It Ain't Half Hot, Mum

As I came back from Holiday, where it was both Scorchio! (link added for the 'young' ones) and also humid to the point that merely stepping out of an air conditioned area, resulted in my slipping on a puddle of my own sweat... I remember last year's heat wave, the "robust" communications with contractors, and seeing every person and their dog trying to flog system add-ons which will apparently eliminate all hot weather issues a system has...

And it bothers me.

Why does it bother me so much? The vast majority of hot weather issues encountered by all refrigeration stakeholders in the UK are easily preventable. There is no excuse to be resigned total "outages" as simply the nature of the beast.

This is important because this increases the costs of refrigeration to every stakeholder. It increases the environmental impact. It also robs people of their summer, increases stress, and drives people out of the refrigeration industry.

So lets kill a few sacred cows that are pervasive through the industry, hopefully enlighten a few people, annoy a few others, and maybe people much more knowledgeable than me, can chip in too.

The pack's design ambient is 32°C, and it was 33°C - nothing we can do, Squire...

The most common response I saw from service providers was last year was - "pack tripped, ambient temperature above rated condition, will return when cooler", usually with a request to sign off a request for money for attending.

As a former pack designer - lets get something straight right now.

The design ambient temperature is not the maximum ambient temperature you can expect a pack system to operate in.

If you had a car with a rated for a 70 mph on a flat motorway, consuming 50 mpg - would you expect the car to simply stall and breakdown on the hard shoulder once presented with a 1% gradient incline? Hopefully not. You would expect either to increase the throttle and fuel consumption to increase, or the speed to decrease.

And this should be the case with a well designed, well maintained refrigeration system.

Lets just have a look at what a Copeland ZB45 Scroll compressor does on R404A (For reasons)

We see when we increase the condensing temperature of the typical scroll compressor? For every °C increment:

• Duty reduces by approximately 2%.
• PI (Power Input) increases by 2%.
• THR (Total Heat of Rejection) reduces by 0.5%.

Those lines are flat. If a refrigeration system can not handle a short term 2% reduction in maximum refrigeration capacity - you have a significant problem regardless of hot weather. A refrigeration design engineer will typically look to have 10% contingency in hand in sizing any system and that's before you get to considering what the actual load is at any one time.

The THR actually reduces. Condensers that have been designed with 10% contingency (a typical factor of safety), shouldn't be running out of capacity once the ambient and condensing temperature increases by 1-2 K (other factors will limit condenser capacity, and we well discuss that later).

If we look at the ZB45's operational envelope - even at 50°C condensing temperature, the compressor can handle it quite handily. And this shouldn't come as a surprise. Manufacturers of refrigeration components want components to perform (to varying degrees)in various global regions. From the tip of Norway, to Death Valley.

I'm using a ZB45 scroll for its ubiquitous nature in the UK*, but it should be noted that different compressors and compressor types (From Scroll, to Rotary, to Reciprocating Piston, Screw, Centrifugal) can have different characteristics, (e.g. isentropic and volumetric efficiency) over the same envelope.

Regardless small increases of ambient temperature above the rated condition shouldn't have any significant effect on operation of a well designed and maintained system. This is important, because it means that something is actually wrong, and it can be fixed in most cases..

Why do refrigeration systems fall over in hot weather?

Hot weather causes refrigeration systems to fall over because hot weather creates conditions that expose latent (hidden or dormant) issues in the following (not in any particular order):

• Poor Maintenance.
• Poor Commissioning.
• Poor System Design/Manufacture/Installation.

Poor Maintenance

Latent issues likely become visible in hot weather. Such issues as:

• Refrigerant charges that just scrape through a cold spring with a liquid level just above the minimum receiver volume, will struggle (and fail) when hot weather occurs because you need 30-50% more refrigerant simply to do the same duty in the hotter weather. Note the Pressure Enthalpy Diagram (or PH Chart) below.

• The compressor which had been isolated for months whilst a replacement is waited to be fitted.
• The oil levels which were good enough to keep one and half compressors running at anyone time, not being sufficient to run 3,
• The two failed fans on the condenser , with the third barely keeping condensing pressures under control during cooler conditions.
• The coldstore evaporator which resembles a the Snowman, more than a piece of cooling equipment.

Things do break and malfunction and it is important for stakeholders to ensure that systems are properly maintained, whether with appropriate service and predictive, planned, and reactive maintenance solutions being procured by the end user, to technicians carrying out those regimes diligently and competently, to problems being identified and rectified promptly.

Poor Commissioning

This topic refers to the final state of commission for a particular system. One common reason why systems might be more likely to fail in hot weather is a "disconnect" between system designers and the technicians commissioning the system, and sometimes, a disconnect between both sets of people and physical reality. It's important to recognise this, because this can cause systems to fail without being undersized, or failed parts. It creates costs and loss of sales, for no reason than a lack of consideration.

Inappropriate commissioning can result inbuilt inefficiencies, increased costs of ownership, but relevant to this article poor states of commission will result in refrigeration outages, when there is no valid need to shut down. Be it inappropriately set pressure switches through to valve control philosophy that simply creates a refrigeration outage, because the requirements of both the system and the end users haven't been understood at all levels.

