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Guidelines for hot gas defrost on plate freezers. These guidelines are prepared for A/S Dybvad Stålindustri by Per Skærbæk Nielsen, Cooling Consult in cooperation with the Refrigeration and Heat Pump technology centre at the Danish Technological Institute in Aarhus, Denmark, and are intended as a help for our costumers. The guidelines is based on many years of experience in optimizing refrigeration systems for our plate freezers, and we recommend to follow the advice as far as possible, because that will give a safe and effective operation of the installation. At the same time we want to make it clear that we don’t want to dictate how to design the refrigeration system for our freezers, and that the responsibility of the design and installation of a correct and workable refrigeration system still is resting on the supplier of the refrigeration system.

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Plate Freezer Hot Gas Defrost. General guidelines: 1) Never inject the hot gas into the liquid feed line of the plate freezer. 2) Always use slow opening hot gas valves or open hot gas line in two steps. First open a small bypass solenoid valve and after 5- 10 sec open the big main hot gas solenoid valve. 3) Always make a pump down and / or draining before injecting hot gas. Newer inject hot gas into a plate freezer which can be nearly full or full of liquid. 4) If possible open drain line before opening hot gas line. 5) Do not open hot gas line before suction line valve is fully closed. Note: Hot gas operated normally open (NO) valves can have long closing times. 6) Bleed down the high pressure from the hot gas defrost in the plate freezer before opening suction valve again.

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Explanations: 1) Never inject the hot gas into the liquid feed line of the plate freezer: The hot gas can “shoot” the liquid in the plate freezer into the suction stop valve, pressure regulating valve or the hose connection between pipe system and freezing plates. The cold vapour and vapour bubbles in front of the liquid will collapse or implode into liquid and create very high pressure “spikes” of several hundreds of bars pressure. The pressure “spikes” will hit where the last vapour bubbles collapse or implode as the movement of the liquid becomes focused at this point and at the same time becomes incompressible, when no vapour bubbles is left. Possible damage: Suction and control valves: Internal parts in valves damaged, leaking gaskets. Joints and assemblies: Bolts stretched so nuts and assemblies gets loose and / or broken gaskets. Hose connections: Hoses can be “stretched” by the pressure “spikes” and eventually leak.

2) Always use slow opening hot gas valves, or open hot gas line in two steps. First open a small bypass solenoid valve and after 5- 10 sec open the big main hot gas solenoid valve. The hot gas line must open slowly or “soft” so the pressure in the plate freezer rises slowly. When the pressure rises slowly the pressure “spikes” from collapsing or imploding cold vapour bubbles becomes smallest possible. The pressure “spikes” will hit where the last vapour bubbles collapse or implode as the movement of the liquid becomes focused at this point and at the same time becomes incompressible, when no vapour bubbles is left.

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It is possible to get an idea of the magnitude of the pressure spikes where they hit, by looking at the change of momentum of the moving liquid. (mass) ( original velocity – 0) M x (V1-V2) pA= Time needed to bring liquid to rest = pA= T1-T2 Where: p= pressure (bar) , A= area (m2) From the formula can be seen: If T1 – T2 goes against zero (T1-T2 →0) the value of pA goes against infinity (pA→∞) The value of (T1-T2) will depend on the compressibility of the liquid and the materials in the construction. If the Mass and velocity rises, the value of pA will rise: The velocity (V1-V2) will be very high when vapour bubbles collapse or implode. It is the same phenomenon as cavitations, which can destroy the surface on hard materials with even very small bubbles imploding. The mass will be given by the amount of liquid moving in the pipes and plate freezer. Possible damage: Suction and control valves: Internal parts in valves damaged, leaking gaskets. Joints and assemblies: Bolts stretched so nuts and assemblies gets loose and / or broken gaskets. Hose connections: Hoses can be “stretched” by the pressure “spikes” and eventually leak.

