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View Full Version : Multiple ref. units for cold stores







smpsmp45
29-05-2007, 12:06 PM
In one case, there are Five no 40 Hp ref. units - monoblock type for a cold store of size 50 x 35 x 12m. The cold store has racking inside.

The ref units are controlled by remote panel. Where to fix up the temp thermostat sensor. All five units are controlled through One T/Stat.

Other point is sensors in parallel / series & their behaviour.

Typically the sensor is located behind the cooling coil ( return air temperature)

Experts comments please

US Iceman
29-05-2007, 03:26 PM
What does the remote panel contain and what does it contribute to the cold store operation?

smpsmp45
30-05-2007, 12:25 PM
The Remote panel is essentially to set up the various parameters like temp, defrost cycle change - like time, manual defrost mode, temp indicators , indication for which cycle is on - compressor on, defrost mode, temp cut off, any error in the system etc.

So instead of changing these parameters on the units( which are located at 12 mt height) , those can be adjusted say just at the door entry.

For multiple units, there shall be a bank of these indicators individually for each unit. But temperature control / indication / location of the temp sensor is always a debatable issue

US Iceman
30-05-2007, 03:21 PM
Most of the installation I have seen place the air temperature sensor in the return air path back to the evaporator.

If you are maintaining the interior air temperature, the return air should be the warmest air in the room. Except when the door is open!:)

If you have five individual systems, why not use a temperature sensor on each evaporator to control a specific compressor unit?

smpsmp45
03-06-2007, 06:35 AM
If we do that, the unit near the door is alwasy running. We also wnat to stagger the operations of the unit. Plus many a times we find there are zones of hot ait pockets if we control individual units. The unit which is on Defrost mode, the near by units get wrong signals for temp. We are planning to put sensors in more than 4 places & connect them - Now if those should be in series or parallel is the issue & how the average temp is found out?

US Iceman
03-06-2007, 02:03 PM
If we do that, the unit near the door is always running.


Where is the evaporator in relation to the door? Is it located above the door, or away from from it with the air discharge blowing towards the door?



The unit which is on Defrost mode, the near by units get wrong signals for temp.


That is a problem when the coils are in close proximity to one another. If the coils are too close together, the air flow across the room may be uneven. This could contribute to your temperature distribution problem.

smpsmp45
04-06-2007, 08:00 AM
The Evaporator is not above the door. But still one of the units is nearer to the door compared to others. That unit generally runs all the time. Plus we need a start timer delay so that all the units do not start in one shot after electrical supply problems.

At present if we run the system as such, there is no problem in terms of air temp all over the room. But we wnat to ensure that technically correct system is installed. Many thanks for your keen interest in the situation

US Iceman
04-06-2007, 04:18 PM
One other way to address the total running time of the systems can be addressed by using a separate refrigeration system on the entrance side of the doors.

If you can dehumidify the air before it enters the cold store this can reduce the cooling load on the main systems. The dehumidifying system can run at a much higher evaporating temperature so that the coils can run wet instead of frosted.

By removing the water before it enters the cold store you can reduce the defrost times of the units in the cold store.

You may already be doing this, but I thought I would mention it anyway.

Samarjit Sen
04-06-2007, 07:27 PM
Considering that the size of the Cold Storage is pretty big, I would not like to use monoblock system. The monoblock system is ideal for small cold rooms. In this case there will be a number of air pockets as the air distribution is not proper.

It also depends on the technical specifications of the Evaporators. What is the air throw, the velocity and the Static Pressure. These points we tend to forget, but these are important for proper temperature which can be attained by having proper air circulation.

Another thing is that an Evaporator should never be installed above a door or near a door. The layout of the Evaporators are important.

We have executed similar projects, but have never incorporated the monoblock units. It is better to have seperate condensing units with seperate evaporators located such, so that there is an uniform distribution of air.

It is good to locate the temperature at the rear of the evaporator as you will get the return air temperature which will be warmer.

US Iceman
04-06-2007, 09:50 PM
Considering that the size of the Cold Storage is pretty big, I would not like to use monoblock system. The monoblock system is ideal for small cold rooms. In this case there will be a number of air pockets as the air distribution is not proper.

It also depends on the technical specifications of the Evaporators. What is the air throw, the velocity and the Static Pressure. These points we tend to forget, but these are important for proper temperature which can be attained by having proper air circulation.

Another thing is that an Evaporator should never be installed above a door or near a door. The layout of the Evaporators are important.

We have executed similar projects, but have never incorporated the monoblock units. It is better to have seperate condensing units with seperate evaporators located such, so that there is an uniform distribution of air.

It is good to locate the temperature at the rear of the evaporator as you will get the return air temperature which will be warmer.

This sums up my concerns nicely. Especially the those two statements highlighted in red and underlined. The other comments are noteworthy too.

I may be bit particular on the two main points, but I can't emphasize them enough nor the following: Do not locate evaporators over doors.

Sledge
21-06-2007, 06:25 AM
Why is the running time of one unit, compared to the rest, a concern?

I understand that equipment is not designed to be running absolutely non-stop all the time., but I dont understand the concern over balancing run-time of equipment.
If you try and run all the equipment equally, you will have minimal equipment wear-out in the short-term, and then you will be hit with major issues all at once as all of your equipment will be worn out.
I am a strong supporter in avoiding total catastrophic failure, by dealing with things in a controlled way.

smpsmp45
21-06-2007, 08:50 AM
Oh! I like this aspect & I am sure all our clients too shall like this.

You said it. It was the major concern for all the clients that one unit running & wearing out fast.

Many thanks for the appropraite answer.

Samarjit Sen
21-06-2007, 06:02 PM
The Cold Storage should have uniform temperature throughout its space. If your unit is performing properly then thay shall be shutting down at the set temperature. You must have provided on delay timers to the restart of the compressors so that all compressors do not start at one time.

I am interested to know how have you located the evaporators inside the chamber.