Common refrigeration defrost management

All cool rooms and freezers use evaporators or coolers to help regulate temperature inside the room.

When people and equipment enter and exit a room they let varying amounts of ambient air into the space which needs to be brought back down to the regulated temperature. This air generally contains a significantly higher level of water vapour than the room. The water vapour or moisture in the air is naturally drawn to the coldest element in the room which just happens to be the fin and tube surfaces within the evaporator.

The coolant circulating through an evaporator or cooler is always colder than the air circulating around the room. Depending upon the criteria or special circumstances used by the installer to create your system, this coolant can be anywhere between 3°c and 8°c colder. After a period of time the accumulated ice and frost on these surfaces restricts the air flow over the unit and also acts as an insulator.

This significantly reduces the efficiency of your evaporator and creates a significant risk of consequential damage to compressors downstream by way of liquid refrigerant floor back and oil wash out. Both of these scenarios can have a measurable impact on the costs of operation.

In order to allow the refrigeration service to get back to working at optimal levels, you need to remove the frost build up as quickly and effectively as practical. There are many methods employed, all with their own strategic advantages and disadvantages.

OFF-CYCLE DEFROST:

The simplest and cheapest method is generally only applied to coolrooms operating above +2°c. In this mode any ice formed during the normal running time of the system is melted by the cyclic stopping of the cooling system, whilst maintaining the room air flow. The room air melts the ice which falls to the drop tray and away to waste. The number of stoppages and the length of the off cycle time could be either unforced or forced depending upon the room controls and system demands.

It is not uncommon for room temperatures to rise by several degrees under this methodology and so consideration needs to be given to the effect on product quality and storage certification needs.

ADVANTAGES: Cheap to install and operate. Simplest
DISADVANTAGES:

Little temperature control authority, suitable only for some cool rooms, risk of water droplet carry over into the room and product.

ELECTRIC DEFROST:

One of the most common methods in defrost management is to utilise AC electrical elements on or within the evaporator or cooler. This utilised a heating element within the coil block. Used predominantly in freezer rooms and cool rooms where off-cycle defrost is unreliable or too slow.

During a defrost cycle, determined by the automatic system controls or a manual initiation, the circulation fans are switched off and the heating element on.

The number of defrost cycles per day, length of defrost, defrosting temperatures, dropping time post defrost are determined by the system controls. These controls may employ the very simple time start: time terminate or the more complex system response controller.

ADVANTAGES:

 

Moderate in complexity and speed to defrost, good control authority, suitable for all small to medium sized rooms.
DISADVANTAGES:

Moderate cost to install, high energy cost to defrost and recover.

REFRIGERANT GAS DEFROST:

Another way of performing a defrost is through the use of the heated refrigerant already in your system.

“Hot Gas Bypass” systems can be found in both larger systems and small package equipment and will generally require the attentions of a refrigeration maintenance engineer to ensure it is both effective and reliable. These systems stop the cooler fans and divert a percentage of the hot discharge vapour from the refrigeration compressor outlet directly to the evaporator to life the temperature of the heat exchanger. The evaporator coils are heated from within, melting the ice and frost.

Like the electric defrost system, the number of defrost cycles per day, length of defrost, defrosting temperatures, dripping time post defrost are determined by the system controls. These controls may also employ the very simple time start: time terminate or the more complex system response controller.

Since this method also changes the refrigeration system complexity, careful consideration needs to be given to the system balance and lubrication issues that might be otherwise adversely affected.

ADVANTAGES:

 

Speed to defrost, good control authority, suitable for all small and larger sized rooms, lower energy cost to defrost and recover.
DISADVANTAGES:
High cost to install, moderate in complexity.

“REVERSE CYCLE”:

“Reverse cycle” systems are most often found in packaged equipment since they require the attentions of a refrigeration services designer to ensure it is both effective and reliable. These systems stop the cooler fans and literally reverse the system operation so the cooling is now performed outside and all the rejected heat is sent to the internal cooler. Since the cooler or evaporator fans are not running, the heat is almost wholly retained within the unit melting the ice and frost from within.

Like the electric defrost system, the number of defrost cycles per day, length of defrost, defrosting temperatures, dropping time post defrost are determined by the system controls. These controls may also employ the very simple time start: time terminate or the more complex system response controller.

Since this methods also changes the refrigeration system complexity, careful consideration needs to be given to the system balance and lubrication issues that might be otherwise adversely affected.

ADVANTAGES:

 

Highest speed to defrost, high control authority, suitable for package units only, lower energy cost to defrost and recover.
DISADVANTAGES:
Moderate cost to install, increasing complexity.

The important component of any of these three common methods of defrost management is to take the time to ensure that the system is correctly configured to suit your particular establishment.

Each and every defrost costs and the need for defrosting is a simple consequence of daily operation. A periodic review of effectiveness and bottom line impact can be a very worthwhile exercise and best results accomplished by engaging operational and refrigeration maintenance personnel.

If you are looking for assistance navigating these issues or looking for a defrost management solution for your refrigeration system please give us a call on 03 9793 6088 or contact us below an obligation free chat with one of our refrigeration maintenance technicians today.

