Hi Everyone,

We are currently in the concept design stage of a food storage and processing facility with the majority of the spaces being operated around 4 degC (around 8,000-10,000sqm). Only around 400 sqm of the facility is freezer store operated around -25degC. Various 4degC spaces are occupied permanently and hence outside air treatment air handling units are required to provide conditioned air into the space.

The facility is still in concept design stage, but we are throwing around some ideas regarding servicing the various areas.

For the freezer stores, we are most likely going down the route of sub-critical DX R-744 cascaded against some sort of high stage loop. This plants may be local to the freezer stores, which would help with the provision of warm glycol defrost to the freezer store evaporators.

For the 4degC spaces, we would like to work towards a high stage ammonia system (in plant room only) with low pressure receiver and flooded heat exchanger arrangement (noting that the site is in close proximity of residential and commercial areas). With regards to MT reticulation, we are considering a secondary propylene glycol loop vs a pumped CO2 system. I understand the CO2 system will be operating quite a bit more efficiently, but I'd be keen to hear some thoughts/feedback on these options, especially around the following topics:

1. Cost
Pipework and evaporators smaller for CO2, however more cost involved with regards to valve sets, heat exchangers, controls?

2. Risk of Ammonium Carbamate
In order to keep the ammonia charge to a bear minimum, a flooded cascade (plate or shell and plate) heat exchanger with low pressure receiver/separation vessel appears to be the way forward. Double sheet shell in tube appears to result in significantly larger ammonia charges. How can we mitigate the risk? What is the latest in terms of detection? I read that ammonium carbamate detectors can be used, but where do you locate these in a flooded HX arrangement? Is the intent to detect the substance in the ammonia wet return line and isolate the HX if detection occurs?

3. Ammonia Wet Scrubber
Given we are in a densely populated area, we are looking at providing wet scrubbers in the exhaust ventilation system for the ammonia plant room. Has anyone ever used one of these? How big/costly are these? How efficient are they? Who makes them?

4. Room Temperature and Air Handling Unit Supply Air Temperature Control
Some of the rooms may be required to operate under variable operating conditions. In addition, the outside air treatment air handling unit would also require to be fitted with a glycol/CO2 coil. Has anyone every used CO2 to control the supply air temperature of an AHU via using a motorised evaporator pressure regulator on the CO2 outlet of the coil (vary the CO2 pressure/temperature in the coil to maintain supply air temperature at set-point? Any ideas on how it can be done otherwise? This would be no issue with a glycol system, but keen to see how it can be done with CO2?

5. Thermal Energy Metering
It appears some of the cool rooms are to be leased by tenants and consideration should be given to providing thermal energy metering. Thermal energy metering with glycol (flow meter plus differential temperature sensor) is not an issue. Does anyone know of an accurate way to measure liquid CO2 flow rate? Any suitable products out there that are reliable and have been used for such applications?

Apologies for the multitude of questions. Hopefully some of you have had experience with the above! Any responses would be much appreciated.

Thanks and kind regards,
M