD is the major factor.
In the Plant I cited however the circumstances are different. I used this Plant as an example, it's design is not one that has an oil return problem.
The Plant is Pump Recirculated, two stage. It has one large evap in the -10F (-24C) area. (This Evap is fed continuously, except during defrost.There are twelve evaporators in the Medium Temp areas.
The normal running mode is with a Screw Booster running at about 90% during the Summer, 30% in the
Winter. The High Stage operates at about 90% Summer/50% Winter. The Compressors cannot be cycled On/Off.
The Winter Average Temp is down 42F (23C). The indication of the reduced load is the reduced load on the Compressors.
But this is not caused by the outside load directly. The Evaporators cycle by the opening\closing of a Liquid Solenoid. It is not uncommon to see only one or two Medium Temp Evaps working in the plant at any one time during the Winter.
So you are correct that the Mass Flow through the Evaps does not change during the time the coil is operating. However the flow through the common suction does.
Cycling the Compressors is the difference in this case.
But lets us look at the case of a system using TX Valves. (Not my best area of expertise, I might say, so feel free to correct me)
During a cooling cycle the Liquid Line Solenoid will come on and admit Liquid to the TX Valve. At first it will feed quite heavily. As the load drops off with the room temperature, the TX Valve lowers the feed rate to maintain the Superheat Setting. The Mass Flow drops off. It can only fall to a certain point as the Liquid Line Solenoid closes when the temperature is satisfied at the Thermostat. In most cases this will still maintain sufficient Mass Flow to return the Oil.
I have seen Blast Freezing Systems (poorly designed/installed) that would not return oil at the end of the freezing cycle. These systems are left to run at low temperatures without any provisions for cycling.
In the example I used of the Summer/Winter loads in this Plant there are many factors involved in the reduced Winter load, besides the outside Temperature.
Infiltration, forklift usage, occupants and other factors all come into play here.

[Peter_1]

NH3LVR, it was not my intention of pointing on an error.
It was more sometimes I allways think on when someone says that load decreases when it becomes cold. This seems not so acceptable for me but it's sometimes difficult to explain your doubts.

[Peter_1]

But this is not caused by the outside load directly. The Evaporators cycle by the opening\closing of a Liquid Solenoid. It is not uncommon to see only one or two Medium Temp Evaps working in the plant at any one time during the Winter.
So you are correct that the Mass Flow through the Evaps does not change during the time the coil is operating. However the flow through the common suction does.

You're completely right that the speed in the main suction header will decrease a lot at reduced load because this header is calculated for max capacity.

How we solve this: we open on a regulary base (4 to 6 times /day) all the SV's of the whole plant for some time to ensure oil return.
Main header is also calulated at minumum 12 m/s (sometimes 15 m/s) at max load conditions.

We once had a freezing tunnel with a pack with big troubles with the problems you mentioned: oil not coming back. The service tech filleld the oil receiver back up again and that was his solution. After some weeks, fill it up once gain and eventually once again.
Till one compressor tripped on overload and we opened the compressor: pistons were broken and we even found pistons turned 90° in their housing.
This freezing tunnel started at 0° (32 °F) and went to -32°C (-25.6°F), so a very big decrease of load.

Another thing that botters me and I'm not sure my point of view is followed by other members: load on the evaporator in a cooling room or a freezing room varies almost never: the heat content of air at 3°C (37.4°C), 2°C (35.6°F) and 1°C (33.8°F) is so small that this can be seen as each time the same load.
So I don't see the need for the so praised EEV's which can adapt fast and precise to various load conditions. But the load doesn't change, only the running time.

[stan1488]

good description of plant peter lol, now who has info on energy reduction w vfd control? soft starting less cycling steady load control etc , has anyone done analysys before and after?

[NH3LVR]

Peter_1
We are digressing from the original point of this thread. I wonder if it would be better if we continued this on another a new thread, related to the percentage of total load caused by the box load. Others could contribute much easier than I. I have not worked these calculations in a long time, and with the software many members use, they could run examples for us very quickly.

[NoNickName]

It looks like that marketing strategies concerning VFD are taking their toll also on engineers and technicians.
I don't see a single reason why VFD here should be applied, as Peter explained, the load for a industrial plant rarely changes throughout the year.

[Peter_1]

We've installed several VFD's on packs but with -at least for me - valid reasons:
1. Main reason: the Belgium government is paying almost 75% of the VFD. This is because they then can - or better they think they can - this way fulfill the demands of the Kyoto protocol. You have to fill in some papers and make an estimate how much you will save. (how many hours will you run on 100% and how many hours on partial load. Nobody is verifying this numbers) They (government) can say then that this plant will use less energy due to the VFD they sponsored.
2. They were installed on packs in meat cutting plants where some coldrooms are switched off completely for some hours, even some days.
3. In some rooms are hanging evaporators so that we can cool very fast the meat and then afterward once everything is cooled down switch off some evaporators so that the running cooling capacity meets more the actual load.

Another way - like I and many others see it a very stupid and shortsighted way - how we fulfill the demands of the Kyoto protocol: we simply buy fresh air from those who still are far away from the Kyoto specs. So we're 'buying' fresh air from Russia where there is a lot of not explored land, land where nobody is living.
Heard that the UK, France, Germany and many others is doing it also the same way.

[US Iceman]

If the process is a production line the cooling loads for the product are almost stable throughout the year. The biggest shift in loads may be due to any loads from air cooling/dehumdification.

In the winter or cooler months the air cooling/dehumidification loads drop off quite drastically, so you might see a significant reduction in cooling demand due to this.

Another application might be production freezing for a short time of the year. The system could have a high capacity installed for that short period, but then if the only loads are for storage, again you could see a large reduction.

A lot of this I think depends on the number and sizes of compressors installed. If there are sufficient sizes and capacities available, a VFD may not be required.

So in conclusion, I would have to say... it depends on the installation as to whether a VFD makes sense.