Taking some examples from Transcritical CO2 and how the High Pressure Valve (HPV) is controlled has been a common point of needless refrigeration outages. Daniel Clark of Isentra goes into detail about how inappropriate HPV control points create needlessly high volumes of flash gas, which then (amongst other things) overwhelms the capacity of the Flash Gas Bypass Valve (FGBV)!

A very common issue in 2022 was the how some default HPV control settings would result in the gas cooler pressure exceeding the pressure of the high pressure safety switch, stopping the system.

This occurred solely due to the HPV being commissioned to control to the optimal CoP within it's commissioned control band. That the control band overlapped the HP Safety switch was not picked up by the designers or the commissioners.

This is one example result of a disconnect between designers, technicians, and a failure to understand what all parties want the system to do, in hot weather.

Poor System Design/Manufacture/Installation

Thinking about poor designs, manufacture, installations, there have been more than a few that I've been responsible for, one that required liberal application of an angle grinder, to my esteemed former colleague Mike Humphries shoehorning a 1.5 metre high pack control panel into 800 mm at short notice.

But I'm not going to focus on those, because we're talking about HOT WEATHER - I'm going to focus on something much more fundamental.

One major issue that has affected many stakeholders - if you procured, selected, installed, commissioned, maintained a UK manufactured HFC refrigeration plant for retail refrigeration in the last 20 years you probably worked on a system which was designed fire and foremost with R404A. That system probably came with a 24.8 bar Pressure Relief Valve on the Liquid Receiver.

That particular setting of 24.8 bar, happens to correspond to the 55°C high side saturated refrigerant temperature (bubble) for an air cooled condenser in a 32°C Ambient temperature that is stated in Table 2 in Section 6.2.2.1 Maximum Allowable Pressure (PS) of BS EN 378-2:2016.

That selection point is designed to account for "maximum ambient temperature;?the possible accumulation of non-condensable gases; the setting of any pressure relief device;?the method of defrosting;?the application (e.g. cooling or heating application); solar radiation; (e.g. impact on ice rinks during standstill);? fouling;"

It is likely that significant amount of systems procured throughout the UK, regardless of the desired design ambient temperature, regardless of the refrigerant (potentially with glide) - that unit, came with a 24.8 bar PRV on the liquid receiver. There are notable exceptions, but all too often, it came with a 24.8 PRV as standard, regardless of the ambient temperature, the application, and so on.

The good news is the system remains safe.

The mediocre news... is the system might been subject to a slight envelope reduction if you've retrofitted R407F to extant R404A systems. In heat recovery applications, the performance might be significantly worse...

The bad news is that the system probably has no more ability to operate in ambient temperatures than a system designed explicitly for 32°C ambient temperatures.

The additional capacity compressor capacity that has been purchased for operation in a 38°C ambient is worthless, unless it has been designed, installed, and commissioned to reflect the higher ambient operating requirement, which according to the table below, should be determining a high side pressure for air cooled refrigeration systems corresponding to a saturated temperature of 59°C.

Was that checked that? Did the Application Engineer note it? The Mechanical Design Engineer select the right PRV? The Installing contractor check compliance to specification? Did the client or their agents?

All too often these systems slipped through. This isn't just the responsibility of one person to ensure compliance, but it should be noted that simply selecting enough compressors to operate in a 35°C ambient, is not enough to ensure high reliability and resiliency in hot weather.

This is one example - but complete design of a refrigeration system doesn't just sit with the plant manufacturer. Sub-optimal design choices by all stakeholders can result in refrigeration systems that are not as resilient as they should be. Some times these choices are unavoidable and need to be mitigated but mostly , sub-optimal designs are entirely avoidable.

What do I need to do, so my refrigeration system does not fail in Hot Weather?

1. Do not automatically accept the refrain "It's too hot". It's rarely too hot. It's mostly latent issues which manifest themselves in hot weather to the point of mischief. Automatically accepting that "it's too hot" means that the same latent issues will cause issues the following period of hot weather.
2. Set in place appropriate service and maintenance regimes, and work with competent partners so that latent issues (many which may be patent (visible) on a PPM, are dealt with. It will save everybody money if we identify the root cause of issues quickly and put in place the remedial actions promptly.
3. Identify the problematic sites and systems, and inspect why they are falling over. What are the trends? Is everything working? What are the failure conditions? What can we do with the existing system to make it work?
4. Make sure that the appropriate plant is selected, the refrigeration system is designed, installed, and commissioned appropriately.

It's a lot of work for anybody - which is why you should partner with people who know how to get to the root cause of the issue and at the best value and engineering practice. Colin Chapman CBE of Lotus Cars is quoted as saying (emphasis mine)

Simplify, then add lightness.

Thanks for reading this if you made it to the end, and I hope you found it interesting, if not useful.

*(as ex. Arctic Circle employee, with their own compressor manufacturer, we had a love hate relationship with the black souled hermetic compressor, both being well packaged and very efficient where you needed it to be)