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Heuch - Frost Heave Prevention and Slab Heave.

Frost heave

Frost heave can be found in walls, floors and any other areas associated with freezer rooms and colder environments, both constructed and natural.

 

What is Frost heave?

Frost heave (sometimes referred to as ice heave, slab heave and concrete heaving) occurs when sub-zero ambient temperatures penetrate the earth or a structure and freeze pockets of moisture. As these pockets freeze, they expand. This potentially enormous pressure has to go somewhere, so it pushes the earth or structure outwards toward the point of least resistance. When considering ground based frost heave, this is usually up to the surface, and creates little mounds. Areas that exhibit Freeze-Thaw-Refeeze cycles can be particularly persuasive and cause slab heave.

 

Why is frost heave important?

A bit of frozen moisture doesn’t sound like much, but it has the ability to seriously damage floors, building foundations, roads and driveways. Much in the same way that a little steam doesn’t sound like anything to worry about, until you contain it, then it can drive a train, or electricity producing turbines!

Moisture is always drawn to the cold. So when a pocket of ice forms under the soil, underground moisture is drawn to it, freezes and adds to the pocket. This is a problem that the people of Canada (for example) have to deal with on a seasonal basis. The frost heave often pushes sideways and damages the foundations of their houses and basements.

In the commercial sector, if the floor of a freezer isn’t heated, the frost heave will lift and crack floors, create a very uneven surface, prejudice walls and structures and make for a very unsafe work environment. It can penetrate poorly sealed walls, freezing and bulging, damaging the structural integrity of the very walls themselves.

 

How to spot frost heaves.

It starts with a bit of a sinking feeling. Often, businesses we visit will comment that the foundations of their building are sinking, but a quick inspection points out that actually, the floor is lifting! Sometimes the floor heating has failed, and other times it hasn’t been installed at all.

Early signs to look out for include:

  • The corners and edges of the freezer room appear to be sinking.
  • Uneven spots in a once very flat freezer floor.
  • Cracks in the concrete that are only getting worse.
  • Bulging or warped walls of the freezer room.

 

How to fix frost heave

At the first moment you suspect frost heave is occurring, check that your floor heating is working!  If the walls are the problem, check that your freezer room has been vapour sealed correctly and effectively. Is there forklift damage to the external skin of the cool room panels? Do the door jambs need repair?

 

Floors:

If your floor is electrically heat traced, check the circuit breaker to make sure it’s still working. If it can’t be reset, check for a second circuit. Often a second heater is laid in the concrete to add a level of redundancy and peace of mind.

If the floor relies on outside air ventilation, check that the pipes or channels supplying air flow aren’t blocked or supporting nesting rodents and their families.

If your floor uses hot gas or reticulated glycol from your refrigeration system to heat the floor, this will need reviewing by your refrigeration services company.

The bad news is that if these have failed, the heating was poorly installed, or never installed, then the fix may become expensive.

The first step is shutting the freezer room down to let the ice pockets thaw and the floor to settle. This can take weeks in some cases and obviously disrupts productivity, and is only a very temporary measure. The results are variable and there is no guarantee this action will not in itself cause consequential damage.

To fix the problem from here, you might have to have the freezer floor re-laid. Getting a contractor in to tear up the existing floor and insulation, lay floor heating and re-pour a concrete floor. But once it is done, is something you no longer have to have in the back of your mind.

Walls:

Have a coolroom panel professional inspect the joins and penetrations in your walls. They will be able to detect if your vapour seal is adequate, and identify where the moisture is getting in.

If it’s caught early, and the vapour seal is repaired, the room might be able to simply returned to service.

If not, the coolroom panels may have to be replaced.

 

The best fix, like almost anything, is PREVENTION! 

Heuch - Frost Heave and Slab Heave.

Notice the pallet racking leaning due to an uneven surface caused by frost heaves.

How to prevent frost heave.

Best prevention is to have a regular maintenance program by knowledgeable techs and make sure floor heating is being looked at and included in your programmed maintenance service.

If you are going to be buying or leasing a new facility, check with the current occupants/landlord to find out if the freezer room has floor heating, and make sure that it works!

If you are having a new freezer room built, discuss the floor heating methodology with your supplier.

  • What floor heating system are they offering?
  • What backup method if offered?
  • How is it being installed?
  • Can it be repaired or what redundancy is included?
  • How much energy will it consume? Remember this heat energy is required 24 hours a day, 365 days a year!
  • Is waste energy reclaim used?
  • How much floor insulation will be laid?
  • What type and specification is the vapour barrier system?
  • How will the installer implement the vapour barrier?
  • What insulated cool room panels joining system is proposed?
  • When will these steps happen in the construction program? Planning inspection visits during construction is a good idea.

A little vigilance and a little investment now will save you many times the expense in down times in productivity, lost storage space and will ensure a safer workplace for you and your colleagues, as well as giving you the invaluable gift of peace of mind!

Heuch can help make an assessment and develop a cost effective programmed maintenance schedule. 

Talk to Heuch’s engineering team when considering your next refrigeration system expansion.

Further references: www.ashrae.org; www.astm.org; www.bradyips.com.au; ASTM D5918-06

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