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3) Always make a pump down and / or draining before injecting hot gas. Newer inject hot gas into a plate freezer which can be nearly full or full of liquid. If the plate freezer is nearly full or full of liquid, there will be nearly no free cold surfaces to condense the incoming hot gas. The fast condensation of the incoming hot gas on the cold surfaces will reduce the pressure chock on the liquid in the plate freezer. The pump down and / or draining will also reduce the mass of the moving liquid in the pipe system and plate freezer And by this reduce the pressure “spike” from collapsing or imploding bubbles. Efficient draining can be done by closing the suction line and liquid line and open the drain line, so liquid is sucked out through drainline. Note: To be able to perform a pump down, there must be a solenoid valve in the liquid line, so liquid supply can be switched off during pump down Possible damage: Suction and control valves: Internal parts in valves damaged, leaking gaskets. Joints and assemblies: Bolts stretched so nuts and assemblies gets loose and / or broken gaskets. Hose connections: Hoses can be “stretched” by the pressure “spikes” and eventually leak.

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4) If possible open drain line before opening hot gas line. If the drain line can be opened, it must be opened before the hot gas line opens as the last collapse or implosion of vapour bubbles often takes place at or in the valve in this line. If the line is open the liquid can shoot thru the valve, and there will be less risk of damage to valve and joints. Possible damage: Drain valve: Drain valve cone, drain valve flanges, gaskets and connections, Hoses: Hoses and hose connections.

5) Do not open hot gas line before suction line valve is fully closed. Note: Hot gas operated normally open (NO) valves can have long closing times. Hot gas operated normally closed (NC) valves can have long closing times. The hot gas keeping the valve open will condense and must be drained out as evaporating liquid thru a small pilot valve to the suction line, before the spring can close the valve. If the suction line is not closed, the hot gas will by pass to the pump separator and raise the pump separator pressure without defrosting the plate freezer, until the suction valve is closed. The closing process can take long time and will depend on the valve size, the refrigerant in question and running conditions of the system. The larger the valve is the more time will it take to close. Ammonia (R717) is the refrigerant with the longest closing times for the valves, as small amounts of liquid produce large amounts of gas.

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Some of the large valves (DN 125-DN150) can have closing times of 10 to 12 minutes. To get more information on these valves closing times the valve manufacturer should be contacted. NOTE: The delayed closing of the suction valve can be a reason for the pressure in the plate freezer to rise slowly during hot gas defrost, and by this actually help preventing “liquid hammer” in the plate freezer, when opening the hot gas valve. Possible problems and / or damage: Hot gas operated suction valve: Gas and liquid is send thru the valve at very high velocities and can damage the valve. The two step suction valves are especially vulnerable if the second step in the valve is spring loaded. These types of valves can hammer so hard with the second step closing cone that they break completely. Energy loss: The bypassing of hot gas in the system without achieving any defrost means energy loss. The rising pressure in the pump separator can also make compressors start and stop again unnecessarily during defrost.

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6) Bleed down the high pressure from the hot gas defrost in the plate freezer before opening suction valve again. The high pressure from defrosting the plate freezer should be taken down slowly to close to suction pressure before opening the main suction valve to avoid liquid hammer and pressure shocks in the suction line. This bleed down can be done thru a little by pass solenoid valve over the main suction valve, or thru the drain line, if this can be opened by a solenoid valve. Note: The use of differential pressure depending two step suction valves might lead to long opening times for this type of valves on plate freezers. Plate freezers normally start up with very high capacity and with liquid in the plates. This means it can take a long time to lower the pressure in the plate freezer thru the first step limited valve capacity to the pressure where step two opens and full valve capacity is available.

Per Skærbæk Nielsen Thomas Lund Cooling Consult. Danish Technological Institute Bjødstrupvej 77 Refrigeration and Heat pump technology 8310 Tranbjerg Kongsvang Alle 29 Denmark 8000 Århus C Tlf +45 28720673 / +45 86721990 Denmark Email skaer@mail.dk Tlf +45 72201266 Email thomas.lund@teknologisk.dk