Reversible cycle defrost for cold storage

[tadipatrir]

Dear All,

I am planning to go for a reversible cycle defrost for cold storage sub zero application.

Can you please tell me what all things I need to look at.
I suppose since condenser will act as evaporator duting defrost cycle and since it is exposed to ambient, suction pressure may go up and also lot of superheating may take place.

I will be using a 4-way valve for this application.

Waiting for your replies.

Regards

[smpsmp45]

Use this , if this is a must.

Lots of controls & expansion devise needs to be done carefully.

You can still think of a simple system like Hot gas defrost or elec heaters. The system shall be very easy to operate.

[tadipatrir]

Dear Smpsmp45,

Thanks for your reply. The reason of going for this system for simplicity & lower cost involved.

I thought of going for hot gas defrost initially but it was becoming complex in terms of controls and cost. Crank case pressure regulator is also must. You can suggest if you have any simpler system in hot gas defrost.

Cheers

[Mr Cilltech]

i think hot gas defrost will be a cheaper and straight forward option. there are heaps of digital controller out in the market which will give u options to control hot gas solenoid , i can send u a wiring diagram if needed.

[gas_n_go]

Im a little bit confused can someone link me or explain whats the difference between reverse flow defrost and hot gas defrost? I thought hot gas was one form of reverse flow and kool gas the other?

[Mr Cilltech]

hello!

for reverse cycle hot gas defrost, a reversing valve will be used but it involves a bit to much pipework modification whereas for hot gas defrost , just Tee a pipe from discharge and install a solenoid in the middle somewhere and join on inlet of evap. use digital controller to control solenoid and use evap coil sensor to terminate defrost as soon as evap temp is around 6-8 degC.

[Peter_1]

...The reason of going for this system for simplicity & lower cost involved.

Simplicity and lower costs?? I don't think so.
It's more prone to failure and the additional valves and controls you will have to install will cost a lot of money.

[mohamed khamis]

Dear All,

I am planning to go for a reversible cycle defrost for cold storage sub zero application.

Can you please tell me what all things I need to look at.
I suppose since condenser will act as evaporator duting defrost cycle and since it is exposed to ambient, suction pressure may go up and also lot of superheating may take place.

I will be using a 4-way valve for this application.

Waiting for your replies.

Regards

Hi tadipatrir

It is not favorable and practical to use a reversed cycle as defrost cycle. The reason of that is when the evaporator acts as a condenser there is a problem which hastens the process of system failure which is during the defrost cycle the former evaporator is fully iced up there fore the present condenser rejects its all heat to this ice "and this ur aim" because there is a blocked air flow over the coil. however, due to the huge difference between the refrigerant temperature and the iced surface, this causes a high confidence of possibility to thermal stress leads to a crack after short service hours in the tube surface due to the difference in the temperatures. And there are some factors activates the strength of this thermal stress which are :

1- the frequency of using the coil as condenser once and another as a evaporator afterwards.

2- the area of the evaporator coil as acting as condenser "in defrost cycle" is small and this pushes the condenser to relay on the coolant temperature "the iced surface".

I wish it could help

Best regards

[tadipatrir]

Dear All,

Lot of good feedback. Thank you for that.

I will keep in mind about thermal stress of tubes.

I will be interested to have the wiring diagram of hot gas defrost if Mr. Cillteck can send me.

At present we are using electrical defrost. Due to manufacturing difficulties and setting times involved we wanted to go for defrost using discharge gas.
In normal hot gas defrost needs atleast 2 solenoids and a crank case pressure regulator in the suction line. Even in this case also themal stress of the tubes is possible.

In reversible cycle I will be using 4-way valve and an expansion valve with biflow function replacing the existing expansion valve. Addition of tubes is involved in both the cases.

Correct me if I am off the track.

Cheers!

[mohamed khamis]

Dear All,

In normal hot gas defrost needs atleast 2 solenoids and a crank case pressure regulator in the suction line. Even in this case also themal stress of the tubes is possible.

Correct me if I am off the track.

Cheers!

Hi tadipatrir

Yes, There is thermal stress in case of hot gas defrost but it is trivial comparing to that in case of ur proposed reversed cycle. The reason is in hot gas defrost it is strongly recommended to use a low pressure hot gas and flow rate and according to ASHRAE fundamentals it is recommenced to design hot gas defrost by 1/3 the system capacity by selecting the 3-way valve which will be installed on the hot gas line which gives a condensing temperature of 11 to 18°C. Thus, if u use reversed cycle as u mentioned and the evaporator will subjected to ambient in India, inevitable the condensing temperature above 45°C "if i m not mistaken" and u can judge now which method has the significant thermal stress. In addition, the higher condensing temperature used in defrosting cycle can not only melt the frost but also boil off into the cold room and increase the humidity inside the room and it is not applicable in case of low-humidity application. Furthermore, the steam formed from the boiling off can accumulate on the fan blades and during system operation this condensed steam can be freezed making small ice balls and certainly unbalance operation of the evaporator fans. I suggest if u would like to use a reversed cycle defrosting u put by-pass valve circuit on the condenser "in defrost cycle" to bypass unwanted amount of hot gas over the condenser
, this will protect u from these problems and the bypassed hot gas can be exploited to heat the drain pan of the frost melting i.e. it replaces the job of the heater which immersed in the drain pan under evaporator in hot gas defrost but take care to measure the length of the bypassed gas line which will be immersed in the drain pan to design it to can warm up the drain only with bolling it off. i think also this will make a rather drop in evaporating temperature due to inefficient liquid feeding of TXV to the evaporator and in turn lowering the condensing pressure/temperature. i wish it could help

Best regards

[mohamed khamis]

Hi tadipatrir

I came across today the book of"Princples of Refrigeration" authoized by Dossat. He describes a reversed defrost cycle by adding automatic expansion valve before the condenser in case o defrost cycle to meter the proper amount of refrigerant to defrost the coil. I wish it could help

Regards

[tadipatrir]

Hi Mohamed,

Thanks for your advice. I also have book of Dossat. I will go through and follow your advise.

Cheers!

[mohamed khamis]

U welcome at any time tadipatrir